Hydrosilylation of Ketones, Imines and Nitriles Catalysed by Electrophilic Phosphonium Cations: Functional Group Selectivity and Mechanistic Considerations

The electrophilic phosphonium salt, [(C6F5)(3)PF][B(C6F5)(4)], catalyses the efficient hydrosilylation of ketones, imines and nitriles at room temperature. In the presence of this catalyst, adding one equivalent of hydrosilane to a nitrile yields a silylimine product, whereas adding a second equivalent produces the corresponding disilylamine. [(C6F5)(3)PCl][B(C6F5)(4)] and [(C6F5)(3)PBr][B(C6F5)(4)] are also synthesised and tested as catalysts. Competition experiments demonstrate that the reaction exhibits selectivity for the following functional groups in order of preference: ketone>nitrile>imine>olefin. Computational studies reveal the reaction mechanism to involve initial activation of the SiH bond by its interaction with the phosphonium centre. The activated complex then acts cooperatively on the unsaturated substrate.