(CAAC)CuCl: A Competent Precatalyst for Carbonyl and Ester Hydrosilylation

Cu-catalyzed carbonyl hydrosilylation involves a ligated "[(L)CuH]" as the active catalyst, where the ligand L has a crucial role toward the stability, stereoselectivity, and enhancement of the hydridicity. Strongly sigma-donating N-heterocyclic carbenes (NHCs), their ring-expanded form, and an abnormal NHC as ligands have yielded robust and efficient Cu catalysts. However, cyclic(alkyl)(amino)carbenes (CAACs), despite being stronger sigma-donors than NHCs and already having a salient Cu-(I) chemistry, are yet to be reported as a similar ligand platform for this purpose. We establish here the familiar [((Me2)CAAC)CuCl] as a powerful precatalyst in this regard. Additionally, it also catalyzes the more challenging ester hydrosilylation, which is a rare feat for a Cu catalyst. Apart from the stronger sigma-donating ability, the more steric "openness" of CAACs than bulky NHCs also seems to be advantageous. To corroborate, three new (CAAC)CuCl complexes [((ArCH2,Me)CAAC)CuCl] (Ar = Ph, 1-naphthyl, and 1-prenyl) are devised, where the effective steric around the copper is practically unaltered from the case of [((Me2)CAAC)CuCl]. All three are equally active in carbonyl and ester hydrosilylation as [((Me2)CAAC)CuCl]. Computation suggests the carbonyl insertion into a "(CAAC)Cu-H" as the rate-limiting step. To elucidate the involvement of a "(CAAC)CuH", "((PhCH2,Me)CAAC)CuH" is generated in situ and is trapped as its BH3 adduct ((PhCH2,Me)CAAC)CuBH4.