Solvent-Dependent Asymmetric Synthesis of Alkynyl and Monofluoroalkenyl Isoindolinones by CpRh ^III -Catalyzed C−H Activation

The development of new methods for the direct functionalization of unactivated C-H bonds is ushering in a paradigm shift in the field of retrosynthetic analysis. In particular, the catalytic enantioselective functionalization of C-H bonds represents a highly atom- and step-economic approach toward the generation of structural complexity. However, as a result of their ubiquity and low reactivity, controlling both the chemo- and stereoselectivity of such processes constitutes a significant challenge. Herein we comprehensively review all asymmetric transition-metal-catalyzed methodologies that are believed to proceed via an inner-sphere-type mechanism, with an emphasis on the nature of stereochemistry generation. Our analysis serves to document the considerable and rapid progress within in the field, while also highlighting limitations of current methods.

Metal complexes coordinated by a single cyclopentadienyl (Cp) ligand are widely used, versatile catalysts, but their application to asymmetric reactions has been hindered by the difficulty of designing Cp substituents that effectively bias the coordination sphere. Here, we report on a class of simple C(2)-symmetric Cp derivatives that finely control the spatial arrangement of the transiently coordinated reactants around the central metal atom. Rhodium(III) complexes bearing these ligands proved to be highly enantioselective catalysts for directed carbon-hydrogen (C-H) bond functionalizations of hydroxamic acid derivatives.

Transition-metal catalyzed C-H functionalizations became a complementary and efficient bond-forming strategy over the past decade. In this respect, Cp*Rh(III) complexes have emerged as powerful catalysts for a broad spectrum of reactions giving access to synthetically versatile building blocks. Despite their high potential, the corresponding catalytic enantioselective transformations largely lag behind. The targeted transformations require all the remaining three coordination sites of the central rhodium atom of the catalyst. In consequence, the chiral information on a competent catalyst can only by stored in the cyclopentadienyl unit. The lack of suitable enabling chiral cyclopentadienyl (Cp(x)) ligands is the key hurdle preventing the development of such asymmetric versions. In this respect, an efficient set of chiral Cp(x) ligands useable with a broad variety of different transition-metals can unlock substantial application potential. This Account provides a description of our developments of two complementary classes of C2-symmetric Cp(x) derivatives. We have introduced a side- and back-wall concept to enforce chirality transfer onto the central metal atom. The first generation consists of a fused cyclohexane unit having pseudo axial methyl groups as chiral selectors and a rigidifying acetal moiety. The second ligand generation derives from an atrop-chiral biaryl-backbone and which possesses adjustable substituents at its 3,3'-positions. Both ligand families can be modulated in their respective steric bulk to adjust for the specific needs of the targeted application. The cyclopentadienes can be metalated under standard conditions. The corresponding chiral rhodium(I) ethylene complexes are relatively air and moisture and represent storable stable precatalysts for the targeted asymmetric Rh(III)-catalyzed C-H functionalizations. These complexes are then conveniently oxidized in situ by dibenzoyl peroxide to give the reactive Cp(x)Rh(III)(OBz)2 species. For instance, this catalyst is used for directed C-H activations of aryl hydroxamates and the subsequent enantioselective trapping with olefins, providing dihydroisoquinolones in very high enantioselectivities. In addition, we have established highly selective intramolecular trapping reactions with tethered higher substituted alkenes giving dihydrobenzofurans with quaternary stereogenic centers. Concerning intermolecular reactions, allene coupling partners allow for an enantioselective hydroarylation yielding substituted allylated compounds. A trapping process of the cyclometalated intermediate with diazo reactants enables the enantioselective construction of isoindolinones. Moreover, the catalysts can be used for the construction of atropchiral biaryl motives using a dehydrogenative Heck-type reaction. The development of flexibly adjustable chiral Cp(x) ligands is described in this Account showcasing their applicability for a variety of Rh(III) catalyzed C-H functionalization reactions. These Cp(x) derivatives hold promise as powerful steering ligands for further transition-metals used in asymmetric catalysis.