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

The 1,4 hydrosilation of a variety of simple alpha,beta-unsaturated enones as catalyzed by B(C6F5)(3) (1-2%) is described. For substrates with no steric hindrance near the beta-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. (C) 2002 Elsevier Science Ltd. All rights reserved.