C-O bond homolysis in a tungsten alkoxide: The mechanism of alcohol deoxygenation by WCl2(PMe3)(4) and WH2Cl2(PMe3)(4)

Reactions of alcohols with WCl2(PMe3)(4) (1) or WH2Cl2(PMe3)(4) (2) yield W(O)Cl-2(PMe3)(3) (3), PMe3, and hydrocarbons. Cyclopropanemethanol is deoxygenated to give 1-butene and a trace of trans-2-butene as the organic products; benzyl alcohol yields toluene and bibenzyl. These products indicate the intermediacy of organic radicals. Benzyl radicals in the reaction of 1 with PhCH2OH can be trapped by added 2 or by 9,10-dihydroanthracene (DHA), leading to increased yields of toluene vs bibenzyl. With WD2Cl2(PMe3)(4) (2-d(2)) or DHA-d(12), PhCH2D is formed. The reaction of methanol with 1 proceeds similarly in the presence of DHA, forming 3 and methane. Kinetic studies on the reaction of 1 with benzyl alcohol indicate that the reaction proceeds via alkoxide intermediates. A mechanism involving homolysis of the C-O bond in an alkoxide intermediate is suggested by these results. The thermodynamics of this unusual transformation are discussed.