1
Introduction
1.1
General
Reactivity of Alkyne-Gold(I) Complexes
For centuries, gold
had been considered a precious, purely decorative
inert metal. It was not until 1986 that Ito and Hayashi described
the first application of gold(I) in homogeneous catalysis.
1
More than one decade later, the first examples
of gold(I) activation of alkynes were reported by Teles
2
and Tanaka,
3
revealing
the potential of gold(I) in organic synthesis. Now, gold(I) complexes
are the most effective catalysts for the electrophilic activation
of alkynes under homogeneous conditions, and a broad range of versatile
synthetic tools have been developed for the construction of carbon–carbon
or carbon–heteroatom bonds.
Gold(I) complexes selectively
activate π-bonds of alkynes in complex molecular settings,
4−10
which has been attributed to relativistic effects.
11−13
In general, no other electrophilic late transition metal shows the
breadth of synthetic applications of homogeneous gold(I) catalysts,
although in occasions less Lewis acidic Pt(II) or Ag(I) complexes
can be used as an alternative,
9,10,14,15
particularly in the context of
the activation of alkenes.
16,17
Highly electrophilic
Ga(III)
18−22
and In(III)
23,24
salts can also be used as catalysts,
although often higher catalyst loadings are required.
In general,
the nucleophilic Markovnikov attack to η2-[AuL]+-activated alkynes 1 forms trans-alkenyl-gold
complexes 2 as intermediates
(Scheme 1).
4,5a,9,10,12,25−29
This activation mode also occurs in gold-catalyzed
cycloisomerizations of 1,n-enynes and in hydroarylation
reactions, in which the alkene or the arene act as the nucleophile.
Scheme 1
Anti-Nucleophilic Attack to η2-[AuL]+-Activated Alkynes
Structurally, Au(I) predominantly forms linear two-coordinate
complexes,
although higher coordination numbers are also possible.
30
A significant number of alkyne-gold complexes
have been characterized
31,32
and studied either
in solution
32,33
or theoretically.
34
This selective activation of the alkyne moiety
can explain a vast majority of the results experimentally observed
for gold(I)-catalyzed cyclization of 1,n-enynes.
Nevertheless, complexes of gold(I) with the alkene moiety of the enynes
are also formed in equilibrium with the alkyne-gold complexes.
35
Indeed, well-characterized complexes of gold(I)
with alkenes have been reported,
36
as well
as with allenes
37
and 1,3-dienes.
38
Despite the fact that simple gold salts
such as NaAuCl4 or AuCl are active enough to catalyze several
transformations, gold(I)
complexes bearing phosphines or N-heterocyclic carbenes as ligands
have found more wide-ranging applications.
39
The active species are often generated in situ by chloride abstraction
from [LAuCl] upon treatment with a silver salt bearing a weakly coordinating
anion.
Complexes [LAuY] only exist as neutral species when Y– is a coordinating anion (halides,
carboxylates, sulfonates,
and
triflimide). The corresponding complexes with less coordinating anions,
such as SbF6
–, PF6
–, or BF4
–, are in most of the cases
not stable. Although, species [AuL]+ (also known as “naked
gold complexes”) are often suggested in mechanistic proposals,
structural proof for their existence as stable, isolable species is
still lacking. Here, for the sake of simplicity in mechanistic schemes
throughout this review, LAu+ is used as a surrogate of
[LAuL′]+ complexes, where L′ states for a
relatively weakly bound ligand such as the substrate (alkyne or alkene),
product, or solvent molecule.
It is important to remark that
when the catalytically active species
are generated in situ by chloride abstraction from complexes [LAuCl]
in the absence of the alkyne or other unsaturated substrate, much
less reactive chloride-bridged dinuclear species [LAuClAuL]Y are readily
formed.
40
Formation of these dinuclear
complexes could explain, at least partly, the erratic results that
have been ascribed as the “silver effects” in reactions
in which Ag(I) salts are used in situ to activate neutral gold(I)
complexes [LAuY].
41
Often, the most
convenient catalysts for the activation of alkynes
are complexes [LAuL′]X or [LAuX] bearing weakly coordinating
neutral (L′)
42
or anionic ligand
(X–).
43
These complexes
can enter catalytic cycles by ligand exchange with the unsaturated
substrate, which proceed by associative mechanisms as observed for
Au(I) and other diagonal d10 metal centers.
44
Thus, large negative activation entropies characteristic
of associative mechanisms have been determined for the rate determining
ligand exchange reactions of substituted alkyne
45,46
and alkenes
36o
on commonly used Au(I)
catalysts. Although nitriles are frequently used as weakly coordinating
neutral ligands, 1,2,3-triazole
46,47
or other related ligands
48
have also been employed.
The properties
of gold(I) complexes can be easily tuned sterically
or electronically depending on the ligand, consequently modulating
their reactivity in the activation of alkynes, alkenes, and allenes.
27,29f,49
Thus, complexes containing more
donating N-heterocyclic carbenes (3) are less electrophilic
than those with phosphine ligands (4, 5)
(Figure 1).
28
Complexes
with less donating phosphite ligands (6) and related
species are the most electrophilic catalysts.
Figure 1
Increase in electrophilicity
with decreased donating ligand ability
in gold(I) complexes.
Gold(I) complexes bearing weak-coordinated ligands such as
Me2S, thiodiglycol, or tetrahydrothiophene (tht) have been
widely
used for the preparation of soluble gold(I) complexes, commonly starting
from a gold(III) source.
50
Complex [Au(tmbn)2]SbF6 (tmbn = 2,4,6-trimethoxybenzonitrile), in
which gold(I) is supported by two nitrile ligands, can be used for
the in situ preparation of a variety of chiral and achiral cationic
complexes [LAu(tmbn)]SbF6, including complexes immobilized
on a polymeric support.
42a
Other immobilized
gold(I) complexes have also been prepared.
51
The use of gold complexes bearing chiral ligands has led to the
development of efficient asymmetric gold-catalyzed transformations.
52
Less common precatalysts used in gold(I)-catalyzed
transformations are gold hydroxo complex IPrAuOH, which is activated
in the presence of Brønsted acids,
53
open carbenes,
39c,54
and other related complexes,
55
which give rise to selective catalysts of moderate
electrophilicity. Cyclopropenylylidene-stabilized phospenium cations,
which behave similarly to classical triaryl- and trialkylphosphines,
have also been used as ligands in gold-catalyzed reactions.
56
The effect of the counteranion has been
studied in detail for several
gold(I)-catalyzed transformations.
57,58
Thus, for
the intermolecular reaction of phenylacetylene with 2-methylstyrene
catalyzed by [t-BuXPhosAu(NCMe)]Y, it was found that
yields increase depending on the counteranion in the order Y = OTf– < NTf2
– < BF4
– < SbF6
– < BARF (BARF = 3,5,bis(trifluoromethyl)phenylborate). By using
the bulky and noncoordinating anion BARF, yields are increased by
10–30% compared to those obtained when Y = SbF6
–, probably due to a decrease in the formation of the
unproductive σ,π-(alkyne)digold(I) complexes from the
initial alkyne.
57
1.2
Scope
and Organization of the Review
Homogeneous gold(I)-catalysis
has experienced an outbreak in the
past decade leading to the discovery of a remarkable amount of new
synthetically useful transformations. Thus, in recent years many groups
have used gold catalysis in key steps of total synthesis taking advantage
of the unique catalytic ability of gold to build molecular complexity
under mild reaction conditions.
Several reviews have been published
on gold(I)-catalyzed reactions of alkynes, enynes, and related substrates,
5,7,25−28,59
as well as on gold(I)-catalyzed reactions of allenes
60
and cascade gold-catalyzed reactions.
61
Moreover, specific reviews focused on gold-catalyzed
carbon-heteroatom bond formation
62
and
on the use of gold catalysis in total synthesis
63
have also been published. In this review, we will cover
reactions of alkynes activated by gold(I) complexes, including recent
applications of these transformations in the synthesis of natural
products. According to the aim of this thematic issue, the main focus
is on the application of gold(I)-catalyzed reactions of alkynes in
organic synthesis, although reactions are organized mechanistically.
Reactions of gold(I)-activated alkenes and allenes, as well as gold(III)-activated
alkynes, will not be covered.
The discussion has been primarily
organized based on the different
reactions catalyzed by gold(I) complexes that alkynes can undergo.
When possible, inter- and intramolecular processes, as well as the
applications in total synthesis, are treated in specific subsections.
2
Addition of Heteronucleophiles to Alkynes
2.1
Addition of O-Nucleophiles to Alkynes
The effectiveness
of gold(I) complexes for the activation of alkynes
toward inter- and intramolecular nucleophilic attack has been demonstrated
for a variety of heteronucleophiles. Due to relativistic effects,
cationic gold complexes possess, besides a high π-acidity, a
low oxyphilicity,
7,8
and therefore they are able to
activate unsaturated C–C bonds in the presence of H2O, alcohols, or any other oxygen-containing
functional groups. Hence,
once the alkyne is activated, the nucleophilic oxygen can attack forming
a new C–O bond.
2.1.1
Intermolecular Addition
of O-Nucleophiles
to Alkynes
Hydration and alkoxylation of alkynes are industrially
important processes for the synthesis of carbonyl compounds. A well-known
method for the addition of water and alcohols to alkynes uses toxic
Hg(II) salts under acidic conditions.
64
Other less harmful although expensive transition-metal-based catalytic
systems have also been described,
65
including
the use of Au(III) salts.
66
The first
examples of gold(I)-catalyzed addition of alcohols and water to alkynes
were reported by the groups of Teles
2
and
Tanaka,
3
respectively, employing air stable
gold complexes [AuMe(L)] (L = phosphine, phosphite or arsine), which
were activated in situ by protic acids to form acetals such as 7 or ketones 8 and
9 (Scheme 2). Markovnikov-type addition was observed in all
cases, being reactive under these reaction conditions both terminal
and internal alkynes. Despite its efficiency, this method suffers
from several drawbacks, including the use of concentrated solutions
of strong acids and relatively high catalyst loadings.
Scheme 2
Hydration
and Acetalization of Alkynes with Ph3PAuMe and
Protic Acids
Although the catalytic
hydration of alkynes with neutral N-heterocyclic
carbene gold(I) chlorides or carboxylates in the presence of B(C6F5)3 was already
demonstrated in 2003,
67
later in 2009 it was found that the use of gold(I)
complexes bearing bulkier N-heterocyclic carbene ligands allowed to
catalyze this process at loadings as low as <10 ppm under acid-free
conditions (Scheme 3).
68
This catalytic system showed wide versatility, since both terminal
and internal alkynes possessing any combination of alkyl and aryl
substituents were suitable substrates. However, the reaction required
high temperatures to proceed and for unsymmetrical internal alkynes
only moderate regioselectivities were obtained. Alkynes can also be
hydrated at room temperature without any acidic cocatalyst in the
presence of gold(I)-phosphine complexes.
69
Scheme 3
Hydration of Alkynes with Low-Catalyst Loadings
The regioselective hydration of haloalkynes
was recently reported
to afford α-halomethylketones 12 in excellent yields
under very mild reaction conditions (Scheme 4).
70
This procedure was applied on the
gram-scale, and the catalyst loading could be decreased up to 2 mol
% with little effect on the yield. However, ortho-substituted arylacetylenes provided
only trace amounts of the carbonyl
compounds.
Scheme 4
Hydration of Haloalkynes
Cyclic acetals are important building blocks present in
many natural
occurring biologically active compounds, and, moreover, they are very
useful as protecting groups.
71
They are
usually obtained by the addition of a diol to the corresponding aldehyde
or ketone under Brønsted acid conditions. When diols are added
to alkynes in the presence of a gold(I) catalyst, cyclic acetals 13 are regioselectively
formed instead of the unprotected
carbonyls (Scheme 5). This reaction proceeds
with complete regioselectivity in the case of terminal alkynes and
with moderate selectivity for internal alkynes, yielding a mixture
of cyclic acetals 13/13′.
72
The reaction can also be expanded to the preparation
of acetals from 1,5-diols.
Scheme 5
Acetalization of Alkynes
In a more recent example, α-bromo cyclic
and acyclic acetals 14 and 15 were obtained
from terminal alkynes
and NBS (Scheme 6).
73
The reaction proceeded via formation of bromoacetylenes followed
by addition of the O-nucleophiles to the triple bond.
Scheme 6
Bromo-Acetalization of Alkynes
The role of the anion in the intermolecular alkoxylation
of alkynes
catalyzed by N-heterocyclic carbene-based gold(I) complexes [(NHC)AuX]
(X = BARF–, BF4
–, OTf–, OTS–, TFA–, AcO–) has recently been dissected, with the conclusion
that both coordination ability and basicity have a great impact on
this transformation.
74
The most important
factor seems to be the ability to abstract the proton from the alcohol
during the nucleophilic attack, which is directly related to the anion
basicity.
Gold(I)-catalyzed intermolecular additions of alcohols
to alkynes
can be coupled with other tandem reactions, therefore increasing the
degree of molecular complexity in the final product.
75
The addition of allylic alcohols to alkynes followed by
a Claisen rearrangement of the resulting intermediates (16) has recently been developed,
leading to the formation of γ,δ-unsaturated
ketones 17 in an efficient one-pot procedure from simple
starting materials (Scheme 7).
76
Scheme 7
Synthesis of Homoallylic Ketones by Nucleophilic Addition-Claisen
Rearrangement
2.1.2
Intramolecular
Addition of O-Nucleophiles
to Alkynes
The high activity of gold(I) salts and complexes
for C–O bond formation has been widely exploited intramolecularly
to construct oxygen containing heterocycles. In this context, one
of the most studied transformations is the intramolecular gold(I)-catalyzed
cyclization of unactivated alkynols.
60b,77
As an example,
alkynediols 18 underwent a gold(I)-catalyzed cyclization
leading to furans 20 after elimination of H2O from intermediate 19 in the presence
of very low catalyst
loadings (Scheme 8).
78
The alcohol moiety may already be present in the precursor of the
gold(I)-catalyzed transformation
79
or may
be generated in situ from epoxides by ring opening.
80
Carbonyls can also act the nucleophiles.
81
Thus, alkynyloxiranes 21 were converted into
trisubstituted furanes 22,
80b
and alk-4-yn-1-ones 23 gave rise to 2,4,5-trisubstituted
furans 24 or substituted 4H-pyranes 25 depending on the substitution pattern of the
substrate
(Scheme 9).
81b
Scheme 8
Synthesis of Furans by Cyclization of Alkynediols
Scheme 9
Cyclization of Epoxy and Ketoalkynes
Furan derivatives were obtained from Z-enynols 26 (Scheme 10).
82
Enynols bearing a secondary alcohol
afforded substituted
furans 27, and those containing a tertiary alcohol gave
dihydrofurans 28. In the presence of gold(I) catalysts
bearing N-heterocyclic carbene ligands these substrates were also
converted into furan structures allowing the use of low catalyst loadings
and very mild reaction conditions.
83
Scheme 10
Synthesis of Furan Derivatives from Z-Enynols
Oxazocenones 30 have been recently synthesized via
gold(I)-catalyzed 8-endo-dig hydroalkoxylation of
alkynamides 29 (Scheme 11).
84
Scheme 11
Synthesis of Oxazocenones
The reaction of alkynediols to give bicyclic
acetals by gold(I)-catalyzed
intramolecular hydroalkoxylation of terminal alkynes using bis-homopropargylic
alcohols 31 was first described in 2005, opening a door
to an interesting family of strained acetals 32 (Scheme 12).
85
Notably, the addition
of methanol to alkene intermediates did not occur under the reaction
conditions. Recently, the selective conversion of acetonide-tethered
alkynes into bridged acetals through an analogous process using Ph3PAuCl and AgOTf
in the presence of water was reported.
86
Scheme 12
Intramolecular Acetalization of Alkynes
As described for intermolecular
additions of O-nucleophiles, the intramolecular version
can also be combined with
subsequent tandem reactions in order to increase molecular complexity
of the resulting products. One of the simplest examples is the tandem
cycloisomerization/hydroalkoxylation of homopropargylic alcohols in
the presence of an external alcohol to form tetrahydrofuranyl ethers 33 (Scheme 13).
87
Bishomopropargylic alcohols also react with gold(I) in
a similar fashion, giving rise to methylene tetrahydrofuran motifs,
which in the presence of an external nucleophile are the entry to
a variety of structurally different products.
88
Scheme 13
Tandem Cycloisomerization/Hydroalkoxylation of Homopropargylic
Alcohols
The formation of
bicyclo[3.2.0]heptenes 35 was reported
involving an intramolecular addition of an oxygen-containing nucleophile
to a cyclopropyl alkyne 34 activated by gold(I) (Scheme 14).
89
The same group
also developed an unprecedented gold(I)-catalyzed tandem reaction
to form polysubstituted dihydrofurans from acetal-protected propargylic
alcohols and carbonyl compounds.
90
Scheme 14
Synthesis of Bicyclo[3.2.0]heptenes by Intramolecular Addition to
a Cyclopropyl Alkyne
The synthesis of tricyclic cage-like structures was described
starting
from diyne-diols by trapping the intermediate dienol ether, which
results from a double intramolecular hydroalkoxylation with external
nucleophiles.
91
The synthesis of
3-alkoxyfurans 37 from acetal-containing
propargylic alcohols 36 has recently been reported.
92
Thus, these synthetically useful substrates
can be easily prepared in two steps from readily available aldehydes,
alcohols, and 3,3-diethoxypropyne (Scheme 15).
Scheme 15
Synthesis of 3-Alkoxyfurans from Acetal-Containing Propargylic
Alcohols
In the case of alkynylketones
or esters, the mechanism of the gold(I)-catalyzed
hydration or alkoxylation involves the anchimeric assistance of the
carbonyl group forming an intermediate that is opened back by one
water or alcohol molecule leading to the final carbonyl compounds.
93
Diethynylketones 38 were converted
into naphthol derivatives 40 by this ketone-assisted
hydration followed by cyclization (Scheme 16).
Scheme 16
Synthesis of Naphthol Derivatives from Diethynylketones
In the formation of tricyclic
spiroketones 44/44′ by tandem hydration/Conia-ene/aldol
condensation,
93b
the first step also takes
place via anchimeric
assistance of the carbonyl moiety to give intermediate 42 and not through direct addition
of H2O to 41 (Scheme 17).
Scheme 17
Synthesis of Tricyclic
Spiroketones by Tandem Hydration/Conia-Ene/Aldol
Condensation
2-Alkynyl-1,5-diketones 45 undergo intramolecular
oxygen transfer in the presence of AuCl via [4 + 2] cycloaddition
to form cyclopentenylketones 48 (Scheme 18).
94
The mechanism of this transformation
was proposed based on DFT calculations and also demonstrated by isotopic
labeling experiments, determining that the reaction proceeds via tandem
oxycyclization/[4 + 2] cycloaddition.
Scheme 18
Synthesis of Cyclopentenylketones
from 2-Alkynyl-1,5-diketones
A metal-dependent catalytic method has been developed
for the synthesis
of cyclohexadienes from enynals 49 and alkenes (Scheme 19).
95
When Cu(OTf)2 was used as the catalyst, 2,4-cyclohexadienes 52 were formed, whereas
in the presence of InCl3 or gold(I)
complexes, 1,3-cyclohexadienes 53 were obtained instead.
The gold(I)-catalyzed transformation proceeds via intramolecular addition
of the carbonyl to the alkyne, followed by a Diels–Alder reaction
between the resulting pyrylium intermediate 50 and the
alkene, and final demetalation.
Scheme 19
Catalyst-Dependent Synthesis of 1,3-Cyclohexadienes
Cascade-type sequences have
also led to the formation of pyrane
derivatives.
96
Tricyclic 2,3-benzopyranes 56 were synthesized by a tandem intramolecular hydroalkoxylation/hydroarylation
to obtain benzo-fused cyclic ethers.
97
Shortly
later, a gold(I)-catalyzed tandem intramolecular hydroalkoxylation/Prins-type
cyclization was described, affording oxygen-containing [3.3.2] bicyclic
compounds 58 diastereoselectively (Scheme 20).
98
Scheme 20
Synthesis of Oxygen-Containing
[3.3.2] Bicyclic Compounds by Cascade
Sequences Involving Intramolecular Hydroalkoxylations
Spiroketals, which are key structural units
in many biologically
active natural products, can also be obtained by gold(I)-catalyzed
intramolecular hydroalkoxylation of alkynediols and alkynetriols.
79,99
The use of an acetonide moiety to function as a regioselectivity
regulator in the spiroketalization process has been proposed to address
the possible regioselectivity issues in the formation of monounsaturated
spiroketals.
100
The first multicomponent
enantioselective gold(I)-catalyzed synthesis of spiroketals 59 has been recently described
using a gold-phosphate catalytic
system in a three-component coupling between alkynols, anilines, and
glyoxylic acid (Scheme 21).
101
More recently, another synthesis of spiroketals has been
reported combining a gold(I)-catalyzed cycloisomerization of an alkynol
with an inverse-electron-demand hetero Diels–Alder mediated
by Y(OTf)3.
102
Scheme 21
Enantioselective
Synthesis of Spiroketals by Coupling of Alkynols,
Anilines, and Glyoxylic Acid
Carboxylic acid derivatives,
103
carbonates,
104
carbamates,
105
sulfoxides,
106
boronic
acids,
107
and related O-nucleophiles
108
can also add to alkynes
(Scheme 22). Acetylenic carboxylic acids 60 undergo selectively
an exo-cyclization providing functionalized γ-lactones 61.
103a
Analogously, tert-butyl carbonates derived from homopropargyl alcohols 62 cyclize
to afford cyclic enol carbonates 63(104b) and N-Boc propargylamines 64 yield 1,3-oxazin-2-ones
65.
105b
Gold(I) also promotes the rearrangement of homopropargyl
sulfoxides 66 to give benzothiepinones 67(106a) and the formation of boron enolates
70 from alkynes, which can be further transformed into other
derivatives such as diols 71.
107
Scheme 22
Intramolecular Addtion of O-Nucleophiles to
Alkynes
A gold(I)-catalyzed
tandem sequence has been developed for the
synthesis of dihydropyridones 76 from homopropargylic
carboxamides 72 (Scheme 23).
109
After an intramolecular carbonyl addition of
the homopropargylic amide to the alkyne, the formation of a σ-complex
of the gold salt with the intermediate oxazine 73 promotes
a nucleophilic addition of an external alcohol to form 74 followed by a Petasis-Ferrier
rearrangement via 75.
Based on this concept, a new protocol for the synthesis of benzyl
alcohols has been described generating the benzylating agent upon
treatment of N-Cbz-N-benzyl-propargylamine
with IPrAuNTf2.
110
This reaction
takes place under very mild conditions and eliminates the need of
base additives.
Scheme 23
Synthesis of Dihydropyridones by Intramolecular Addition
of Homorpropargylic
Amides to Alkynes
1-Oxo-4-oxy-5-ynes 77 react with gold(I)
to form s-trans-methylene(vinyl)oxonium
intermediates 78, which in the presence of an external
alkene undergo a
formal [4 + 2] cycloaddition giving rise to 9-oxabicyclo[3.3.1]nona-4,7-dienes 79
(Scheme 24).
111
Scheme 24
Reaction of 1-Oxo-4-oxy-5-ynes with Alkenes To Form
9-Oxabicyclo[3.3.1]nona-4,7-dienes
2.1.3
Addition of O-Nucleophiles to Alkynes in
Total Synthesis
The gold(I)-catalyzed formation of O-heterocycles is a powerful tool in the field
of total
synthesis of natural products. In the synthesis of (−)-atrop-abyssomicin
C (82),
112
the bridged bicycle 81 was obtained via intramolecular 6-exo-dig cyclization of alkynol
80 (Scheme 25). Another remarkable example is found in the total synthesis
of bryostatin 16 (85),
113
in
which a gold(I)-catalyzed oxycyclization of alkynol 83 generates the dihydropyran
cycle present in the natural product
(Scheme 26). In the formal total synthesis
of didemniserinolipid B, a gold(I)-catalyzed 6-endo-dig alkynol-cycloisomerization
was considered as the key step to
construct the bicyclic acetal skeleton.
114
Another example of gold(I)-promoted cycloisomerization of alkynols
was recently reported in the total synthesis of (±)-cafestol.
115
In this work, the furan ring is constructed
in a late-stage of the synthesis via intramolecular 5-endo-dig cycloisomerization
followed by dehydration.
Scheme 25
Synthesis of (−)-atrop-Abyssomicin
C
Scheme 26
Synthesis of Bryostatin 16
The formation of spiroketal
skeletons catalyzed by gold(I) has
also been successfully applied in several total syntheses.
116
Okadic acid (86) is a natural
occurring polyether containing three spiroketal motifs. The C15–C38
fragment was synthesized taking advantage of the high activity and
selectivity of AuCl for the synthesis of spiroketals starting from
alkynediols (Scheme 27).
117
Scheme 27
Construction of the Spiroketal Fragments in the Total
Synthesis of
Okadic Acid
The ability of carboxylic
acid derivatives to add to alkynes in
the presence of gold(I) complexes was exploited in the first total
synthesis of neurymenolide A (94)
118
(Scheme 28) and in the synthesis
of psymberin (97)
119
(Scheme 29).
Scheme 28
Synthesis of Neurymenolide A
Scheme 29
Synthesis of Psymberin
A gold(I)-catalyzed tandem
reaction of 1,7-diynes bearing a propargylic
carboxylic acid was developed for the total synthesis of drimane-type
sesquiterpenoids,
120
involving a first
intramolecular alkoxylation to give a furanone intermediate that undergoes
an alkoxycyclization with benzyl alcohol. A similar tandem alkoxylation/alkoxycyclization
sequence was also recently used in the synthesis of cladiellins.
121
2.2
Addition of N-Nucleophiles
to Alkynes
2.2.1
Intermolecular Addition of N-Nucleophiles
to Alkynes
Despite the fact that gold(III)-catalyzed hydroamination
of terminal alkynes had been known since 1987,
122
it was not until 2003 that Hayashi and Tanaka developed
the first gold(I)-catalyzed intermolecular amination of alkynes with
anilines to form imines 98/98′ (Scheme 30).
123
Another more recent
example was developed using gold(I) complexes bearing 1,2,4-triazole-based
N-heterocyclic carbene ligands for the formation of the corresponding
ketimines from terminal alkynes and aniline derivatives.
124
A different approach for the formation of imines
was based on gold(I)-phosphine complexes bearing a low coordinating
bis(trifluoromethanesulfonyl) imidate counterion, namely SPhosAuNTf2.
125
These complexes are active
catalysts for the regioselective intermolecular hydroamination of
both internal and terminal alkynes under mild reaction conditions,
showing a regioselectivity based on electronic rather than steric
factors. This electronic control on the regioselectivity of alkyne
hydroamination reactions had earlier been reported for the hydroamination
of unsymmetrical electron-poor and electron-rich alkynes with anilines
(Scheme 31).
126
More
challenging hydroaminations of less-reactive internal alkynes an unprotected
aliphatic amines could be performed using a novel series of 1,2,3-triazole-based
cationic gold(I) complexes,
47a
which turned
out to have a superior thermal stability than other catalysts previously
reported.
Scheme 30
Intermolecular Amination of Alkynes to Form Imines
Scheme 31
Regioselective Hydroamination of
Unsymmetrical Alkynes
Nowadays, as happens for the addition of O-nucleophiles
to alkynes, gold(I)-catalyzed hydroamination of alkynes is not limited
to the synthesis of imine or enamine products by formal addition of
N–H reagents onto triple bonds.
123,127
Instead,
a number of tandem sequences involving alkyne-hydroamination steps
have been reported leading to the formation of more complex structures
such as azaflavanones 101(128) or quinoline derivatives 102(129) (Scheme 32). Another
example refers to a
tandem intermolecular hydroamination/transfer hydrogenation catalyzed
by gold(I) combined with a chiral Brønsted acid to form enantioenriched
secondary amines 103 (Scheme 33).
130
This methodology tolerates a wide
range of aryl, alkenyl, and aliphatic alkynes, as well as a range
of anilines with different electronic properties.
Scheme 32
Synthesis of Azaflavanones
and Quinoline Derivatives
Scheme 33
Enantioselective Hydroamination/Transfer Hydrogenation Synthesis
of Secondary Amines
An enantioselective tandem hydroamination/hydroarylation
of alkynes
has recently been reported using a gold(I)/chiral Brønsted acid
binary system as catalyst.
131
In this transformation,
the gold(I)-activated alkynes react with a range of pyrrole-based
aromatic amines to give pyrrole-embedded aza-heterocyclic scaffolds
bearing a quaternary center. Isoxazoles can also add to ynamides in
the presence of gold(I) in a formal [3 + 2] cycloaddition to give
polysubstituted 2-aminopyrroles.
132
The gold(I)-catalyzed intermolecular reaction between 2H-azirines and ynamides provides
highly substituted pyrroles 105 in a formal [3 + 2] cycloaddition (Scheme 34).
133
Scheme 34
Synthesis of Polysubstituted
Pyrroles from 2H-Azirines and Ynamides
2.2.2
Intramolecular Addition of N-Nucleophiles
to Alkynes
The intramolecular hydroamination of unactivated
alkynes is a very useful tool to construct nitrogen-containing heterocycles.
An efficient method to obtain pyrroles comes from the intramolecular
dehydrative cyclization of amino-3-alkyn-2-ols mediated by gold(I).
78b
A similar catalytic system catalyzed a tandem
direct amination/cycloisomerization from (Z)-2-en-4-yn-1-ols
in the presence of an external amine to give substituted pyrroles
under mild reaction conditions.
134
Alkynyl
amidoalcohols undergo a gold(I)-catalyzed spiroamidoketalization giving
rise to spiro-N,O-ketals with 5-
and 6-membered rings presumably via tandem intramolecular 5-exo-dig hydroamidation/intramolecular
oxycyclization.
135
Aryl-substituted N-tosyl
alkynylaziridines 106 are prone to undergo a gold(I)-catalyzed
ring expansion to form 2,5- (107) or 2,4-disubstituted
pyrroles (108) depending on the solvent and the counterion
of the gold complex (Scheme 35).
136
Similarly, the system Ph3PAuCl/AgOTf
is able to catalyze the rearrangement of propargylic aziridines forming
trisubstituted and cycloalkane-fused pyrroles.
137
This transformation involves an unusual tandem cyclization-opening/Wagner-Meerweein
sequence. The selective formation of 2,5-disubstituted pyrroles catalyzed
by the same Au(I)/Ag(I) system from acetylenyl aziridines was also
described using THF as solvent,
138
finding
that the presence of protic species such as MeOH increased the reaction
rate and the yield of the pyrrole products.
Scheme 35
Synthesis of 2,5-
or 2,4-Disubstituted Pirroles from Aryl-Substituted N-Tosyl Alkynylaziridines
A new method for the synthesis of substituted pyrroles 111 based on an intermolecular
hydroamination process has
recently been
reported.
139
The reaction takes place by
a tandem process consisting of an initial addition of a gold-acetylide
to an acetal moiety to form intermediate 110 followed
by a gold(I)-catalyzed 5-endo-dig cyclization and
final aromatization (Scheme 36).
Scheme 36
Synthesis
of Substituted Pyrroles by an Intermolecular Hydroamination
The synthesis of N-heterocycles bearing −CN
and −CF3 groups at the α-position has been
reported by intramolecular hydroamination coupled with the addition
of external nucleophiles to the resulting enamine intermediate.
140
Depending on the relative position of
the alkyne and the nucleophilic
nitrogen, the cyclization can proceed by 5-exo-dig
and 6-endo-dig pathways. In general, the electronic
properties of the substituent of the alkyne play a crucial role in
the control of regioselectivity. The presence of electron-donating
substituents on the alkyne tends to favor 6-endo-dig
cyclization pathway, whereas electron-withdrawing substituents favor
the 5-exo-dig cyclization. One example of this regioselectivity
reversal was reported in the cyclization of o-alkynylbenzyl
carbamates 112 (Scheme 37).
141
However, other factors such as steric interactions
have to be taken into account to predict the regiochemical outcome
of intramolecular hydroaminations of internal alkynes having these
two competing reaction pathways. In the case of terminal alkynes,
most of the heterocyclizations afford 5-exo-dig products
as a consequence of the better stabilization of the positive charge
on the internal carbon of the triple bond.
Scheme 37
Cyclization of o-Alkynylbenzyl Carbamates
Another example of exclusive formation of the 6-endo-dig cyclization product gave
isoquinoline derivatives
by a gold(I)-mediated
hydroamination of (2-alkynyl)benzyl carbamates.
142
Moreover, the hydroamination of alkynyl carbamates bearing
an acetal or enone was applied to the synthesis of tetracyclic heterocycles
such as 119 via a gold(I)-catalyzed tandem hydroamination/cyclization
(Scheme 38).
Scheme 38
Tandem hydroamination/Cyclization
of Alkynyl Carbamates
Alkynylureas can also undergo intramolecular hydroamination
processes
in the presence of gold(I) catalysts. In general, o-ethynylarylureas bearing an internal
alkyne lead to N1-attack-5-endo-dig cyclizations, regardless of
the gold(I) complex employed. In the case of 3-substituted 1-(o-alkynylphenyl)ureas
120, indole derivatives
are formed (Scheme 39). O-Ethynylphenylureas
can selectively undergo N3-attack-6-exo-dig cyclization when NHC-stabilized gold(I)
complexes are used.
143
In contrast, in a similar gold(I)-catalyzed
reaction previously reported, 1-(ortho-ethynylaryl)ureas
bearing a terminal alkyne in aqueous media led exclusively to the
corresponding indole under microwave heating through N1-5-endo-dig cyclization.
144
This method provided indole-1-carboxamides in moderate yields tolerating
a variety of functional groups. 1H-Imidazol-[1,5-a]indol-3(2H)-ones were also prepared
in
the presence of a gold(I) catalyst starting from urea derivatives.
145
On the other hand, acyclic alkynylureas
undergo O-attack-5-exo-dig cyclization
in the presence of
gold(I).
146
This feature was exploited
for the development of an asymmetric three-component tandem reaction
from imines, terminal alkynes, and sulfonylisocyanates in which a
single gold(I) species can catalyze both the alkynylation of aryl–aryl
imines and the subsequent 5-exo-dig cyclization to
afford enantioenriched five-membered carbamimidates.
Scheme 39
Regiodivergent
Cyclization of o-Ethynylarylureas
Indenyl-fused and 2,3-disubstituted indoles
have been obtained
from 2-tosylaminophenylpro-1-yn-3-ols in the presence of gold(I) through
a common vinyl gold intermediate.
147
Furthermore,
aniline-substituted diethynylarenes 121 give aryl-annulated
carbazoles 124 by a hydroamination/hydroarylation cascade
in the presence of gold(I) complexes (Scheme 40).
148
Scheme 40
Synthesis of Aryl-Annulated Carbazoles
by Hydroamination/Hydroarylation
Spiro-N,O-ketals with 5- and 6-membered
rings
have been recently synthesized under mild conditions via gold(I)-catalyzed
spiroamidoketalization of alkynyl amidoalcohols.
149
The analogous intramolecular hydroamidation of the corresponding
alkynylamide leading to a seven-membered ring was not successful.
2.2.3
Addition of N-Nucleophiles to Alkynes in
Total Synthesis
The potential of gold(I) to construct nitrogen
containing heterocycles has been demonstrated in several total synthesis.
The intramolecular gold(I)-catalyzed alkyne hydroamination reaction
provides an efficient entry to tetrahydroisoquinolines, as highlighted
in the total synthesis of (−)-quinocarcin (127).
150
In the presence of [JohnPhosAu(MeCN)]SbF6, 125 undergoes regioselectively a 6-endo-dig
hydroamination forming the corresponding dihydroquinoline,
which after reduction with NaBH3(CN) formed the desired
tetrahydrosioquinoline 126 (Scheme 41). In another remarkable example, the key step
in the total
synthesis of (−)-rhazinicine (132) and (−)-rhazinilam
(133)
151
was a gold(I)-catalyzed
cascade cyclization of 128 initiated by an intramolecular
6-exo-dig nucleophilic addition of the nitrogen atom
of the amide to the gold-activated alkyne to build the highly substituted
indolizinone skeleton 131 (Scheme 42).
Scheme 41
Synthesis of (−)-Quinocarcin
Scheme 42
Synthesis of (−)-Rhazinicine and (−)-Rhazinilam
2.3
Addition
of Other Heteronucleophiles to Alkynes
The gold(I)-catalyzed
addition of heteronucleophiles to alkynes
other than oxygen and nitrogen is by far a less developed transformation.
Nevertheless, it has been described the attack of the sulfur atom
of aryl thioethers to alkynes affording benzothiophenes 135 (Scheme 43).
152
This transformation has also been described in the presence of gold(III).
153
It was shown that aryl thiosilanes 136 can act as both sulfur nucleophiles and silicon
electrophiles capturing
the vinyl-gold intermediate in the gold(I)-catalyzed intramolecular
reaction to afford 3-silylbenzothiophenes 138 (Scheme 44).
154
When enantiomerically
pure o-alkynylphenyl-1-aryl ethyl sulfides were used
as substrates for this transformation, complete chirality transfer
was observed.
155
In an analogous transformation,
the gold(I)-catalyzed alkoxyboration of alkynes provides a method
for the preparation of O-heterocyclic boronic acid
derivatives 140.
156
Scheme 43
Synthesis
of Benzothiophenes by Cyclization of Aryl Thioethers
Scheme 44
Synthesis of 3-Silylbenzothiophenes and O-Heterocyclic
Boronic Acid Derivatives
Moreover, in the presence of gold(I) complexes, dithiols
can also
add to alkynes to form cyclic thioacetals,
72
and homopropargylthiols bearing a propargylic alcohol lead to thiophenes
after addition of the thiol to the gold-activated alkyne followed
by dehydration.
78
A novel rearrangement
was described for propargylic 1,3-dithianes 141 when
they were heated in the presence of Ph3PAuCl/AgSbF6. In this transformation, eight-membered
dithiosubstituted
cyclic allenes 142 were formed with good yields and remarkable
stability (Scheme 45).
157
Scheme 45
Synthesis of Eight-Membered Cyclic Allenes
N-Heterocyclic carbene gold(I)
bifluoride complexes have been shown
to be efficient catalysts in the hydrofluorination of symmetrical
and unsymmetrical alkynes.
158
This reaction
proceeds in good to excellent yields with high stereo- and regioselectivity
to afford fluorinated stilbene derivatives and fluorovinyl thioethers.
3
Gold(I)-Catalyzed Reactions of Alkenes with
Alkynes
3.1
Cycloisomerization of Enynes
3.1.1
General Mechanistic Aspects
Cycloisomerizations
of 1,n-enynes are probably one of the most illustrative
carbon–carbon bond forming reactions catalyzed by electrophilic
metal complexes. These transformations are very useful in synthetic
organic chemistry, since they provide access to complex molecular
architectures from readily assembled starting materials through mechanistically
complex, fully intramolecular processes.
The first example of
electrophilic activation of enynes was reported back in the 1980s
using palladium catalysts,
159
which promoted
an intramolecular Alder-ene reaction. The Alder-ene cycloisomerization
of enynes requires simultaneous coordination of both the alkyne and
the alkene to the metal, followed by a two-electron oxidation of the
metal, which is favorable for palladium(II) and also platinum(II).
160
However, the oxidation of gold(I) to form a
gold(III) metallacycle is highly improbable under ordinary conditions.
161
In addition, [AuL]+ cations are
isolobal to H+, which makes the simultaneous coordination
of the alkene and the alkyne highly unlikely. Therefore, in contrast
to Pd(II)
159
and Pt(II),
162
gold(I)-catalyzed cycloisomerizations of enynes do not
proceed via Alder-ene reaction. Instead, activation of the alkyne
by gold(I) forms a (η2-alkyne)metal complex 143 that reacts as an electrophile with
the alkene moiety
either by 5-exo-dig or 6-endo-dig
pathways to form the corresponding cyclopropyl gold carbenes 144 or 145 (Scheme 46),
45,163
which, in the absence of internal and external nucleophiles, evolve
by different skeletal rearrangements. It is important to emphasize
that these species show highly delocalized structures, which are intermediate
between cyclopropyl gold(I) carbenes and gold(I)-stabilized cyclopropylmethyl/cyclobutyl/homoallyl
carbocations. In general, π-back-donation from gold(I) to the
carbene center is poor,
164,165
although it becomes
more significant in complexes [LAuCR2]+ with
highly donating N-heterocyclic carbene ligands.
166
Indeed, a few complexes with carbene-like structures showing
relatively short Au(I)–C bonds have been structurally characterized.
167
The best-studied gold(I)-catalyzed reactions
from a mechanistic
point of view are cycloisomerizations of 1,6-enynes, which have often
been used as model substrates for the discovery of new reactions and
new catalysts activity. According to DFT calculations, the single-cleavage
skeletal rearrangement occurs via opening of 144 to form
η2-diene 146-gold(I) complexes in a
single step by a 1,3-migration of the terminal carbon of the alkene
to C(1) of the alkyne.
163b
Six-membered
ring compounds 147 arise from an alternative endo-type single-cleavage rearrangement
in which the internal
carbon of the double bond migrates toward C(1) of the alkyne.
168
Although formation of 1,3-dienes by single
cleavage in metal-catalyzed cycloisomerization of enynes could also
be explained by a conrotatory ring opening of cyclobutene intermediates 148, experimental
evidence and theoretical calculations suggest
that for enynes with di- and trisubstituted alkenes this transformation
takes place by a direct reaction of cyclopropyl gold(I) carbenes bypassing
the formation of cyclobutene intermediates.
163b
Thus, the rearrangement of 1,6-enynes with two methyl groups at
the alkene terminus proceeds smoothly at temperatures as low as −40
to −60° using cationic catalysts [(R3P)Au(MeCN)]SbF6, which would imply an abnormally
low activation energy for
the hypothetical conrotatory opening of a cyclobutene.
163b
Scheme 46
General Pathways for the Cycloisomerization
of 1,6-Enynes
For the double-cleavage
rearrangement, intermediates 144 can evolve by a formal
insertion of the terminal carbon of the alkene
into the alkyne carbons. These new carbenes 149 undergo
α-proton elimination to afford dienes 150. In this
process, both the alkene and the alkyne are cleaved in an intramolecular
transformation.
On the other hand, intermediates 145 from 6-endo-dig cyclization can lead to bicyclo[4.1.0]hept-2-ene
derivatives 151 by protodeauration, which are the products
of an intramolecular cyclopropanation of the alkene by the alkyne.
161a,169
Alternatively, isomerization of gold(I) carbene 145 by ring expansion of the cyclopropane
gives (η2-cyclobutene)-gold(I) complexes 152, which can isomerize
to give cyclobutenes 153. Gold(I)-complexes of 152 have been observed by NMR spectroscopy,
170
and a bicyclo[3.2.0]hept-5-ene formed gold(I)-catalyzed
by cycloisomerization has been characterized by X-ray diffraction.
163c
The opening of these gold(I) complexes can
also form complexes 154, which are direct precursors
of 1,3-dienes 146, the product of a single cleavage rearrangement.
Analogously, 1,5-
171
and 1,7-enynes undergo
gold(I)-catalyzed rearrangements through somewhat related pathways.
163b,172
3.1.2
Cycloisomerization of 1,6- and Higher Enynes
The pathway followed by a particular enyne is highly influenced
by its substitution pattern. DFT calculations
173
support that the formation of 5-membered cyclic compounds
is generally kinetically favored for terminal alkynes, while the formation
of 6-membered rings becomes preferred for internal alkynes together
with the ones with heteroatoms at the tether. Gold(I)-catalyzed single-cleavage
rearrangements of enynes proceed under very mild conditions to form
1,3-dienes.
43,161a,163a,163b,174
Hence, enynes 155 bearing a terminal alkyne and a disubstituted
alkene undergo a cycloisomerization reaction in the presence of Ph3PAuCl and AgBF4
to give exclusive formation of
single-cleavage rearrangement 1,3-dienes 156 (Scheme 47).
163a,163b
In most of the cases,
these rearrangements are stereospecific processes in which the configuration
of the alkene is retained. However, 1,6-enynes such as 157 bearing strongly electron-donating
groups at the terminal alkene
carbon lead to Z-configured 1,3-dienes 158 regardless the configuration of the starting
enynes (Scheme 48).
175
1,6-Enynes 159 containing alkyl-substituted alkynes undergo double-cleavage
rearrangements in the presence of Ph3PAuCl and AgBF4 (Scheme 49).
161a,163b
Scheme 47
Single-Cleavage Rearrangement of 1,6-Enynes
Scheme 48
Single-Cleavage Rearrangement of 1,6-Enynes with Electron-Donating
Groups at the Alkene
Scheme 49
Double-Cleavage Rearrangement of 1,6-Enynes with Alkyl-Substituted
Alkynes
1,6-Enynes bearing
a terminal alkene and/or tethered with heteroatoms
such as 161 and 164 provide six-membered
rings 162 and 165 by endo-type single-cleavage rearrangement as the major products
(Scheme 50).
161a
1,6-Enynes boronated
at either the alkyne or the alkene react in a similar vein with Ph3PAuCl and AgSbF6.
176
Scheme 50
Endo-Type Single-Cleavage Rearrangement
Regarding gold(I)-catalyzed
6-endo-dig cyclizations,
when 1,6-enynes are tethered by an ether or a sulfonamide group, oxa-
or aza-bicyclo[4.1.0]hept-4-ene derivatives 168 and 171 are formed as the major products
as a result of a formal
intramolecular cyclopropanation of the alkene by the alkyne (Scheme 51).
161a,177
Scheme 51
Synthesis of Oxa-
and Aza-Bicyclo[4.1.0]hept-4-enes by Cycloisomerization
of 1,6-Enynes
A tandem allylation/cycloisomerization
has been developed for the
synthesis of 3-oxabicyclo[4.1.0]hept-4-ene derivatives using gold(I)
catalysts.
178
The enantioselective synthesis
of bicyclo[4.1.0]hept-4-enes has also been described with excellent
enantioselectivities using (R)-DTBM-MeO-biphep(AuCl)2/AgOTf
179
(DTBM = 4-MeO-3,5-(t-Bu)2C6H2) or more recently
gold(I) complexes with chiral phosphoramidite ligands.
52b,180
NHC-capped cyclodextrins coordinated to AuCl also catalyze the formation
of 3-azabicyclo[4.1.0]hept-4-enes, although this catalyst provides
only modest enantioselectivities.
181
3-Alkoxy-1,6-enynes in the presence of gold(I) form 1,4-cycloheptadienes
by a nucleophilic attack of the alkoxy group onto the alkyne presumably
followed by a [3,3]-sigmatropic rearrangement.
182
O-Tethered 1,6-enynes 172 that contain a strained
ring react with gold(I) by cycloisomerization followed by a 1,2-alkyl
carbocationic shift resulting in ring expansion (Scheme 52).
183
A two-step method
involving this transformation was developed for the synthesis of ketomacrolactones
174, which are scaffolds present in several natural products.
The tricyclic skeleton of natural products crotobarin and crotogoudin
has very recently been obtained via a 1,6-enyne gold(I)-catalyzed
cycloisomerization followed by intramolecular trapping of the resulting
gold(I) carbene by a carboxylic acid.
184
Scheme 52
Cycloisomerization Followed by a 1,2-Alkyl Carbocationic Shift
A gold(I)-catalyzed polycyclization
of linear dienediynes 175 has been developed for the
construction of fused 5,7,6-tricyclic
ring systems 177 in one step with high diastereocontrol
(Scheme 53).
185
The
polycyclization takes place through gold(I)-catalyzed intramolecular
cyclopropanation of the diene with the diyne followed by Cope rearrangement
to give strained allene intermediate 176, which subsequently
undergoes a C–H activation, followed by 1,2-H and G- (H- or
AcO) shifts.
Scheme 53
Polycyclization of Dienediynes To Form 5,7,6-Tricyclic
Ring Systems
The skeletal rearrangements
of 1,7-enynes have been much less studied
than those of 1,5- and 1,6-enynes. Gold(I) complexes are in general
the best catalysts for the cycloisomerization of these substrates,
leading to 1,3-dienes 179 through a single-cleavage process
(Scheme 54).
172
Related
enynes bearing aryl substituents at the alkene give mixtures of products
of single-cleavage along with seven-membered ring compounds by an endo-type single-cleavage
rearrangement.
186
Scheme 54
Single-Cleavage Rearrangement of 1,7-Enynes
Gold(I) complexes also catalyze
Conia-ene reactions, which can
be considered as cyclizations of 1,6-enynes via the corresponding
enol tautomers (Scheme 55).
187
Thus, cyclopentane derivatives 180 were obtained
in excellent yields and good diastereoselectivities. The reaction
has also been efficiently carried out using a gold catalyst bearing
a bulky phosphine ligand
188
or a catalyst
generated in situ from a cyclic thiourea-AuCl complex and a silver
salt.
189
The cyclization of aldehydes with
alkynes in the presence of secondary amines proceeds analogously via
the corresponding enamines generated in situ.
190
Scheme 55
Formation of 5-Membered Rings by Conia-Ene Cyclization
Silyl enol ether derivatives
react with alkynes in a similar way.
191
Substrates featuring a 1,6- or 1,7-relationship
between the silyl enol ether and the alkyne such as 181 undergo an exo-cyclization
to form five- (182) or six-membered rings (183), respectively
(Scheme 56).
192
However,
it was found that it is possible to tune the exo-
or endo-selectivity by changing the ligand on gold.
193
Thus, gold(I) complexes containing very bulky
phosphine ligands favor the 6-endo-dig cyclization,
whereas complexes with NHC ligands lead to 5-exo-dig
cyclization products preferentially. Enantioselective versions of
this reaction have been developed using chiral gold(I) complexes.
194
Scheme 56
Exo-Cyclization of Silyl
Enol Ethers with Alkynes
Conia-ene cyclizations have also been performed starting
from 1,6-diynes
that react first with methanol of another alcohol in the presence
of gold(I) to form in situ the corresponding enol ethers, which then
undergo the cycloisomerization process to afford the corresponding
5-membered-ring products.
195
An analogous
process has been developed by the intramolecular addition of other
nucleophiles such as carboxylic acids or nitrogen nucleophiles to
1,6-diynes.
196
Gold(I)-complexes
with a semihollow-shaped triethynylphosphine
ligand promote a 7-exo-dig cyclization of 1,7-enynes
bearing silyl enol ethers.
197
Related 1,8-enynes
also give seven-membered-ring products in the presence of this catalyst
as a result of a 7-exo-dig cyclization.
198
The 8-endo-dig pathway to
form eight-membered carbocycles could be promoted by using a modified
triethynylphosphine-based ligand.
1,7-Enynes 184 also undergo a formal gold(I)-catalyzed
8-endo-dig cyclization to give benzoxocines 187 (Scheme 57).
199
It has been proposed that the reaction proceeds through
a 7-endo-dig cyclization, followed by ring expansion
to form the benzylic carbocation 186 that after elimination
and protodeauration leads to the final product. As a proof of this
mechanism, tricyclic compounds deriving from the protodeauration of 185 have been
isolated in some cases as the minor products.
Scheme 57
Synthesis of Benzoxocines by Cyclization of 1,7-Enynes
Certain 7-aryl-1,6-enynes undergo
gold(I)-catalyzed cycloisomerization
reactions leading to bicyclo[3.2.0]hept-6-enes.
163c,169c,170,200
In the case of 7-aryl-1,6-enynes 188 having an amide
or an ester at the tether, gold(I) promotes the exclusive formation
of cyclobutenes 189 that result from a formal [2 + 2]
cycloaddition between the alkene and the alkyne followed by isomerization
of the double bond (Scheme 58).
201
It has been recently found that, when these
reactions are carried out in the presence of air, tricarbonyl compounds 190 are formed
instead.
202
Scheme 58
Formal [2 + 2] Intramolecular Cycloaddition Between Alkene and Alkyne
1,6-Ene-ynamides undergo highly
diastereoselective gold(I)-catalyzed
cycloisomerizations forming cyclobutanones or carbonyl compounds containing
a 2-azabicyclo[3.1.0]hexane subunit, depending on the substitution
pattern.
203
1,7-
163b
and 1,8-enynes
204
also form
cyclobutene derivatives in the presence
of gold(I) catalysts. In the case of 1,9-enyne 191, 10-membered-ring 192 was obtained
in the presence of large amounts of gold(I)
(Scheme 59).
205
Scheme 59
Synthesis of a 10-Membered-Ring Compound by Cycloisomerization of
a 1,9-Enyne
The macrocyclization
of higher 1,n-enynes (n = 10–16)
affords benzene-fused 9- to 15-membered
rings incorporating a cyclobutene ring as a result of a formal [2
+ 2] cycloaddition (Scheme 60).
206
Scheme 60
Macrocyclization of 1,n-Enynes
3.1.3
Cycloisomerization
of 1,5-Enynes
In general, 1,5-enynes react by endocyclic
pathways. This tendency
can be rationalized in terms of the more favorable formation of a
bicyclo[3.1.0]hexane system in the endo-cyclization,
which is less strained than the bicyclo[2.1.0]pentane system that
would result from the exo-cyclization.
244b
The first example of a cyclization of 1,5-enynes
was described for the gold(III)-catalyzed synthesis of pyridines from
ketones and propargyl amines via in situ generated enamine.
207
1-Alkynyl-2-alkenylbenzenes 195 in the presence of gold(I) mainly undergo endo-dig
cyclizations affording substituted naphthalenes 196 (Scheme 61).
208
The formation
of products 197 through exo-cyclization
becomes more relevant when R1 = H or I.
208a
In an analogous reaction, (Z)-hexa-1,3-dien-5-ynes
lead to highly substituted benzene derivatives through a gold(I)-catalyzed endo-cyclization
followed by a 1,2-alkyl shift.
209
Scheme 61
Synthesis of Naphthalenes by endo-dig Cyclization
of 1,5-Enynes
The selectivity of
the cyclization of 2-(alkynyl)-α-methylstyrenes
could be switched from 6-endo-dig to 5-exo-dig by adding an alcohol to the reaction
media.
210
Moreover, 2-alkynylstyrenes disubstituted at the β-position
of the styrene moiety selectively afford 1H-indene
derivatives via 5-endo-dig cyclization. An enantioselective
cycloisomerization of 2-alkynylstyrenes 198 has been
described using a chiral dinuclear gold(I) catalyst with the (S)-3,5-xylyl-MeO-biphep
ligand.
211
In this particular case, only exo-cyclization products
were observed, providing access to 1-alkenyl-1H-indenes 199 in excellent yields and
with good enantioselectivities
(Scheme 62).
Scheme 62
Enantioselective
Cycloisomerization of 2-Alkynylstyrenes
Nitrogen-tethered 1,5-enynes have been used to form N-heterocycles. The synthesis
of 4-aryl-2-pyridones was
reported by
gold(I)-catalyzed endo-cyclization of 1,5-enynes
bearing an amide at the tether.
212
Analogously,
azaanthraquinones 201 were assembled from N-propargylaminoquinones 200 (Scheme 63).
213
The same strategy was later
employed for the synthesis of pentacyclic pyrido[4,3,2-mn]acridin-8-ones.
214
Alkyne-tethered dihydropyrimidones
undergo an endo-cyclization in the presence of AuCl
yielding pyridopyrimidones.
215
Scheme 63
Synthesis
of Azaanthraquinones from N-Propargylaminoquinones
Alkyl- and aryl-substituted
1,5-enynes undergo gold(I)-catalyzed endo-dig cycloisomerization
to afford bicyclo[3.1.0]hexene
derivatives 202 in a stereospecific manner (Scheme 64).
216
Under these reaction
conditions, substrates bearing a quaternary center at the propargylic
position evolve differently. Thus, cycloalkyl-substituted enyne 203 undergoes a ring
expansion by 1,2-alkyl shift to give
tricyclic compound 205, whereas 204 bearing
a larger cycloalkyl substituent goes through C–H bond insertion
to afford tetracyclic compound 206 (Scheme 65).
217
Scheme 64
Synthesis
of Bicyclo[3.1.0]hexenes by endo-dig Cycloisomerization
of 1,5-Enynes
Scheme 65
endo-dig Cycloisomerization of 1,5-Enynes Followed
by Ring Expansion and C–H Insertion
In the presence of gold(I), 1,5-enynes 207 bearing
a cyclopropylidene moiety undergo an enantioselective ring-expanding
cycloisomerization providing bicycle[4.2.0]octanes 210 (Scheme 66).
218
Scheme 66
Enantioselective Ring-Expanding Cycloisomerization of Cyclopropylidene
1,5-Enynes
The gold(I)-catalyzed
cycloisomerization of 3-hydroxylated-1,5-enynes
may follow divergent reaction pathways depending on their substitution
pattern (Scheme 67).
219
The reaction of nonprotected 3-hydroxy-1,5-enynes with gold(I) gives
bicyclo[3.1.0]hexan-3-ones 211, whereas those benzyl-protected
give the analogous product 212 together with the product
of skeletal rearrangement 213. Boc-protected 3-hydroxy-1,5-enynes
form cyclohex-4-ene-1,2-diol derivatives 214.
220
Gold(I) complexes also catalyze formation of
benzene derivatives 215 from 3-hydroxy-1,5-enynes,
221
which can also be generated in situ by the
addition of allylsilanes to alkynals.
222
Scheme 67
General Pathways for the Cycloisomerization of 3-Hydroxylated-1,5-Enynes
The gold(I)-catalyzed cyclization
of 3-silyloxy-1,5-enynes proceeds
with concomitant semipinacol rearrangement leading to carbonyl compounds.
223
6-Silyloxy-1,5-enynes such as 216 and 218 react with AuCl by an endo-pathway to give
1,4- (217) or 1,3-cyclohexadienes (219) depending on the substitution at the propargylic
position
(Scheme 68).
171
Analogously,
chiral 1,5-enynes were used to construct tricyclic structures bearing
a spirofused 1,3-cyclohexadiene ring.
224
Scheme 68
endo-dig Cycloisomerizations of 6-Silyloxy-1,5-enynes
Propargyl cyclopropenes react
with gold(I) to form substituted
benzene derivatives (Scheme 69).
225
In this transformation, the substituents in
the cyclopropane ring have a crucial effect on the reaction pathway
and therefore on the regiochemical outcome. Thus, cyclopropenes bearing
larger substituents such as aryl groups (220) lead to
rearranged products in which neither the alkene nor the alkyne is
cleaved (222). In contrast, those cyclopropenes bearing
smaller hydrogen or alkyl groups (221) favor a double
cleavage pathway. According to DFT calculations, the mechanism of
this reaction involves an unprecedented two consecutive 1,3-cationic
alkylidene migrations of nonclassical carbocation intermediates.
226
Scheme 69
Synthesis of Substituted Benzenes by Cycloisomerization
of Propargyl
Cyclopropenes
Alkynylcyclopropanes 224 undergo a gold(I)-catalyzed
rearrangement to afford alkynyl cyclohexadienes 226 and 227 (Scheme 70).
227
The mechanism of this rearrangement has been recently revised
experimental and theoretically, giving clear evidence for the involvement
of η1-allylic gold(I) cationic intermediate 225.
228
Scheme 70
Synthesis of Alkynyl
Cyclohexadienes by Rearrangement of Alkynylcyclopropanes
Enols such as 229 deriving from β-keto esters
can also participate in gold(I)-catalyzed 1,5-enyne cycloisomerizations,
namely Conia-ene carbocyclizations (Scheme 71).
229
Silyl enol ether derivatives react
similarly with alkynes.
191
As for 1,6-enynes,
enantioselective versions of this reaction have been described.
194
Bicyclo[m.n.1]alkenone frameworks 232 with a bridged ketone were
synthesized through a Conia-type reaction of 1,5-enynes 231 (Scheme 72).
230
Scheme 71
Conia-Ene Carbocyclizations of β-Keto Esters
Scheme 72
Synthesis of Bicyclo[m.n.1]alkenone
Frameworks by Conia-type Cyclization
3.1.4
Cycloisomerization of 1,n-Enynes in Total Synthesis
Cycloisomerizations of 1,n-enynes, generally coupled with tandem reactions, allow
for an increase of molecular complexity in one step under very mild
conditions. Therefore, these transformations have been widely used
in the field of total synthesis.
63
The total synthesis of sesquiterpene ventricosene 236 was accomplished through a key
gold(I)-catalyzed cyclization of
1,6-enyne 233, which proceeds with concomitant ring expansion
via 234 to form cyclobutanone 235 (Scheme 73).
231
Scheme 73
Synthesis
of Ventricosene
The total syntheses
of hyperforin (239) and papuaforins
A-C (240-242) have been achieved by using
a gold(I)-catalyzed Conia-type reaction from 1,5-enynes 237 (Scheme 74).
232
Similar cyclizations were used in the syntheses of the alkaloids
(+)-lycopladine A (245)
233
and (+)-fawcettimine (249) (Scheme 75),
234
as well as in the total synthesis
of platencin.
235
This reaction has also
been applied to the cyclization of 1,7-enynes in the total synthesis
of the marine sesquiterpene (±)-gomerone C 252(236) and also in the total synthesis
of Daphniphyllum alkaloid daphenylline 255 (Scheme 76).
237
Scheme 74
Syntheses
of Hyperforin and Papuaforins A–C
Scheme 75
Synthesis of Alkaloids (+)-Lycopladine and Fawcettimine
Scheme 76
Syntheses of the Sesquiterpene (±)-Gomerone
C and Alkaloid Daphenylline
The cyclization of 3-silyloxy-1,5-enyne 256 followed
by semipinacol rearrangement was used to construct the hexahydro-1H-inden-4(2H)-one
core of (+)-sieboldine
A (259), which contains an unprecedented N-hydroxyazacyclononane ring (Scheme 77).
238
Scheme 77
Synthesis of (+)-Sieboldine A
3.2
Intermolecular
Reactions of Alkenes with Alkynes
Gold(I)-catalyzed intermolecular
reactions between alkenes and
alkynes constitute a real challenge since all the conceivable products
are by themselves potential substrates for gold(I), which in consequence
may compete with the initial alkene leading to oligomerization products.
59
The first reaction developed in this area
was a formal [2 + 2] cycloaddition between terminal alkynes and substituted
alkenes that led to the formation of cyclobutenes 261 (Scheme 78).
239
The regiochemical outcome of this process is in agreement with a
reaction pathway through highly distorted cyclopropyl gold(I) carbenes 260 that finally
undergo a ring expansion.
Scheme 78
[2 + 2] Cycloaddition
of Alkynes with Alkenes
Interestingly, the intermolecular reaction of propiolic
acid with
alkenes does not form cyclobutenes. Instead, this reaction leads to
1,3-dienes (262) or lactones (263) depending
on the nature of the alkenes (Scheme 79).
240
Asymmetrically substituted alkenes lead to
the formation of lactones by attack of the carboxylic acid to the
most substituted carbon of the alkene. On the other hand, alkenes
with two electronically identical or very similar substituents afford
stereospecifically 1,3-dienes by 1,3-migration, following a pathway
that is similar to the one occurring in the single-cleavage rearrangement
of 1,6-enynes.
Scheme 79
Formation of Lactones or Dienes by Reaction of Propiolic
Acid with
Alkenes
Propiolic esters
241
and sulfonylacetylenes
242
react in a different manner with allylic ethers
to provide 1,4-dienes by nucleophilic addition of the ether oxygen
onto the alkynes, followed by [3,3]-sigmatropic rearrangement. The
presence of α-substituents in the allyl ether promotes a competing
[1,3]-sigmatropic rearrangement concomitant with the [3,3]-rearrangement.
Terminal ynamides react intermolecularly with enol ethers in the
presence of gold(I) to give enamines 264 as a result
of a [2 + 2 + 2] cycloaddition (Scheme 80).
243
Arylynamides react intermolecularly with alkenes
forming a cyclopropyl gold(I) carbene, which is opened by a Friedel–Crafts
attack of the aryl group forming 1,2-dihydronaphthalenes.
243
Scheme 80
[2 + 2 + 2] Cycloaddition between Ynamides
and Alkenes
3.3
Addition
of Heteronucleophiles to Enynes
Gold(I) complexes catalyze
the addition of amines, alcohols, or
water to enynes leading to products of amino-, alkoxy-, or hydroxycyclization
under much milder conditions than other metal catalysts.
161a,163a,211,244
The overall process is an anti-addition of an electrophile
(alkyne-gold(I) complex) and a heteronucleophile to a double bond
in a stereospecific process.
3.3.1
Intermolecular Addition
of Heteronuclephiles
to Enynes
In the presence of an external heteronucleophile,
the cyclopropyl gold(I) carbenes generated as intermediates in the
cycloisomerization of 1,n-enynes are opened by attack
to the cyclopropane ring. These additions take place following the
Markovnikov regiochemistry, giving rise to products of exo-trig (266) or endo-trig
cyclization
(268) (Scheme 81).
161a,163a
Similar results have been obtained using NHC-gold(I) or gold(III)-complexes
as catalysts.
174a,244a,245
The enantioselective hydroxy- and alkoxycyclization of 1,6-enynes
catalyzed by a chiral biphosphine-gold complex
246
or by NHC-gold(I) complexes
247
proceeds with moderate to good enantioselectivities. 1,5-Enynes
also react with alcohols or water in the presence of gold(I) catalysts
to give the corresponding adducts.
244c
The
hydroxy- and alkoxycyclization of 1,7-enynes takes place similarly.
172
It is remarkable that the hydroxycyclization
process is usually much faster than the direct addition of water to
terminal alkynes to form the corresponding methyl ketones.
Scheme 81
Alkoxycyclization
of 1,6-Enynes
7-Substituted-1,6-enynes 269 bearing a fused aromatic
ring at the tether undergo predominantly gold(I)-catalyzed hydroxycyclization
by a 6-endo-dig pathway instead of the 5-exo-dig pathway usually observed for 1,6-enynes
with trisubstituted
alkenes to afford bicyclic compounds 270 (Scheme 82).
248
Scheme 82
Alkoxy
or Hydroxycyclization of 1,6-Enynes by 6-endo-dig
Pathway
The reaction of allylsilylalkynes
such as 271 catalyzed
by gold(I) in the presence of external alcohols gives vinylsilanes.
249
Depending on the choice of the nucleophile,
either cyclic or acyclic vinylsilanes were obtained (Scheme 83). DFT calculations
suggest that this transformation
takes place through silicenium cations 272 formed by
a pericyclic reaction of the allylsilylalkynes coordinated to gold(I).
250
The attack of methanol to these intermediates
gives cyclic vinylsilanes 273, whereas the attack of
a weaker nucleophile such as phenol takes place at the silicon leading
to acyclic vinylsilanes 274.
Scheme 83
Synthesis of Vinylsilanes
from Allylsilylalkynes
Propargyl vinyl ethers in the presence of gold(I) and
water or
alcohols undergo a Prins-type reaction affording dihydropyrans.
251
Furthermore, N-heteronucleophiles
such as carbamates and anilines also react intermolecularly with 1,6-enynes
to form amino-functionalized carbo- or heterocycles 275 (Scheme 84).
252
Scheme 84
Aminocyclization of 1,6-Enynes
1,6-Enynes also react with aldehydes to give products
of formal
[2 + 2 + 2] cycloaddition 276 together with a metathesis-type
reaction of the enyne with the aldehyde that forms 1,3-dienes 277 (Scheme 85).
253
1,7-Enynes also undergo a [2 + 2 + 2] cycloaddition with
carbonyl compounds in the presence of gold(I) giving rise to analogous
heterocyclic products.
254
Scheme 85
[2 +
2 + 2] Cycloaddition of 1,6-Enynes with Carbonyl Compounds and
Formation of Metathesis-Type Products
Cyclopropenones react with enynes in a ring-expanding
spiroannulation
incorporating a molecule of water to afford spirocyclic cyclopentenones 278 by a mechanistically
related process (Scheme 86).
255
Scheme 86
Reaction
of 1,6-Enynes with Cyclopropenones
1,6-Enynes bearing a monosubstituted alkene react with
aldehydes
and ketones in a different way, presumably via trapping of the rearranged
carbene that results from the enyne to give intermediate 279, followed by Prins reaction,
to afford tricyclic compounds 281 (Scheme 87).
256
In a similar vein, 1,5-enynes also undergo intermolecular
reactions with carbonyl compounds.
253
Scheme 87
Reaction of 1,6-Enynes with Carbonyl Compounds via Rearrangement
3.3.2
Intramolecular
Addition of Heteronuclephiles
to Enynes
The alkoxycyclizations can also take place intramolecularly
starting from hydroxyl-1,6-enynes,
161a,244d
and the
resulting adducts may further evolve increasing molecular complexity.
As an example, the synthesis of 4-oxa-6-azatricyclo[3.3.0.02,8]octanes 285 was reported
by a complex gold-catalyzed
cycloisomerization of alkynyl hydroxyallyl tosylamides 282 in the presence of Ph3PAuCl
and AgSbF6 (Scheme 88).
257
Scheme 88
Synthesis
of 4-Oxa-6-azatricyclo[3.3.0.02,8]octanes from
Alkynyl Hydroxyallyl Tosylamides
The intramolecular amino- or alkoxycyclization of amino-
or hydroxyl-1,5-enynes 286 yields spirofused heterobicyclic
compounds 287 (Scheme 89).
258
1,5-Enynes bearing carbamates
259
or carbonates
260
also undergo
gold(I)-catalyzed tandem cyclizations
in a mechanistically related transformation. The analogous reaction
of 1,6-enynes bearing a carboxylic acid leads stereospecifically to
lactones.
244d,261
Phenols can also add to 1,5-enynes
in substrates of type 288 with a gold(I) catalyst to
give tricycles 289 stereospecifically (Scheme 90).
262
The enantioselective
version of the addition of phenols to 1,6-enynes has also been described.
261
Similarly, the intramolecular reaction of hydroxypropargyl
vinyl ethers 290 catalyzed by a trinuclear gold(I)-oxo
complex leads to 5,6- and 6,6-spiroketals 291 and 292 with good stereocontrol (Scheme
91).
251
This transformation constrasts with
the reactivity previously shown for propargyl vinyl ethers, which
in the presence of the same trinuclear gold(I)-oxo complex underwent
a Saucy-Marbet rearrangement giving rise to allenes.
263
Scheme 89
Intramolecular Amino- or Alkoxycyclization of 1,5-Enynes
Scheme 90
Intramolecular Addition of Phenols
to 1,5-Enynes
Scheme 91
Synthesis of 5,6-
and 6,6-Spiroketals from Hydroxypropargyl Vinyl
Ethers
Oxo-1,6-enynes such
as 293 also react in the presence
of gold(I) complexes to give oxatricyclic compounds 295 by a tandem sequence in which
two C–C bonds are formed together
with one C–O bond (Scheme 92).
264
This formal [2 + 2 + 2] alkyne/alkene/carbonyl
cycloaddition proceeds by attack of the carbonyl to the cyclopropyl
gold carbene intermediate followed by Prins cyclization to give 294, which forms the
final oxatricyclic derivative after deauration.
Oxo-1,5-enynes also undergo an intramolecular reaction to give tricyclic
derivatives.
265
Scheme 92
Cyclization of Oxo-1,6-enynes
by [2 + 2 + 2] Alkyne/Alkene/Carbonyl
Cycloaddition
Terminal alkynes
and oxoalkenes undergo an analogous [2 + 2 + 2]
cycloaddition reaction by intermolecular cyclization of the alkyne
and the alkene followed by intramolecular attack of the carbonyl group
to form 8-oxabicyclo[3.2.1]oct-3-enes 296 (Scheme 93).
266
Scheme 93
Intermolecular
[2 + 2 + 2] Cycloaddition of Terminal Alkynes with
Oxoalkenes
Dienynes with a methoxy
or other OR group at the propargylic position
such as 297 react with gold(I) by an intramolecular 1,5-OR
migration to form tricyclic compounds 299 (Scheme 94).
267
Substrates substituted
with other OR groups at the propargyl position also undergo this 1,5-migration.
Scheme 94
Cyclization of 1,6-Enynes via 1,5-OR Migration
3.3.3
Addition of Heteronucleophiles
to Enynes
in Total Synthesis
Gold(I)-catalyzed intramolecular [2 +
2 + 2] alkyne/alkene/carbonyl cycloadditions
264
have been exploited for the synthesis of several oxygen-bridged
sesquiterpenoids. Ketoenyne (
E
)-300 in the presence of gold(I) reacted intramolecularly
to give oxatricyclic compound 301, which was converted
into (+)-orientalol F (302) in three additional steps
(Scheme 95).
268
The
key step of the synthesis of pubinernoid B (304) proceeded
analogously starting from (
Z
)-300.
Scheme 95
Syntheses of (+)-Orientalol F and Pubinernoid
B
The stereospecific [2 + 2 +
2] alkyne/alkene/carbonyl cycloaddition
was also applied to the synthesis of antitumor sesquiterpene (−)-englerin
A (309) in two independent syntheses (Scheme 96).
269
It is remarkable
that in both approaches an unprotected aldol subunit could be used
as the substrate for the gold(I)-catalyzed reaction.
Scheme 96
Synthesis
of (−)-Englerin A
The formation of alkenylsilanes from allylsilylalkynes
in the presence
of an external alcohol
249
was used in the
total synthesis of (−)-amphidinolide V.
270
Gold(I)-catalyzed reaction of 1,6-enyne 310 by cyclization
followed by 1,5-acetoxy migration from 312 forms an α,β-unsaturated
carbene 314,
267
which reacts
intermolecularly with alkene 311 to afford 315 with only 5% loss of enantiomeric excess
(Scheme 97).
271
This transformation was reported
as part of the total synthesis of antiviral sesquiterpene (+)-schisanwilsonene
A (316). It is interesting that the cyclization/1,5-acetoxy
migration is faster than the alternative 1,2-acyloxy migration, which
would lead to racemization.
Scheme 97
Synthesis of (+)-Schisanwilsonene
A
The total syntheses of three
sesquiterpenes, namely (−)-4β,7α-aromadendranediol
(320), (−)-epiglobulol (321), and
(−)-4α,7α-aromadendranediol (322),
have been accomplished by a stereodivergent gold(I)-catalyzed reaction
from a single precursor 317 (Scheme 98).
272
The reaction can take place
intramolecularly by 1,5-migration of OBn giving 318 or
intermolecularly in the presence of allyl alcohol as an external nucleophile,
obtaining allyl ether 319.
Scheme 98
Synthesis of Aromadendrane
Sesquiterpenes
3.4
Addition of Carbonucleophiles to Enynes
3.4.1
Cyclopropanation
of Enynes
The
gold(I)-catalyzed cyclization of dienynes 323 leads stereoselectively
to tetracyclic compounds 325 by cyclopropanation of intermediate
gold(I) carbenes 324 (Scheme 99).
163a,169a
These intramolecular cyclopropanations afford
very complex ring systems when the starting dienynes are cyclic substrates.
273
Intramolecular cyclopropanations of 1,5-enynes
take place similarly through an endo-carbene.
274
Scheme 99
Intramolecular Cyclopropanation of 1,6-Enynes
Intermediate cyclopropyl gold
carbenes resulting from the cyclization
of 1,6-enynes can also be trapped intermolecularly by cyclic or acyclic
alkenes to form adducts 326 stereoselectively (Scheme 100).
275
Scheme 100
Intermolecular
Cyclopropanation of 1,6-Enynes
1,6-Enynes bearing a terminal alkene react by a different
mechanism
through rearrangement of the initially formed cyclopropyl gold(I)
carbene 327 to form 328, which reacts with
an external alkene to afford 329 (Scheme 101).
Scheme 101
Intermolecular Cyclopropanation of 1,6-Enynes via
Rearrangement
In the presence
of gold(I), 7-alkynylcyclohepta-1,3,5-trienes generate
highly fluxional barbaralyl gold(I) cations 330, which
can be intercepted intramolecularly with alkenes to form 331a–b (Scheme 102).
276
These two species are in equilibrium by Cope
rearrangement both in the solid state and in solution.
Scheme 102
Intramolecular
Cyclopropanation from 7-Alkynylcyclohepta-1,3,5-trienes
3.4.2
Friedel-Craft
Arylation of Enynes
Electron-rich aromatic and heteroaromatic
compounds such as indoles
can undergo a stereospecific intermolecular addition to 1,6-enynes
in the presence of gold(I) catalysts (Scheme 103).
252,265,277
This transformation
proceeds by opening of the intermediate cyclopropyl gold(I) carbene
in a process mechanistically related to the hydroxy- and alkoxycyclization
of 1,6-enynes. The enantioselective version of this reaction has also
been reported.
278
The enantioselective
intramolecular addition leads to enantiomerically enriched complex
ring systems in a single step.
261
Scheme 103
Intermolecular
Addtion of Indole to 1,6-Enynes
1,6-Enynes bearing an aryl substituent at the alkyne such
as 333 react stereospecifically with gold(I) in a formal
[4 +
2] cycloaddition reaction under very mild reaction conditions (Scheme 104).
169b,244a,279
This reaction proceeds via initial exo-cyclization
followed by opening of the cyclopropyl gold(I) carbene by a Friedel–Crafts-type
reaction to provide 334. Related 1,6-enynes with 1-thienyl
and 1-indolyl groups at the alkyne also undergo formal [4 + 2] cycloadditions
catalyzed by gold(I) complexes bearing bulky biphenyl phosphine ligands.
280
In the presence of chiral phosphine gold(I)
complexes
281
or gold(I) phosphite complexes
282
these [4 + 2] cycloadditions can be performed
enantioselectively. Related cyclizations of alkynes with allenes
283
and diynes
284
have
also been described. The endo-cyclization also takes
place in certain cases, being the major pathway in the platinum(II)-
or gold(I)-catalyzed cycloaddition of related arylalkynes bearing
enesulfonamides or enamines.
285
Scheme 104
Intramolecular
[4 + 2] Cycloaddition of Arylalkynes with Alkenes
1,5-Enynes 335 with an aryl substituent
at the alkyne
react with gold(I) to form dihydrobenzofluorenes 339 in
a formal [3 + 3] cycloaddition by a 1,2-H shift in intermediate 337, followed by a
Friedel–Crafts alkylation (Scheme 105).
286
Scheme 105
Synthesis
of Dihydrobenzofluorenes by Cycloisomerization and Friedel-Crafts
Alkylation
3.4.3
Addition
of Other C-Nucleophiles
1,3-Dicarbonyl compounds can add
to 1,6-enynes as C-nucleophiles through their enol
tautomers, although some of them
such as cyclohexane-1,3-dione and 2-oxocyclohexanecarboxaldehyde behave
as O-nucleophiles.
277a
In the presence of 1,3-diketones, N-tethered 1,6-enynes
bearing an internal alkyne afford tetrahydropyridines 340 as a result of an endo-cyclization
(Scheme 106). In contrast, analogous 1,6-enynes bearing a
terminal alkyne undergo an exo-cyclization and in
the presence of the nucleophile giving adducts 341 and/or 342 depending on the nature
of the gold(I) catalyst (Scheme 107). Products 341 are favored with
the most electrophilic gold(I) catalysts, whereas less electrophilic
complexes with more donating ligands favor the formation of 342. Gold(I) complexes
also catalyze the addition of allylic
silanes to 1,6-enynes.
277a
Scheme 106
Addition
of 1,3-Diketones to 1,6-Enynes by endo-Cyclization
Scheme 107
Ligand-Controlled Addtion of 1,3-Diketones to 1,6-Enynes
1,3-Dien-8-ynes undergo a formal
intramolecular gold(I)-catalyzed
[4 + 2] cycloaddition reaction.
287
Dienynes 343 cyclize similarly in the presence of gold(I) and heating
by microwave irradiation to form stereoselectively hydrindanes 344 (Scheme 108).
169b,244a,288
Scheme 108
Intramolecular
[4 + 2] Cycloaddition of 1,3-Dien-8-ynes
4
Gold(I)-Catalyzed Reactions
of Propargylic Carboxylates
4.1
Reactions of Propargylic
Carboxylates
Propargylic carboxylates react with gold(I)
complexes undergoing
1,2- or 1,3-acyloxy migrations through 5-exo-dig
or 6-endo-dig cyclizations to form α-acyloxy-α,β-unsaturated
carbenes 345 or allene-gold complexes 347, which are in equilibrium (Scheme 109).
46,289
Similar transformations have been described for propargyl acetals,
290
as well as by using other metal catalysts.
291
Scheme 109
General Pathways in the Isomerization
of Propargylic Carboxylates
Species 348 have been trapped in intermolecular
reactions
with alkenes,
52a,292
ynamides,
293
carbon nucleophiles,
294
imines,
295
and sulfides.
296
Although the reaction of intermediates 348 with alkenes
usually affords vinylcyclopropanes, in the case of propargyl acetals
such as 349, products of [3 + 2] cycloaddition 352 are obtained (Scheme 110).
290b,290c
In this transformation, gold(I) carbene 351 is formed
by 1,2-OMe shift. Intermediate 351 undergoes an intermolecular
cyclopropanation with vinyl acetate to form 350, which
reacts with 351 in a Michael-type addition, followed
by a Prins cyclization to form 352.
Scheme 110
[3 + 2] Cycloaddition
of Propargyl Acetals with Alkenes
The gold(I)-catalyzed synthesis of benzopyrans 356 has been described from propargyl
carboxylates 353 by
intramolecular trapping of gold(I) carbenes 354 by an
ether group followed by rearrangement of the resulting allylic oxonium
ylides 355 (Scheme 111).
297
The synthesis of substituted naphthalenes has
been reported from propargylic esters by a gold(I)-catalyzed sequence
involving a 1,3-acyloxy migration followed by a 1,2-alkyl or aryl
migration and subsequent hydroarylation.
298
Scheme 111
Synthesis of Benzopyrans from Propargyl Carboxylates
Gold allenic intermediates 347 derived from 1,3-acyloxy
migration (see Scheme 109) can be trapped by
other functional groups to give of a range of different compounds.
299
In situ generated ketone-allene substrates 358 were used as substrates for a gold(I)-mediated
tandem
oxacyclization/[4 + 2] cycloaddition cascade to afford highly substituted
oxacycles 359 with excellent stereocontrol (Scheme 112).
300
Nucleophilic
allenes have also been generated in situ by a gold(I)-catalyzed rearrangement
of propargylic esters and then used for intermolecular C(sp3)–C(sp2) bond formation
reactions.
301
Scheme 112
Tandem Oxacyclization/[4 + 2]-Cycloaddition
from in Situ Generated
Ketoallene Substrates
In the presence of AuCl, ω-hydroxy propargylic acetates
undergo
a 1,3-acetoxy migration to form allenyl acetate 360,
which is trapped intramolecularly to form tetrahydropyranes 361 containing an exocyclic
enolacetate (Scheme 113).
302
This transformation
proceeds with remarkable high Z-selectivity in the
final alkenes and the retention of the configuration of diastereomerically
pure substrates. 1,6-Diyne esters also react with gold(I) forming
allenyl gold(I) intermediates, which can react with the pendant alkyne
giving a variety of cyclized products.
303
Scheme 113
Synthesis of ω-Hydroxy Propargylic Acetates via 1,3-Acetoxy
Migration
The gold(I)-catalyzed
acyloxy migration has been applied to the
synthesis of 1-acetoxy-1H-indenes.
304
The mechanism of this transformation involves a 1,3-migration
to form the allenyl intermediate that undergoes an intramolecular
hydroarylation, followed by another final 1,3-acyloxy migration to
generate a more stable substituted indene. The same reaction in the
presence of water leads to α,β-unsaturated ketones.
Terminal halo-substituted propargyl carboxylates 362 react with Ph3PAuNTf2 in anhydrous
CH2Cl2 to form 1-halo-2-carboxy-1,3-dienes 363 by 1,2-migration of the ester (Scheme
114).
289f
It has been recently reported that
the same substrates in the presence of Ph3PAuCl/AgSbF6 and H2O undergo a regioselective
hydration to
give α-acyloxy α′-halo ketones 364.
305
Scheme 114
Migration or Hydration of Halo-Substituted
Propargyl Carboxylates
Gold(I) catalyzes the formation of alkenyl enol esters
or carbonates
from trimethylsilylmethyl-substituted propargyl esters/carbonates
with excellent E-selectivity.
306
Alkynyloxiranes 365 bearing a propargylic
ester rearrange to form divinyl ketones 366 (Scheme 115).
307
The mechanism
of this transformation seems to proceed via anchimeric assistance
of the propargyl ester moiety, although DFT calculations predict a
complex mechanism involving several equilibriums.
Scheme 115
Synthesis of Divinyl
Ketones from Alkynyloxiranes Bearing a Propargylic
Carboxylate
α-Iodoenones
can be accessed by 1,3-migration of propargylic
acetates in the presence of gold(I) and NIS.
308
α-Trifluoromethyl enones have been synthesized by a tandem
1,3-acyloxy migration/trifluoromethylation from 1-arylpropargyl esters
with excellent stereoselectivity.
309
Propargylic
3-indoleacetates undergo a gold(I)-catalyzed tandem [3,3]-rearrangement/[2
+ 2] cycloaddition to afford 2,3-indoline-fused cyclobutanes.
310
Other structures have also been accessed by
reactions of propargyl carboxylates with gold(I), such as dihydrofurans,
311
aromatic ketones,
312
allenes,
313
or polyconjugated δ-diketones.
314
4.2
Cycloisomerizations of
Enynes Bearing Propargylic
Carboxylates
Enynes bearing propargylic α-acyloxy substituents
can react with gold(I) by two parallel pathways, depending on the
order of attack of the acyloxy group or the alkene onto the alkyne
in complexes 367 (Scheme 116).
This transformation is known as the Ohloff-Rautenstrauch (or simply
Rautenstrauch) rearrangement, following the original discovery of
the reaction catalyzed by zinc(II) or palladium(II).
315
If the alkene reacts first, then the cyclopropyl gold carbene 369 can suffer an intramolecular
attack of the acyloxy group
on the carbene, followed by elimination of AuL+ to give 370. On the other hand, the
acyloxy group can first undergo
a 1,2-migration to form carbene 368, which leads to 370 by intramolecular cyclopropanation.
Annulations of enynes 367 may also proceed via the acyloxy allene formed by 1,3-migration.
316
Scheme 116
Parallel Reaction Pathways of Enynes Bearing
Propargylic Carboxylates
The gold(I)-catalyzed cyclization of 1,5-enynes containing
propargylic
carboxylates affords bicyclic structures 373, which form
the corresponding ketones 374 after methanolysis (Scheme 117).
317
The cyclization
of 1,7- and 1,8-enynes with propargylic acetates affords analogous
products by a tandem 1,2-acyloxy migration/cyclopropanation.
318
This process allows the enantioselective synthesis
of 7- and 8-membered-ring compounds by using chiral gold(I) catalysts.
319
Scheme 117
Synthesis of Bicyclo[3.1.0]hexan-2-ones
by Cyclization of 1,5-Enynes
with Propargylic Carboxylates
In a mechanistically related transformation, 1,4-enynes 375 bearing propargylic carboxylates
in the presence of gold(I)
form
cyclopentenones 376 via 1,2-acyloxy migration (Scheme 118).
320
In this Rautenstrauch
rearrangement, enantiomerically enriched propargyl carboxylates give
enantioenriched cyclopentenones. The cycloisomerization of 1,6-enynes
with acyloxymethyl substituents at the alkyne also proceeds by 1,2-migration
to produce bicyclic derivatives with an exocyclic enol acetate group.
321
Scheme 118
Synthesis of Cyclopentenones from 1,4-Enynes
with Propargylic Carboxylates
1,3-Enynes bearing propargylic carboxylates also lead
to cyclopentenones
in the presence of gold(I).
322
This transformation
takes place through 1,3-migration of the carboxylate followed by a
Nazarov-type cyclization.
289d
Propargylic
acetates 377 also react via 1,3-acyloxy migration to
afford bicyclo[3.1.0]hexenes 378 (Scheme 119), which can be converted into cyclopentenones
(R2 ≠ H) or cyclohexenones (R2 = H) by methanolysis.
316,323
Scheme 119
Synthesis of Cyclopentenes from 1,3-Enynes with Propargylic
Carboxylates
The gold(I)-catalyzed
cycloisomerization of 1,6-dien-8-yne carbonates
and esters 379 has been reported to yield cis-cyclohepta-4,8-diene-fused pyrrolidines
381 by a tandem
process involving a 1,2-acyloxy migration/cyclopropanation to form 380 and a final
[3,3] sigmatropic rearrangement (Scheme 120).
324
Scheme 120
Synthesis
of cis-Cyclohepta-4,8-diene-Fused Pyrrolidines
from 1,3-Dien-8-yne Carbonates and Esters
Trienyne 382 cyclizes following a 5-endo-dig pathway through cyclopropyl gold(I) carbene
383, which gives 384 by 1,2-acyloxy migration
followed
by a Cope rearrangement to afford hydrozulenic derivative 385 (Scheme 121).
325
Scheme 121
Synthesis of Hydroazulenes by Cycloisomerization of 1,5-Enynes
and
Cope Rearrangement
N-Tethered 1,7-enyne benzoates such as 386 bearing a substituent at the alkyne react
with gold(I)
by 1,3-acyloxy
migration to form azabicyclo[4.2.0]oct-5-enes 388 by
a formal [2 + 2] cycloaddition of the alkene with the in situ generated
1-acyloxyallene 387 (Scheme 122).
326
Scheme 122
Synthesis of Azabicyclo[4.2.0]oct-5-enes
from N-Tethered
1,7-Enyne Benzoates
Enynes 389 form hydroxy dicarbonyl compounds 391 in wet dichloromethane (Scheme 123).
327
This reaction presumably proceeds
by two consecutive 1,2-acyloxy migrations, followed by a 1,3-dipolar
cycloaddition of the carbonyl ylide to form acetal 390, and final hydrolysis. A similar
mechanism is involved in the gold(I)-catalyzed
reaction of propargylic esters tethered to cyclohexadienones.
328
Scheme 123
Synthesis of Hydroxy Dicarbonyl Compounds
by Two 1,2-Acyloxy Migrations
Cyclopropyl alkynyl acetates without any substituent at
the alkyne
undergo a gold(I)-catalyzed rearrangement to form cyclohexenones with
high degree of enantiospecificity through gold-stabilized carbocations.
329
Cyclopropyl alkynyl acetates with a substituent
at the alkyne terminus react differently by a 1,3-acyloxy shift, whereas
the ones substituted at the internal position of the alkene follow
a different reaction pathway to give cyclohexenones disubstituted
at the 4-position.
330
1,n-Enynes 392 with a conjugated
enone react by a different mechanism involving a 1,3-acyloxy migration
to form acyloxyallenes 393, which undergo a Michael addition
to form cyclic products 394 (Scheme 124).
331
Scheme 124
Cyclization of
1,n-Enynes by 1,3-Acyloxy Migration
and Michael Addition
α,β-Unsaturated ketones can be efficiently
prepared
by a gold(I)-catalyzed Meyer-Schuster-like rearrangement from propargyl
carboxylates
332
or alcohols.
333
In contrast, γ-hydroxy-α,β-acetylenic
esters in the presence of alcohols undergo a gold(I)-catalyzed alkoxylation/lactonization
to form 4-alkoxy-2(5H)-furanones.
334
The gold(I)-catalyzed reaction of alkynylcyclopropanols
and cyclobutanols proceeds through ring expansion to give α-alkylidene
cyclobutanones.
335
4.3
Reactions
of Propargylic Carboxylates in Total
Synthesis
The gold(I)-catalyzed Meyer-Schuster rearrangement
was applied in the total synthesis of (+)-anthecotulide 396 (Scheme 125).
336
Scheme 125
Synthesis of Anthecotulide
The cyclization of an acetoxy-1,6-enyne by 1,3-acyloxy
migration,
followed by a Nazarov-type electrocyclization, was used in the total
synthesis of the marine triquinane sesquiterpene capnellene 399 (Scheme 126).
337
Scheme 126
Synthesis of Capnellene
5
Hydroarylation and Hydroheteroarylation of Alkynes
5.1
Cyclizations of Arylalkynes
Electrophilic
metal catalysts form, upon coordination to an alkyne, electrophilic
complexes that undergo electrophilic aromatic substitution reactions
with arenes. Gold(I)-complexes generally promote reactions according
to this pathway.
338
The direct auration
of electron-rich arenes and heteroarenes by gold(I)
339
and gold(III)
340
is a well-known
process, but the resulting aryl-gold complexes are apparently not
involved in subsequent C–C bond forming reactions with alkynes.
The auration of electron-deficient arenes has also been achieved.
341
Aryl-gold(I) complexes only react with alkynes
in the presence of a palladium(0) catalysts, or a palladium(II) precatalyst,
to afford products of carboauration.
341c
The gold-catalyzed intermolecular hydroarylation of alkynes
leads
to 1,1-disubstituted alkenes or 1,2-disubstituted derivatives in the
case of alkynes bearing electron-withdrawing groups.
342
According to experimental and computational work
on the cyclization
of arylalkynes catalyzed by platinum(II), two pathways with very similar
activation energies compete in intramolecular hydroarylations: a Friedel–Crafts
alkenylation and a reaction proceeding through metal cyclopropyl carbenes.
343
However, comparing the results obtained for
platinum(II)-catalyzed hydroarylation reactions such as the cyclization
of N-propargyl-N-tosyl anilines,
better yields and milder reaction conditions are obtained with cationic
gold(I) catalysts (Scheme 127).
344
A detailed theoretical analysis of the cycloisomerization
of phenyl propargyl ethers catalyzed by a Au38 cluster
has recently been reported.
345
N-Butynyl anilines also form 1,2-dihydroquinolines by 6-exo-dig cyclization followed
by a proton-catalyzed isomerization
of the exocyclic double bond.
346
These
products could be rearranged into functionalized indole derivatives
under photochemical conditions. The intramolecular hydroarylation
of N-propargyl-N′-phenylhydrazines
gives cinnoline derivatives.
347
Scheme 127
Synthesis
of 1,2-Dihydroquinolines from N-Propargyl
Anilines
N-Aryl-2-alkenylpyrrolidine derivatives 402 formed in
situ give rise to pyrrolo[1,2-a]quinolones 403 through a tandem sequence involving
the attack to an external alkyne followed by an intramolecular hydroarylation
(Scheme 128).
348
Unprotected propargylic and homopropargylic aniline derivatives
in the presence of terminal alkynes undergo a tandem hydroamination/hydroarylation
to give dihydroquinolines or quinolones.
129a,349
Scheme 128
Synthesis of Pyrrolo[1,2-a]quinolones by Intramolecular
Hydroarylation
The hydroarylation
of iodo-substituted propargyl anilines 404 gives selectively
4-iododihydroquinolines 405 with more electrophilic gold(I)
complexes, whereas 3-iododihydroquinolines 406 are obtained
with more electron-rich gold(I) catalysts
bearing NHC ligands (Scheme 129).
350
The cyclization of bromo- and iodopropargyl
aryl ethers with IPrAuNTf2 also proceeds with 1,2-halogen
migration to give 3-halo-2H-chromenes.
351
Scheme 129
Hydroarylation of Iodo-Substituted Propargyl
Anilines
The cyclization
of propargyl aryl ethers takes place analogously
yielding 2H-chromenes, even in the cases in which
the aromatic ring bears electron-withdrawing groups.
163a,343,344b,344c,352
However, in the case of substrates
containing both a 1,6-enyne and an aryl propargyl ether, the enyne
cycloisomerization is favored, and no hydroarylation takes place.
353
Aryl alkynoate esters 407 afford coumarin derivatives 408 in the presence of
gold(I) by intramolecular hydroarylation
under anhydrous conditions (Scheme 130).
342b,344b,354
It has been recently reported
that the same substrates under the same reaction conditions swapping
from anhydrous to wet solvent undergo a gold(I)-catalyzed spirocyclization
under mild conditions, giving spirolactones 409 in high
yields.
355
Spiro[4.5]cyclohexadienones
have been also obtained by a gold(I)-catalyzed carbocyclization of
phenols with a terminal alkyne via intramolecular ipso-Friedel–Crafts.
356
Aryl alkynylphosphonates
also undergo intramolecular gold(I)-catalyzed hydroarylation to give
phosphacoumarins.
357
Scheme 130
Synthesis of Coumarins
or Spirolactones from Aryl Alkynoate Esters
The intramolecular hydroarylation has also been applied
in a complex
transformation initiated by a rhodium(II)-catalyzed intramolecular
cyclopropanation of α-aryldiazo ketones with alkenes to give
products 411 that react with silver(I) to form alkynylhydrofurans 412, which in the
presence of gold(I) afford benzo-fused dihydrofurans 413 (Scheme 131).
358
Alkynylaziridines with an aryl group rearrange in the presence
of gold(I) to form spiro[isochroman-4,2′-pyrrolines] via allenylidene
intermediates.
359
Scheme 131
Synthesis of Benzo-Fused
Dihydrofurans from Alkynylhydrofurans
The cyclization of o-alkynyl biphenyl
derivatives
with gold(I), as happens with gold(III), platinum(II), and other metal
catalysts,
360
proceeds preferentially by
the endo-pathway leading to phenanthrenes 414. The use of a new strongly π-acidic
phosphine-bound gold(I)
catalyst has allowed to broaden the scope of this transformation,
leading to excellent yields in short reaction times, even for 4,5-disubstituted
phenanthrenes (Scheme 132).
361
Scheme 132
Synthesis of Phenanthrenes from o-Alkynyl Biphenyls
Interestingly, o-haloalkynebiaryls react
with
AuCl to give phenanthrenes in which the halide has suffered a 1,2-shift.
360c
This type of transformation has been applied
to the synthesis of 5,8-diiodo- and 6,13-diiodobenzo[k]tetraphenes 415 (Scheme 133),
362
as well as to the synthesis of benzo[a]phenanthridines from 3-alkynyl-4-arylisoquinolines.
363
It has been proposed that the most favored
reaction pathway features an initial 6-endo-dig hydroarylation
of the alkyne followed by 1,2-H shift and formation of a gold-carbene
intermediate, which then undergoes a 1,2-halogen shift to finally
give the rearranged product after deauration.
364
The reaction of (o-arylphenyl)alkynylselenides
in the presence of gold(I) and gold(III) affords rearranged phenanthrenyl
selenides very efficiently by migration of the selenide from the terminal
to the internal position of the alkyne.
365
Scheme 133
Synthesis of Diiodobenzo[k]tetraphenes
Gold(I)-catalyzed 6-exo-dig hydroarylation reactions
are much less common. The synthesis of substituted anthracenes in
the presence of gold(I) by exo-cyclization was achieved
from o-alkynyldiarylmethanes.
366
In a mechanistically related transformation, the synthesis
of functionalized phenanthrenes 416 has been described
from o-propargylbiaryls (Scheme 134).
367
Dibenzocycloheptatrienes
were obtained by a related transformation through a 7-exo-dig hydroarylation.
368
Scheme 134
Synthesis
of Phenanthrenes by 6-exo-dig Hydroarylation
The gold(I)-catalyzed hydroarylation
of alkyne-tethered fluorenes
was applied to the synthesis of fluoranthenes and more complex polyarenes.
22
A gold(I)-catalyzed formation of benzofurans
has been coupled with intramolecular hydroarylations to form polyaromatic
ribbons.
369
5.2
Cyclizations
of Heteroarylalkynes
5.2.1
Reactions of Indoles
with Alkynes
The reaction of N-propargyl
tryptophans or tryptamines 417 catalyzed by gold(I) or
gold(III) leads to seven- and
eight-membered rings, respectively (Scheme 135).
370
Eight-membered-ring products 419 are formed by an 8-endo-dig process,
a type of cyclization that had not been observed in any other hydroarylation
of alkynes or cyclization of enynes. These cyclizations have been
coupled with other tandem reactions to obtain polycyclic indole-based
structures.
371
Scheme 135
Formation of Seven-
and Eight-Membered Rings from Alkynyl Indoles
by 7-exo-dig or 8-endo-dig Cyclization
The intermolecular reaction
of indoles with (Z)-pent-2-en-4-yn-1-ols gives intermediate
(Z)-3-(pent-2-en-4-ynyl)indoles,
which undergo a similar cyclization to afford seven-membered-ring
products.
372
Tetracyclic indole derivatives
were synthesized enantioselectively by coupling a organocatalytic
process with a gold(I)-catalyzed 7-endo-dig cyclization.
373
Indoles with the alkynyl chain tethered
at the 2-position can undergo
a gold(I)-catalyzed cyclization to form carbazoles.
374,375
The formation of indoles in situ by intramolecular hydroamination
coupled with an intramolecular hydroarylation has been applied to
the preparation of benzo-fused carbazoles,
148a,376
1,2,3,10-tetrahydroazepino[3,4-b]indoles, and related
cyclic compounds.
148b
Other gold(I)-catalyzed
transformations that yield functionalized carbazoles are the gold(I)-catalyzed
deactylative cyclization of 3-acylindole/ynes
377
and the tandem hydroarylation/6-endo-dig cyclization of alkynes with 2-alkynylindoles.
378
In the case of indoles bearing a nucleophilic
functional group,
either at the 3-position (420) or on the alkynyl chain
(422), the reaction leads to tetracyclic indolines (Scheme 136).
371,379
Other cascade processes
initiated by cyclizations of indoles with alkynes have also been reported.
380
Scheme 136
Synthesis of Tetracyclic Indolines by
Hydroarylation and Intramolecular
Nucleophilic Addition
An Ugi four-component reaction of propargylamines with
3-formylindoles,
acids, and isonitriles can be coupled with a gold(I)-catalyzed cyclization
of the resulting adducts 424 to furnish substituted spiroindolines 425 (Scheme 137).
381
Scheme 137
Synthesis of Substituted Spiroindolines from Ugi Four-Component
Adducts
The intramolecular
gold(I)-catalyzed hydroarylation of alkynes
with indole-3-carboxamides 426 proceeds by 1,2-acyloxy
shift to form dihydroindoloazepinones 428 (Scheme 138).
382
An unusual 1,2-indole
migration has been observed in the gold(I)-catalyzed reaction of 3-propargylindoles.
383
Scheme 138
Synthesis of Dihydroindoloazepinones from
Indole-3-carboxamides
The intermolecular reaction of homopropargyl alcohols
with indoles
in the presence of gold(I) catalysts proceeds differently forming
first 2,3-dihydrofurans, which undergo the addition of two equivalents
of the indole to the resulting enol ether to give bis(indolyl)alkanes 429 (Scheme
139).
384
Scheme 139
Intermolecular Reaction of Homopropargyl Alcohols
with Indoles
5.2.2
Reactions
of Furans with Alkynes
In contrast to the usual Friedel–Crafts
type reaction of arenes
with alkynes, furans usually undergo gold(I)- or gold(III)-catalyzed
intramolecular reactions with alkynes to form phenols in good to excellent
yields (Scheme 140).
91,385
According to experimental and theoretical studies, the phenol synthesis
proceeds initially by a mechanism similar to that of the cyclization
of 1,n-enynes from furans 430 giving
intermediates 431 that evolve by ring opening, cyclization
of the resulting carbene, demetalation, and final rearrangement to
afford phenols 432.
385f,385g,385p,385s,386
Scheme 140
Intramolecular Phenol Synthesis from Alkynyl Furans
Furans generated in situ from
endiynes also undergo cyclization
with alkynes to form phenols that feature the hydroxyl group at the meta position
with respect to the ring junction.
387
The first example of synthesis of phenols
by intermolecular reaction
of a furan with an alkyne was reported using cationic binuclear complex
[(Mes3PAu)2Cl]BF4 as the catalyst,
albeit the reaction is very slow and the resulting phenol was obtained
in low yield.
388
Later it was reported
that phenols 433 can be obtained by intermolecular reaction
of furans and alkynes using gold(I) complexes bearing NHC ligands
(Scheme 141).
389
Scheme 141
Intermolecular Phenol Synthesis from Alkynyl Furans
Furan-yne systems 434 with aromatic tethers react
with gold(III) to form phenol derivatives. However, in the presence
of gold(I) these substrates react to form indene derivatives 435 by exo-cyclization
followed by 1,4-furanyl
migration and cyclization.
390
It has been
recently described that the same substrates undergo an endo-selective cyclization
with concomitant 1,5-migration of the furan
group in the presence of unactivated molecular sieves to yield trisubstituted
alkenes 436 (Scheme 142).
391
Scheme 142
Divergent Reactions of Furan-ynes To Form
Indenes or cis-Stilbene-Type Derivatives
Arylated (Z)-enones 440 are obtained
by intramolecular reaction of in situ formed intermediates 438 to give 439, which
undergo ring opening and final aromatization
(Scheme 143).
392
A related transformation leads to functionalized fulvenes with an
enone or an enal moiety starting from furanynes with a two-carbon
tether between the furan and the triple bond.
393
Scheme 143
Synthesis of Arylated (Z)-Enones
from Furanyl Z-1,3-Enynes
Furans 441 containing an alkynyl ether moiety
in the
presence of gold(I) undergo a furan-yne cyclization by a different
reaction pathway by which, instead of phenols, tetracycles 443 containing two heteroatoms
and two new stereocenters are formed
(Scheme 144).
394
A more complex gold-catalyzed process initiated by an intermolecular
reaction between 1,6-diyn-4-en-3-ols and furans leads to phenanthrene
derivatives.
395
Scheme 144
Furan-yne Cyclization
and Friedel-Crafts Annulation
The gold(I)-catalyzed reaction of 4-silyloxy-4-furyl alkynes
leads
to benzofurans by a Friedel–Crafts mechanism.
396
This cyclization has been expanded for the synthesis of
substituted benzo[b]-furans 445 by reaction
with various external nucleophiles (Scheme 145).
397
Similarly, propargylic alcohol-tethered
furans are converted into benzofuran derivatives in moderate yields.
398
Scheme 145
Synthesis of Benzofurans by Friedel-Crafts
Reaction of 4-Silyloxy-4-furyl
Alkynes
Internal alkynes 446 tethered to a furan through a
protected benzylic alcohol react with gold(I) giving protected 1-naphthol
derivatives 448 via formation of a cationic intermediate 447, followed by ring-opening
of the furan ring and aromatization
(Scheme 146).
399
It was recently reported that, when furan-ynes 449 bearing
a propargylic alcohol moiety were subjected to an analogous reaction,
an additional 1,2-rearrangement takes place, leading to substituted
1-naphthols 451 bearing an enal or enone moiety at the
C-4 position.
400
Scheme 146
Synthesis of Naphthols
by Furan-yne Cyclization
5.2.3
Reactions of Pyrroles with Alkynes
The gold(I)-catalyzed intramolecular post-Ugi hydroarylation of internal
alkynes with pyrroles was developed for the synthesis of pyrrolopyridinones.
401
In the presence of platinum(II)-catalysts this
reaction affords selectively pyrroloazepinones. Ugi-adducts 452 bearing a terminal
alkyne give pyrrolopyridines 454 by 5-exo-dig cyclization followed by
1,2-shift under very mild reaction conditions (Scheme 147).
402
Scheme 147
Synthesis of Pyrrolopyridines
by Intramolecular Hydroarylation of
Pyrroles
The one-pot asymmetric
synthesis of annulated pyrroles 455 has recently been
reported combining cinchona-alkaloid-derived primary
amine and gold(I) catalysts (Scheme 148).
403
Scheme 148
Synthesis of Annulated Pyrroles via Hydroarylation
The analogous intramolecular
reaction for β-yne-pyrrole derivatives
provides access to fused cycloheptapyrroles in the case of internal
alkynes and six-membered-ring fused pyrroles in the case of terminal
alkynes via endo- and exo-selective
cyclizations, respectively.
404
The intermolecular
reaction of alkynes with pyrroles was also developed to form functionalized
pyrrole derivatives 457 or 458 depending
on the nature of the alkyne substituent, which could be useful scaffolds
for additional annulation processes (Scheme 149).
Scheme 149
Intermolecular Hydroarylation of Pyrroles
A new synthesis of indoles 459 proceeds
by formal
gold(I)-catalyzed intermolecular [4 + 2] cycloaddition between 1,3-diynes
and pyrroles (Scheme 150).
405
This reaction involves the hydroarylation of one of the
alkyne moieties of the diyne with the pyrrole, followed by intramolecular
hydroarylation to give 4,7-disubstituted indole. Carbazoles could
also be obtained when indoles were used as the nucleophiles instead
of pyrroles.
Scheme 150
Synthesis of Indoles by Cycloaddition between 1,3-Diynes
and Pyrroles
5.3
Hydroarylation and Hydroheteroarylation Reactions
in Total Synthesis
An application of the formation of 2H-chromenes by intramolecular hydroarylation is
found in
the synthesis of the tetracyclic core of berkelic acid (462) (Scheme 151).
406
Scheme 151
Synthesis of the Tetracyclic Core of Berkelic Acid
The cyclization of aryl propiolates
gives coumarin derivatives,
including the natural products pimpinellin (464), fraxetin,
and purpurasol (Scheme 152).
407
Scheme 152
Synthesis of Pimpinellin
The cyclization of alkynylindoles bearing a nucleophile
at the
3-position has been used in the formal total synthesis of the indole
alkaloid minfiensine.
379a
The gold(I)-catalyzed
synthesis of carbazoles has recently been applied to the first total
synthesis of naturally occurring alkaloid karapinchamine A 466 (Scheme 153).
375b
Scheme 153
Synthesis of Karapinchamine A
An intramolecular gold(I)-catalyzed hydroarylation of
a 3-substituted
furan is used as the key step of the total synthesis of furanosesquiterpenes
crassifolone and dihydrocrassifolone.
408
6
Oxidative Reactions
6.1
Oxidative
Reactions of Alkynes
The
inter- or intramolecular oxidation of alkynes has been described using
sulfoxides,
409
pyridine N-oxides,
410
nitrones,
411
nitroso- and nitrobenzenes,
412
as well as epoxides,
413
as the oxidizing
agent. These processes have been proposed to proceed via α-oxo
gold(I) carbenes 468,
414
although
most likely gold(I) carbenoids 469 are involved by attack
of the nucleophile to the highly reactive gold(I) carbene (Scheme 154).
415
The reaction
of alkynes with pyridine-N-amidines to give 2,4,6-oxazoles
has also been proposed to proceed through α-oxo gold(I) carbenes.
416
Nevertheless, a mechanism involving β-alkoxy
alkenylgold(I) intermediates of type 467 rather than
α-oxo gold(I) carbenes has been proposed in some of these oxidative
reactions of alkynes.
417
Scheme 154
Generation
of α-Oxo Gold(I) Carbenes and Gold(I) Carbenoids
by Oxidation of Alkynes
In the case of terminal alkynes, the gold(I) carbene is
always
positioned at the terminal carbon of the alkyne. However, regioselectivity
becomes a major challenge with internal alkynes. A highly regioselective
oxidation of internal alkynes to α,β-unsaturated ketones
was developed using IPrAuNTf2 as the catalyst and 8-isopropylquinoline N-oxide as
the oxidant.
410c
The intermediate α-oxo gold(I) carbenes can be trapped intramolecularly
by a nucleophile present in the starting alkyne increasing the molecular
complexity of the final products. As an example, propargyl aryl ethers 470 in the
presence of gold(I) and a pyridine N-oxide presumably form α-oxo gold(I) carbene 471,
which undergoes an intramolecular Friedel–Crafts-type reaction
to afford chroman-3-ones 472 (Scheme 155).
418
In a related transformation,
propargylic and homopropargylic alcohols react with gold(I) in the
presence of pyridine N-oxides to give dihydrofuranones
and oxetan-3-ones, respectively.
419
The
related oxidative reaction of o-ethynylanilines gives
3-oxyindoles by intramolecular trapping of the intermediate gold(I)
carbene.
420
Scheme 155
Synthesis of Chroman-3-ones
by Oxidative Cyclization
The strong electrophilicity of the α-oxo gold(I)
carbene
also allows its intermolecular trapping. 2,5-Disubstituted oxazoles 474 were obtained
by a formal [2 + 2 + 1] annulation using
nitriles as the solvent via intermediates 473.
421
When carboxamides were used as the nucleophilic
partners, 2,4-disubstituted oxazoles 476 were obtained
by a formal [3 + 2] annulation between terminal alkynes via intermediates 475 (Scheme
156).
422
Carboxylic acids are also suitable trapping agents for
the in situ generated α-oxo gold carbenes, giving rise to carboxymethyl
ketones.
423
Scheme 156
Synthesis of 2,5-
and 2,4-Disubstituted Oxazoles by Oxidative Cyclization
The intermolecular trapping
of α-oxo gold(I) carbenes by
external nucleophiles such as indoles and anilines in aqueous media
has been reported, revealing that water can dramatically suppress
the undesired overoxidation of the alkyne.
424
A gold(I)-catalyzed tandem cycloisomerization/intermolecular trapping
of an in situ generated α-oxo gold(I) carbene involving this
transformation has been described to form functionalized indoles 477 from o-alkynyl
anilines and ynamides
(Scheme 157).
425
In this transformation, gold(I) serves dual catalytic roles to mediate
both the cycloisomerization of o-alkynyl anilines
and the intermolecular oxidation of ynamides.
Scheme 157
Synthesis of Indoles
from o-Alkynyl Anilines and
Ynamides
Diynes bearing one
terminal and one triarylmethyl-substituted alkyne
were converted into benzofluorenone derivatives via a one-pot process
involving a gold(I)-catalyzed generation of an α-oxo carbenoid
at the terminal alkyne, followed by a photocyclization/oxidation.
426
6.2
Oxidative Cyclizations
of Enynes
The oxidative cyclization of 1,5-enynes in the
presence of gold(I)
and a range of oxidants is a well documented process. Thus, the reaction
of 3,5-dien-1-ynes with pyridine N-oxides leads to
cyclopropa[a]inden-6(1H)-ones 478(427) or cyclopentadienyl aldehydes 479(428) depending
on the structure
of the starting substrate or the particular oxidant used (Scheme 158). In a similar
cyclization, N-allylyamides are converted into 3-aza-bicylo[3.1.0]hexan-2-ones.
429
Scheme 158
Oxidative Cyclization of 3,5-Dien-1-ynes
The gold(I)-catalyzed reaction
of 3,5- and 3,6-dienynes (480) with 8-alkylquinoline N-oxides results
in an oxidative cycloaddition in which a quinoline framework is activated
(Scheme 159).
430
The mechanism of this transformation probably involves an intermediate
α-oxo pyridinium ylide 481, which undergoes a concerted
[3 + 2] cycloaddition with the tethered alkene to form 482. Cycloalkanone-fused cyclopropanes
have been recently obtained by
a gold(I)-catalyzed oxidative cyclization from 1,5-ene-ynes.
431
Scheme 159
Synthesis of Quinoline Frameworks by Oxidative
Cycloaddition
Sulfoxides can also
be employed in oxidative cyclization of enynes
to afford rearranged products bearing a carbonyl moiety via gold(I)-carbenoid
intermediates.
432
Enantiomerically
enriched bicyclo[3.1.0]hexan-2-ones have recently
been obtained by a tandem alkyne oxidation/cyclopropanation in the
presence of gold(I) complexes bearing a chiral phosphoramidite ligand.
433
Similarly, an enantioselective alkyne oxidation/cyclopropanation
sequence of 1,5-enynes by gold(I) complexes bearing a P,N-bidentate
ligand affords bicyclic cyclopropane products.
434
The intramolecular additions of azides to alkynes
are somewhat
mechanistically related transformations that give rise to pyrroles 486 (Scheme 160).
435,436
This reaction proceeds by nucleophilic attack of the azide in 483 to form intermediates
484, which loses N2 in a processs reminiscent of the Schmidt reaction to form
cationic α-imino gold(I)-carbene 485. Final 1,2-H
shift and tautomerization lead to substituted pyrroles. In a similar
reaction, 2-alkynyl arylazides have been converted into indoles,
437
whereas similar substrates bearing propargylic
carboxylates give quinolines.
438
Scheme 160
Synthesis
of Pyrroles by Acetylenic Schmidt Reaction
o-(Azido)ynamides 487 were
efficiently
converted into indoloquinolines 489 in the presence of
gold(I) via α-imino gold(I)-carbenes 488 (Scheme 161).
439
Ynamides bearing
a simple alkene instead of an allyl silane gave cyclopropane-fused
derivatives.
Scheme 161
Synthesis of Indoloquinolines from o-(Azido)ynamides
6.3
Oxidative Reactions in Total Synthesis
An intramolecular oxidation of 490 through its N-oxide forms 4-piperidone 491, an
intermediate
in the total synthesis of the alkaloid (±)-decinine 492 (Scheme 162).
440
Scheme 162
Synthesis of (±)-Decinine
The gold(I)-catalyzed synthesis of optically active γ-lactams
by tandem cycloisomerization/oxidation of homopropargyl amides was
applied in the synthesis of (−)-bgugaine.
441
A similar strategy has been used in the total synthesis
of (−)-irniine.
442
Interestingly,
3-coumaranones such as 493 can also be obtained by gold(I)-catalyzed
oxidative cyclization of o-ethynylanisoles, which
has been applied in the total synthesis of sulfuretin (494) (Scheme 163).
443
Scheme 163
Synthesis of Sulfuretin
The gold(I)-promoted regioselective oxidation of alkynes
410c
was applied on the total synthesis of alkaloids
(−)-citrinadin A (497)
444
and (+)-citrinadin B (500) (Scheme 164).
445
Scheme 164
Total Syntheses
of (−)-Citrinadine A and (+)-Citrinadine B
The gold(I)-catalyzed oxidative cyclization
of 1,5-enynes
427
has been recently used
as the key step for
a concise enantioselective total synthesis of the sesquiterpene (−)-nardoaristolone
B (504) by reaction of dienyne 501 with
IPrAuNTf2 in the presence of 3,5-dichloropyridine N-oxide to form enone 502, along
with the product
of cycloisomerization 503 (Scheme 165).
446
Scheme 165
Synthesis of (−)-Nardoaristolone
B by Oxidative Cyclization
7
Conclusions
Work carried out during
the past decade has demonstrated that gold(I)
complexes and, in particular, cationic complexes bearing bulky phosphines
and NHC ligands are the most active and selective catalysts for the
activation of enynes, even in complex polyfunctional settings. Mechanistically,
reactions catalyzed by gold(I) are similar to those catalyzed by other
electrophilic metal complexes or even Brønsted acids and resemble
carbocation-mediated processes. However, gold(I) provides unique control
on complex transformations by very selectively activating alkynes
and by stabilizing the key carbocationic intermediates by weak, but
still significant, metal to carbene π-back-donation. Although
many gold-catalyzed reactions of alkynes and, in particular, enynes
appear to proceed through gold(I) carbene-like species, the implication
of α-oxo gold(I) carbenes in oxidative cyclizations has been
recently questioned in several contexts. Further work on the mechanism
of these reactions should shed light into the structure of the species
involved in these transformations. Most of the work has been carried
out in intramolecular processes leading to five- or six-membered ring
compounds, although smaller or larger ring systems can also be accessed
by using gold(I)-catalysis. However, the regiochemical control in
many cases is still essentially dependent on the substrate substitution
pattern and not on the ligands on the gold(I) catalyst. Additionally,
the structural characteristics of linear two-coordinated gold(I) complexes,
in which the ligand is very distant from the nucleophilic addition
site to the π-bound substrate, explains the slow development
of general enantioselective transformations of alkynes. Finally, the
developing of broad-scope intermolecular reactions of alkynes for
the formation of carbon–carbon bonds still remains an important
challenge in homogeneous gold(I) catalysis,