Air oxidation of p-substituted benzoin to the corresponding benzil catalyzed by Fe(II)-cysteine peptide complexes

Abstract

The Fe(II)-cysteine-containing peptide complexes were found 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 > CH 3 > MeO, and the isotope effect k H/k D was 3.4. The 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.