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.