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      A Readily Accessible Class of Chiral Cp Ligands and their Application in RuII -Catalyzed Enantioselective Syntheses of Dihydrobenzoindoles

      1 , 1 , 1
      Angewandte Chemie International Edition
      Wiley

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          Chiral cyclopentadienyl ligands as stereocontrolling element in asymmetric C-H functionalization.

          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.
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            Chiral Cyclopentadienyls: Enabling Ligands for Asymmetric Rh(III)-Catalyzed C-H Functionalizations.

            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.
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              A tunable class of chiral Cp ligands for enantioselective rhodium(III)-catalyzed C-H allylations of benzamides.

              The lack of robust and tunable chiral versions of cyclopentadienyl (Cp) ligands hampers progress in the development of catalytic asymmetric versions of a myriad of reactions catalyzed by this ubiquitous ligand. Herein, we describe of a class of chiral Cp ligands with tunable steric parameters. Coordinated to transition metals, the ligand creates a well-defined chiral pocket, able to imprint its chirality onto the metal. The corresponding Rh complexes are shown to be excellent catalysts for enantioselective allylation of N-methoxybenzamides via directed C-H functionalizations at very mild conditions. The obtained enantioselectivities are excellent and demonstrate the viability of chiral Cp complexes as selective transition metal catalysts.
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                Author and article information

                Journal
                Angewandte Chemie International Edition
                Angew. Chem. Int. Ed.
                Wiley
                14337851
                May 04 2018
                May 04 2018
                April 06 2018
                : 57
                : 19
                : 5459-5462
                Affiliations
                [1 ]Laboratory of Asymmetric Catalysis and Synthesis; EPFL SB ISIC LCSA, BCH 4305; 1015 Lausanne Switzerland
                Article
                10.1002/anie.201802244
                a4b31b94-0783-4f9b-88ff-dd3cefd5f486
                © 2018

                http://doi.wiley.com/10.1002/tdm_license_1.1

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