Dichotomous Si–H Bond Activation by Alkoxide and Alcohol in Base-Catalyzed Dehydrocoupling of Silanes

The
activation of silanes in dehydrogenative coupling with alcohols
under general base catalysis was studied experimentally (using multinuclear
NMR, IR, and UV–visible spectroscopies) and computationally
(at DFT M06/6-311++G­(d,p) theory level) on the example of Ph4‑nSiHn (n = 1–3) interaction with (CF3)2CHOH
in the presence of Et3N. The effect of the phenyl groups’
number and H– substitution by the electron-withdrawing
(CF3)2CHO– group on Si–H
bond hydricity (quantified as hydride-donating ability, HDA) and Lewis
acidity of silicon atom (characterized by maxima of molecular electrostatic
potential) was accessed. Our results show the coordination of Lewis
base (Y = Me3N, ROH, OR–) leads to the
increased hydricity of pentacoordinate hypervalent Ph4‑nSi­(Y)­Hn complexes and
a decrease of the reaction barrier for H2 release. The
formation of tertiary complexes [Ph4‑nSi­(Y)­Hn]···HOR is
a critical prerequisite for the dehydrocoupling with alkoxides being
ideal activators. The latter can be external or internal, generated
by in situ HOR deprotonation. The mutual effect of
tetrel interaction and dihydrogen bonding in tertiary complexes (RO–)­Ph4‑nSiHn···HOR leads to dichotomous activation
of Si–H bond promoting the proton–hydride transfer and
H2 release.