McQuade’s Six-Membered NHC–Copper(I) Complexes for Catalytic Asymmetric­ Silyl Transfer

Abstract

A full account of our work on enantioselective silylation of typical prochiral acceptors employing chiral six-membered NHC–copper(I) complexes introduced by McQuade and co-workers is presented. With these precatalysts, asymmetric branched-selective substitution of allylic phosphates and 1,2-addition to imines had become possible for the first time. The successful application of these catalysts in two fundamentally different reactions raised the question whether these are a privileged ligand motif for catalytic asymmetric silyl transfer. To assess their generality, these were utilized in the related 1,2-addition to aldehydes and in conjugate addition to representative α,β-unsaturated acceptors, but with limited success in both cases. This study also includes an optimization of the allylic silylation, now overcoming the limited scope of the previous protocol. The scope of the imine addition is extended to heteroaryl­-substituted aldimines.

Key words

copper - N-heterocyclic carbenes - nucleophilic addition - silicon - transmetalation

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