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

Abstract

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 °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 1 is 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.