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. Furthermore, the formation
of charged complexes through the activation of chiral complex 1 by phosphorane 2a is confirmed by electrical
conductivity experiments.