Quantum Chemical Study of B(C6F5)3-Catalyzed Hydrosilylation of Carbonyl Group

The hydrosilylation reaction of a carbonyl group catalyzed by tris(pentafluorophenyOborane, B(C6F5)(3), is investigated by using the DFT method. M06-2X level calculations suggest the presence of a stable complex between trimethylsilane and B(C6F5)(3). The attack of the carbonyl group in acetone from the back side of the Si H bond prompts the abstraction of the hydride ion by B(C6F5)(3). This reaction path is lower in free energy than the conventional carbonyl-activation path via a four-membered cyclic transition state. The silane-activation mechanism is supported in this case, in agreement with experimental results reported by Piers and by Oestreich. The calculations show, on the other hand, that the silane-activation mechanism does not apply to the reaction catalyzed by BF3. This difference in mechanisms arises from a stronger electrophilicity of the boron center in B(C6F5)(3) than in BF3 toward a hydride ion, as demonstrated by an analysis of reactive orbitals. Attractions between the silane part and the fluorine atoms at the ortho positions of C6F5 groups in the Lewis acid assist the path by making up for the destabilization of the reacting system that is caused by the distortion of the B(C6F5)(3) framework in forming a bond with the hydrogen of silane.