Hypercoordinate Ketone Adducts of Electrophilic η~3-H_2SiRR' Ligands on Ruthenium as Key Intermediates for Efficient and Robust Catalytic Hydrosilation

摘要

The electrophilic η~3-H_2SiRR' σ-complexes [PhBP~(Ph)_3]RuH(η~3-H_2SiRR') (RR' = MePh, 1a; Ph_2, 1b; [PhBP~(Ph)_3]~- = [PhB(CH_2PPh_2)_3]~-) are efficient catalysts (0.01-2.5 mol % loading) for the hydrosilation of ketones with PhMeSiH_2, Ph_2SiH_2, or EtMe_2SiH. An alkoxy complex [PhBP~(Ph)_3]Ru-OCHPh_2 (4b) was observed (by ~(31)P{~1H} NMR spectroscopy) as the catalyst resting state during hydrosilation of benzophenone with EtMe_2SiH. A different catalyst resting state was observed for reactions using PhMeSiH_2 or Ph_2SiH_2, and was identified as a silane σ-complex [PhBP~(Ph)_3]RuH[η~2-H- SiRR'(OCHPh_2)] (RR' = MePh, 5a; Ph_2, 5b) using variable temperature multinudear NMR spectroscopy (-80 to 20 ℃). The hydrosilation of benzophenone with PhMeSiH_2 and 1a was examined by ~1H NMR spectroscopy at -18 ℃ (in CD_2Cl_2), and this revealed that either 1a, 5a, or both 1a and 5a could be observed as resting states of the catalytic cycle, depending on the initial [PhMeSiH_2]:[benzophenone] ratio. Kinetic studies revealed two possible expressions for the rate of product formation, depending on which catalyst resting state was present (rate = k_(obs)[PhMeSiH_2][5a] and rate = k'_(obs)[benzophenone][1a]). Computational methods (DFT, b3pw91, 6-31G(d,p)/LANL2DZ) were used to determine a model catalytic cycle for the hydrosilation of acetone with PhMeSiH_2. A key step in this mechanism involves coordination of acetone to the silicon center of 1a-DFT, which leads to insertion of the carbonyl group into an Si-H bond (that is part of a Ru-H-Si 3c-2e bond). This generates an intermediate analogous to 5a (5a-i-DFT), and the final product is displaced from 5a-i-DFT by an associative process involving PhMeSiH_2.