Mechanism for reduction catalysis by metal oxo: hydrosilation of organic carbonyl groups catalyzed by a rhenium(V) oxo complex

The rhenium oxo complex Re(O)(hoz)2]TFPB], 1 (where hoz = 2-(2'-hydroxyphenyl)-2-oxazoline(-) and TFPB = tetrakis(pentafluorophenyl)borate) catalyzes the hydrosilation of aldehydes and ketones under ambient temperature and atmosphere. The major organic product is the protected alcohol as silyl ether. Isolated yields range from 86 to 57%. The reaction requires low catalyst loading (0.1 mol %) and proceeds smoothly in CH2Cl2 as well as neat without solvent. In the latter condition, the catalyst precipitates at the end of reaction, allowing easy separation and catalyst recycling. Re(O)(hoz)(H), 3, was prepared, and its involvement in an ionic hydrosilation mechanism was evaluated. Complex 3 was found to be less hydridic than Et3SiH, refuting its participation in catalysis. A viable mechanism that is consistent with experimental findings, rate measurements, and kinetic isotope effects (Et3SiH/Et3SiD = 1.3 and benzaldehyde-H/benzaldehyde-D = 1.0) is proposed. Organosilane is activated via eta2-coordination to rhenium, and the organic carbonyl adds across the coordinated Si-H bond 2 + 2] to afford the organic reduction product.