Catalytic Olefin Hydrosilations Mediated by Ruthenium η³-H₂Si σ Complexes of Primary and Secondary Silanes

Unambiguous examples of η3-H2SiH(R) complexes featuring a terminal Si–H bond have been prepared and examined as possible intermediates in olefin hydrosilation. These species were generated by displacement of the secondary silane ligands in [PhBPPh3]RuH[η3-H2SiMePh] (1b) (PhBPPh3 = PhB(CH2PPh2)3− by primary silanes RSiH3 to generate [PhBPPh3]RuH[η3-H2SiH(R)] (R = Cy (1d), CH2CH2Ph (1e), Trip = 2,4,6-iPr3C6H2 (1f)). Complexes 1d and 1e were characterized in solution, whereas 1f was isolated and studied in detail. Complex 1b is not a competent precatalyst for the hydrosilation of 1-hexene with CySiH3, whereas comparable conditions gave reasonable yields for the selective anti-Markovnikov hydrosilations of Cl3SiCH2CH═CH2 (89%), p-chlorostyrene (73%), and allyl chloride (70%). The 1H NMR spectrum of 1f collected at −30 °C displays a downfield signal (δ = 8.26 ppm) for the terminal Si–H bond that suggests electronic similarities between 1f and cationic silylene dihydrides [Cp*(iPr3P)Ru(H)2═SiH(R)]+ that mediate olefin hydrosilations via the direct insertion of the C═C bond into the terminal Si–H bond. However, further mechanistic considerations, including results on the hydrosilation of p-chlorostyrene with the secondary silane Et2SiH2 and [PhBPPh3]RuH[η3-H2SiEt2] (1a) as the catalyst, indicate that an insertion mechanism involving a Ru–H (rather than a Si–H) group is possible. DFT investigations of the hydrosilation of several olefins with CySiH3 using 1d as the catalyst reveal a preferred pathway involving olefin insertion into a Ru–H bond followed by migration of the resulting alkyl group to the silicon atom of an η3-H2SiH(Cy) ligand. The latter process occurs via an unusual transition state in which a Ru–H–Si linkage acts as a pivot point to facilitate a Si–H bond cleavage/Si–C bond formation step that is otherwise similar to those involving the kite-shaped four-center transition states of σ-bond metathesis. Direct insertion into the Si–H bond is the next lowest accessible pathway.