Mechanisms of C−Si and C−H Bond Formation on the Reactions of Alkenylruthenium(II) Complexes with Hydrosilanes

Reactions of the four alkenylruthenium(II) complexes Ru[C(R1)CH(R2)]Cl(CO)(PPh3)2 (R1 = H, R2 = Ph (1b); R1 = H, R2 = t-Bu (1c); R1 = Ph, R2 = Ph (1d); R1 = CHCH(SiMe3), R2 = SiMe2Ph (1e)) with HSiMe2Ph, which constitute the product-forming step of ruthenium-catalyzed hydrosilylation of alkynes, have been examined. Two reaction courses are operative: one provides the C−Si coupling product PhMe2SiC(R1)CH(R2) and RuHCl(CO)(PPh3)3 (path A), and the other forms the C−H coupling product HC(R1)CH(R2) and Ru(SiMe2Ph)Cl(CO)(PPh3)2 (path B). The ratio of the two courses significantly varies with substituents on the alkenyl ligands, particularly with the α-substituent (R1). Thus, 1b and 1c, without an α-substituent, react mainly by path A. In contrast, 1d and 1e, bearing an α-substituent, exclusively undergo path B. Kinetic studies using 1b and its para-substituted styryl ligand derivatives have revealed that path A proceeds by direct interaction of the five-coordinated complexes with hydrosilane, without dissociation of the PPh3 ligand. On the other hand, path B involves dissociation of PPh3 prior to the reaction of 1d or 1e with hydrosilane. Mechanisms of the C−Si and C−H bond formation are discussed with kinetic data in detail.