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.