B(C6F5)3 catalyzed hydrosilation of enones and silyl enol ethers

The 1,4 hydrosilation of a variety of simple α,β-unsaturated enones as catalyzed by B(C6F5)3 (1–2%) is described. For substrates with no steric hindrance near the β-carbon, 1,4 addition of silane is very clean; in other instances, 1,2 hydrosilation is competitive. The reaction is facile with five commercially available silane reagents. For two examples, a novel hydrosilation of the resulting silylenol ethers was also observed. The net trans stereochemistry of H–Si addition to the silylenol ether CC bond was established and points to a stepwise mechanism for this reaction. This was supported by the observation and full spectroscopic characterization of a silylcarboxonium ion intermediate with an [HB(C6F5)3]− counteranion in the hydrosilation of the silylenol ether derived from 4,4-dimethyl-2-cyclohexen-1-one and PhMe2SiH.