Titanium Dioxide Photocatalysis for Radical Alkylation

Abstract The TiO2 photosensitized irradiation of benzyl derivatives (DX = YC6H4CH2X) in the presence of electron-withdrawing substituted alkenes (A) leads to mono- and dibenzylated adducts (DAH or DAD). The addition did not occur (or only poorly) in the absence of the catalyst. The reaction is initiated by hole, electron transfer at the catalyst surface, followed by fragmentation of the radical cation, diffusion and trapping of the resulting benzyl radicals. When either a good donor (Y=OMe) is used with any of the acceptors, or a poor donor (Y=H) is used with an excellent electron scavenger such as tetracyanoethylene, electron transfer is fast and radical cation cleavage is the rate-determining step (X+=Me3Si+ is the best electrofugal group, CO2+H+ is slower, while H+ fragmentation is too slow for causing an useful reaction). With poor donors and moderate acceptors (such as maleic and fumaric acid derivatives) electron transfer is the rate determining step. In both cases the C-C bond formation step involves trapping of the free solvated benzyl radical. As tested in the reaction of p-methoxybenzyltrimethylsilane with maleic anhydride, the alkylation is faster when the catalyst is loaded with a small (0.3-0.75% - almost no effect at 2%) amount of Pt. Under that condition the local concentration of benzyl radicals is higher and larger yields of radical coupling products (DD and DAD) are obtained.