The debut of chiral cyclic (alkyl)(amino)carbenes (CAACs) in enantioselective catalysis

Abstract

Cyclic (alkyl)(amino)carbene (CAAC) metal complexes can also engage in asymmetric transformations, thereby expanding the toolbox of available chiral carbene ligands.

Cyclic (Alkyl)(amino)carbene Ligands Enable Cu‐Catalyzed Markovnikov Protoboration and Protosilylation of Terminal Alkynes: A Versatile Portal to Functionalized Alkenes**

“…Ther eaction would then be expected to proceed under steric control, providing am eans of controlling Markovnikov selectivity in am anner that is independent of the electronic nature of both the alkyne substrate and the diboron reagent (Scheme 1B). Over the past several years, [5] the Bertrand lab has developed an umber of cyclic (alkyl)-(amino)carbene (CAAC) ligands, [6][7][8][9] which show unique reactivity and selectivity profiles in several transition-metalcatalyzed reactions. [10] Given that these ligands are known to be stronger s-donors than analogous NHC ligands, [11,12] we hypothesized that the corresponding LCu(BX 2 )species would undergo preferential Markovnikov-selective addition to terminal alkynes via a-TS.H erein we describe ah ighly aselective protoboration of terminal alkynes catalyzed by CAAC-ligated Cu complexes (Scheme 1C).…”

Indolizy Carbene Ligand. Evaluation of Electronic Properties and Applications in Asymmetric Gold(I) Catalysis

“…Regarding CAACs, the presence of aq uaternary carbon center adjacent to the ylidene carbon is ag ood lever to introduce stereogenicity close to the active site.Bertrand and coll. initially prepared an enantiopure (À)-menthol derived CAAC ligand [7a] but no asymmetric catalysis in ac oppercatalyzed asymmetric conjugated borylation reaction was recorded, [24] this being certainly due to ac onformational inversion of the menthyl group.I nc ontrast, am ore rigid CAAC with extended chirality derived from a5a-cholestan-3one steroid backbone was recently synthesized and the corresponding copper complex Cholest CAACCuCl, promoted encouraging enantioselectivity (55 % ee)i nt he same borylation reaction. [24,25] It should also be noted that racemic chiral CAACAuCl complexes have been prepared by Kumar and Waldmann via aM e 2 SAuCl-promoted cyclization-rearrangement sequence of 1,7-enynes and two examples of catalysis were given.…”

“…initially prepared an enantiopure (À)-menthol derived CAAC ligand [7a] but no asymmetric catalysis in ac oppercatalyzed asymmetric conjugated borylation reaction was recorded, [24] this being certainly due to ac onformational inversion of the menthyl group.I nc ontrast, am ore rigid CAAC with extended chirality derived from a5a-cholestan-3one steroid backbone was recently synthesized and the corresponding copper complex Cholest CAACCuCl, promoted encouraging enantioselectivity (55 % ee)i nt he same borylation reaction. [24,25] It should also be noted that racemic chiral CAACAuCl complexes have been prepared by Kumar and Waldmann via aM e 2 SAuCl-promoted cyclization-rearrangement sequence of 1,7-enynes and two examples of catalysis were given. [26] As part of our ongoing interest in the design of new chiral ligands for gold complexes [27][28][29] we have devised the new family of Indolizy C-ligands E,p ossibly bifunctional [30] (Figure 1).…”

Yellow Circularly Polarized Luminescence from C1‐Symmetrical Copper(I) Complexes

“…Remarkably, Bertrand, Jazzar, Mauduit, and co‐workers have noted that LMent CAAC complexes are unable to induce enantioselective catalysis, due to some conformational flexibility of the menthyl ring . Our results demonstrate that despite this flexibility, the chirality can still induce circularly polarized luminescence, which is consistent with the shorter timescale of light emission.…”