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

Unambiguous examples of eta(3)-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 [PhBP3Ph]RuH[eta(3)-H2SiMePh] (1b) (PhBP3Ph = PhB(CH2PPh2)(3)-by primary silanes RSiH3 to generate [PhBP3Ph]RuH[eta(3)-H2SiH(R)] (R = Cy (1d), CH2CH2Ph (1e), Trip = 2,4,6-(Pr3C6H2)-Pr-i (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 H-1 NMR spectrum of 1f collected at-30 degrees C displays a downfield signal (delta = 8.26 ppm) for the terminal Si-H bond that suggests electronic similarities between if and cationic silylene dihydrides [Cp*((Pr3P)-Pr-i)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 [PhBP3Ph]RuH[eta(3)-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 eta(3)-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 sigma-bond metathesis. Direct insertion into the Si-H bond is the next lowest accessible pathway.