Oxidation of benzoin to benzil and of p-substituted benzyl alcohol to the corresponding benzaldehyde catalyzed by iron(II) thiolate complexes. A proposed reaction mechanism

Abstract

The catalytic p-benzoquinone or air oxidation of benzoin to benzil was carried out in the presence of [Fe II(S 2- o-xyl) 2] 2- ( o-xyl = o-xylene-α, α′-dithiolate) or [Fe II(S- t-Bu) 4] 2- in a ratio [benzoin]/[catalyst] = 20/1 in DMF at 25 °C. Under such mild reaction conditions, the Fe(II) thiolate complex exhibited high catalytic activity. In the reaction catalyzed by [Fe II(S 2- o-xyl) 2] 2-, 64% and 93% of benzoin was oxidized to benzil in 25 h by air and p-benzoquinone, respectively. In the air oxidation of benzyl alcohol catalyzed by [Fe II(S 2- o-xyl) 2] 2-, benzaldehyde was formed specifically without formation of benzoic acid. Furthermore, p-substituent effect (Cl > H > OMe) was found in the catalytic air oxidation of p-substituted benzyl alcohol. The catalytic oxidation is first order with respect to each of substrate and the catalyst at initial stage. A reaction mechanism was proposed and the rate-determining step is at the release of the methine hydrogen as a proton which is based on the kinetic isotope effect (k H/k D was 4.3 by p-benzoquinone oxidation and 5.0 by air oxidation, respectively) and p-substituent effect of substrate in the presence of [Fe II(S 2- o-xyl) 2] 2-.