Dioxomolybdenum(VI) complexes as catalysts for the hydrosilylation of aldehydes and ketones

The dioxomolybdenum(VI) complexes MoO2Cl2] (1), MoO2(acac)2] (2), MoO2(S2CNEt2)2] (3), CpMoO2Cl] (4), MoO2(mes)2] (5) and the polymeric organotin-oxomolybdates (R3Sn)2MoO4] R = n-Bu (6), t-Bu (7), Me (8)] were examined as catalysts for the hydrosilylation of aldehydes and ketones with dimethylphenylsilane. Of these, MoO2Cl2] (1) was the most efficient catalyst, affording quantitative yields of the corresponding silylated ethers at room temperature in acetonitrile. Complexes 2, 4-8 also catalyzed the same reaction but required heating at 80 degrees C and longer reaction times compared with 1. Compound 3 is inactive. The wide scope of molybdenum oxide-mediated hydrosilylation was established with a variety of aldehydes and ketones. Counter intuitively, the activity of is 1 highest in NCMe. In the absence of a carbonyl substrate, MoO2Cl2(NCBu(t))] (10) reacts with HSiMe2Ph affording MoO(OSiMe2Ph)Cl2]2 (11) which has been fully characterized by NMR and IR spectroscopy, elemental analyses and mass spectrometry. Addition of radical scavengers strongly slows down the MoO2Cl2]-based hydrosilylation suggesting the intermediacy of oxygen-centered radicals.