Regioselectivity of Stoichiometric Metathesis of Vinylsilanes with Second-Generation Grubbs Catalyst: A Combined DFT and Experimental Study

The
regioselectivity of metathesis reactions of trisubstituted
vinylsilanes H2CCHSiR3 (SiR3 = SiCl3, SiCl2Me, SiClMe2, SiMe3, Si­(OEt)3) with the second-generation ruthenium
alkylidene complex has been studied theoretically, by density functional
theory (DFT), and experimentally. The DFT results indicate that cycloreversion
is the rate-determining step and the formation of a thermodynamically
stable ruthenium methylidene complex and PhCHCHSiR3 is generally preferred. However, the regioselectivity of the process
can be also governed by the relative stabilities of the ruthenacyclobutane
intermediates, which depend on the electronic and steric properties
of the SiR3 substituent. Higher stability of α,β-disubstituted
ruthenacyclobutanes in comparison to α,α-disubstituted
ruthenacyclobutanes is predicted, in contrast to the corresponding
intermediates formed during metathesis of common α-olefins.
The stabilizing Ru–Cβ interaction in the ring
is strengthened by the electron-donor SiR3 substituent
at Cβ. The experiments performed have shown selectivity
toward styrene formation for SiR3 = SiClMe2,
SiMe3, whereas a preference for the formation of ruthenium
methylidene and PhCHCHSiR3 has been observed for
SiR3 = SiCl3, SiCl2Me, Si­(OEt)3, in accordance with the theoretical predictions.