Studies on the mechanism of B(C6F5)(3)-catalyzed hydrosilation of carbonylfunctions

The strong organoborane Lewis acid B(C6F5)(3) catalyzes the hydrosilation ( using R3SiH) of aromatic and aliphatic carbonyl functions at convenient rat es with loadings of 1-4%. For aldehydes and ketones, the product silyl ethe rs are isolated in 75-96% yield; for esters, the aldehydes produced upon wo rkup of the silyl acetal products can be obtained in 45-70% yield. Extensiv e mechanistic studies point to an unusual silane activation mechanism rathe r than one involving borane activation of the carbonyl function. Quantitati ve kinetic studies show that the least basic substrates are hydrosilated at the fastest rates; furthermore, increased concentrations of substrate have an inhibitory effect on the observed reaction rate. Paradoxically, the mos t basic substrates are reduced selectively, albeit at a slower rate, in com petition experiments. The borane thus must dissociate from the carbonyl to activate the silane via hydride abstraction; the incipient silylium species then coordinates the most basic function, which is selectively reduced by [HB(C6F5)(3)]. In addition to the kinetic data, this mechanistic proposal i s supported by a kinetic isotope effect of 1.4(5) for the hydrosilation of acetophenone, the observation that B(C6F5)(3) catalyzes H/D and H/H scrambl ing in silanes in the absence of substrate, computational investigations, t he synthesis of models for proposed intermediates, and other isotope labeli ng and crossover experiments.