Activation of Chiral (Salen)AlCl Complex by Phosphorane for Highly Enantioselective Cyanosilylation of Ketones and Enones

Phosphoranes 2 are identified as a class of effective Lewis bases to activate chiral (salen)AlCl complex 1 to enhance its electrophilicity. Accordingly, a three-component catalyst system consisting of complex 1, phosphorane 2e, and Ph3PO is developed as a powerful tool for asymmetric ketone cyanosilylation. In particular, an unprecedented highly enantioselective cyanosilylation of linear aliphatic ketones is achieved. A tandem Wittig–cyanosilylation sequence starting from phosphorane 2a and enals 10 is further achieved, which internally utilizes the Ph3PO byproduct and remaining phosphorane 2a as cocatalysts for cyanosilylation of α,β,γ,δ-unsaturated enones, providing atom-efficient access to valuable chiral conjugated dienes and enynes. The high efficiency of the cyanosilylation originates from orthogonal activation of both (salen)AlCl complex 1 and cyanotrimethylsilane by the phosphorane and Ph3PO, respectively. This mechanistic insight is supported by NMR, MS, and ReactIR analyses and DFT calculations....