Group IV metal complex-catalyzed reactions of hydrosilanes: Dehydrocoupling of hydrosilanes and hydrosilylation of ketones

Dehydrocoupling polymerization of phenylsilane with the metallocenes Cp₂MY₂ (M = Ti, Zr, Hf; Y = F, OPh, NMe₂) has been studied. All nine metallocenes produced polysilanes which exhibited a bimodal molecular weight distribution with molecular weights that ranged from 1100 to 4400 and ratios of linear polysilanes to cyclic oligosilanes from 2 to 30. The highest molecular weight polysilanes observed for each metal were obtained from the metallocene amides Cp₂M(NMe₂) ₂ and the highest percentage of linear polysilanes was found with Cp ₂Hf(NMe₂)₂. The near stoichiometric reactions of the metallocenes Cp₂MY₂ with PhSiH₃, PhMeSiH ₂, Ph₂MeSiH were monitored by 1H, 19F and 29Si NMR spectroscopy and in all cases (M-)Y/(Si-)H exchange products containing Si-Y were identified. Metal hydride species could be observed only from Cp₂TiY₂.; These precatalyst systems consisting of a metallocene and a hydrosilane were then utilized for the reduction of ketones. Thus, air and moisture stable Cp₂TiF₂ was examined as a catalyst for the hydrosilylation of ketones with various hydrosilanes to give silyl ethers which produced alcohols after hydrolysis. The optimum conditions at room temperature for the reduction of acetophenone were found with PhMeSiH₂ and 5 mol% Cp₂TiF ₂ in neat and gave a 90% yield of 1-phenylethanol. Selected diaryl, arylalkyl and dialkyl ketones were reduced with PhMeSiH₂ in up to 96% yield. Unsaturated ketones were reduced exclusively at the carbonyl group (1,2-reduction).; The hydrosilylation reaction was extended to the asymmetric reduction of acetophenone by hydrosilanes where the precatalyst complexes were readily obtained from chiral Schiff bases and MY₄ (M = Ti, Zr; Y = OPri, NMe₂). The nature of the hydrosilane and the orientation of the two OPri groups in the complex L*Ti(OPr i)₂ effected the enantioselectivity. The best enantioselectivity (44% e.e. with 99% yield) was obtained with Ph₂SiH₂ and L*Ti(OPri)₂ having a tridentate Schiff base ligand while a tetradentate Schiff base ligand system provided up to 34% e.e. ‘Enantioselective autoinduction’ was observed for L*Ti(OPr i)₂ (L* = tridentate Schiff base ligand). LTi(NMe ₂)₂ and LZrY₂ (L = chiral Schiff base ligand) produced enantioselectivity of less than 16% e.e.