Reactivity of a Titanocene Pendant Si–H Group toward Alcohols. Unexpected Formation of Siloxanes from the Reaction of Hydrosilanes and Ph₃COH Catalyzed by B(C₆F₅)₃

The reaction of [Cp­(η⁵-C₅H₄CH₂SiMe₂H)­TiCl₂] (<b>1</b>; Cp = η⁵-C₅H₅) and methanol in the presence of catalytic amounts of B­(C₆F₅)₃ afforded a complex with a pendant silyl ether group, [Cp­(η⁵-C₅H₄CH₂SiMe₂OMe)­TiCl₂] (<b>2</b>), in good yield. The analogous reaction of <b>1</b> and Ph₃COH resulted in the unexpected formation of [CpTiCl₂{μ-η⁵:η⁵-(C₅H₄)­CH₂SiMe₂OSiMe₂CH₂(C₅H₄)}­TiCl₂Cp] (<b>4</b>). The formation of siloxanes from the reaction of 2 equiv of hydrosilane with Ph₃COH mediated by B­(C₆F₅)₃ has a general applicability and proceeds in two consecutive steps: (i) transfer of the hydroxyl group from the trityl moiety to the silicon atom and (ii) silylation of the silanol formed in situ with the second equivalent of hydrosilane. The different hydrosilane reactivity toward Ph₃COH in comparison with other alcohols can be attributed to the easy generation of the borate salt [Ph₃C]⁺[(C₆F₅)₃B­(μ-OH)­B­(C₆F₅)₃]⁻ (<b>5</b>) under catalytic conditions. The intramolecular Si–H and Ti–Cl exchange in <b>1</b> is catalyzed by B­(C₆F₅)₃ in the presence of no alcohol. This process affords presumably a transient titanocene hydrido chloride, which is either chlorinated to give [Cp­(η⁵-C₅H₄CH₂SiMe₂Cl)­TiCl₂] (<b>3</b>) in CD₂Cl₂ or decomposes into several paramagnetic Ti­(III) species in toluene-<i>d</i>₈. Complex <b>3</b> was independently synthesized from <b>1</b> and Ph₃CCl in a good yield