Photoinduced hydrosilylation through hydrogen abstraction: an NMR and computational study of the structural effect of silane

The hydrosilylation reaction, describing the addition of Si-H bonds to unsaturated bonds, is performed in the presence of catalysts, usually highly active platinum catalysts. This work focuses on the study of a photoinduced hydrosilylation by the use of benzophenone which promotes the addition reaction of olefin on different hydrosilanes. The reactivity of silanes towards addition onto the double bond during hydrosilylation appears to depend on their structure. It was observed that the consumption of Si-H and C&z.dbd;C functional groups increases with the irradiation time, and reaches a maximum of approx. 51% in the case of diphenylsilane. The hydrosilylation products are determined with 1 H NMR, HSQC, DEPT, COSY and 13 C NMR. The main product corresponds to the single adduct of the silyl radical onto the double bond. Substitution of the Si-H bond by two or three phenyls groups (triphenylsilane, diphenysilane) enhances the yield of the reaction, although diphenylsilane was found to be more efficient than triphenylsilane because of its lower steric hindrance. The ketyl radical formed after hydrogen abstraction by the triplet state of benzophenone likely forms benzopinacol, a reaction which reduces the overall yield of the hydrosilylation reaction. All these experiments are in line with DFT calculations of the Gibbs free energy of the reactions involved. This sheds new light on the photoinduced hydrosilylation process and opens the way to more active combinations of photoinitiator/silane/vinylsilane systems. Factors affecting photoinduced hydrosylilation are identified and open new opportunities.