SIDE-CHAIN OXIDATION OF BENZYLTRIMETHYLSILANES BY IODOSYLBENZENE IN THE PRESENCE OF IRON AND MANGANESE PORPHYRINS

Benzyltrimethylsilanes react with iodosylbenzene in the presence of either iron(III) or manganese(III) tetrakis(pentafluorophenyl)porphyrin (TFPPM, with M = Fe, Mn) to give alpha-hydroxybenzyltrimethylsilanes, which are then rapidly converted into the corresponding benzaldehydes in the reaction medium. In these reactions the active species is the metal-oxo complex, TFPPM(V)=O, formed by iodosylbenzene and TFPPM. A relative reactivity study for a series of ring substituted benzyltrimethylsilanes has shown that when M = Fe, the reaction is quite selective (rho = -1.85), with the m-MeO substituent exhibiting a much higher reactivity than expected. When M = Mn, a lower rho value (-1.15) is observed and no anomalous reactivity is found with the m-MeO group. These result suggest that the side-chain hydroxylation of benzyltrimethylsilane by TFPPMn(V)=O occurs by the well known hydrogen atom transfer mechanism. For the corresponding reactions induced by TFPPFe(V)=O a coupled proton/electron transfer mechanism, which might involve the formation of a charge-transfer complex, seems more likely.