AIR OXIDATION OF PARA-SUBSTITUTED BENZOIN TO THE CORRESPONDING BENZIL CATALYZED BY FE(II)-CYSTEINE PEPTIDE COMPLEXES

The Fe(II)-cysteine-containing peptide complexes were found to exhibit catalytic activity for air oxidation of benzoin to benzil and methyl DL-mandelate to methyl benzoylformate. In the presence of [Fe(Z-cys-Gly-Val-OMe)4]2-, the rates of catalytic air-oxidation of p-substituted benzoin indicate a trend, Br > H > CH3 > MeO, and the isotope effect k(H)/k(D) was 3.4. The result indicates that the methine hydrogen of benzoin is released as a proton in the rate determining step. Furthermore, solvent effect was found, in a non-polar solvent, e.g. 1,2-dimethoxyethane (DME), a chelating bidentate cys-peptide complex, [Fe(Z-cys-Pro-Leu-cys-Gly-Val-OMe)2]2- shows higher catalytic activity than that of a non-chelating cys-peptide complex, [Fe(Z-cys-Gly-Val-OMe)4]2-. This is explained by the contribution of intramolecular NH--S hydrogen bonds since such hydrogen bonds stabilize the Fe(III) state which is the active species during the oxidation.