Catalytic mechanism for the biomimetic oxidation of benzoin by p-benzoquinone in the presence of {Fe4S4} complexes with bulky arenethiolate or cysteine-containing tripeptide ligands

Abstract

Catalytic oxidation of benzoin to benzil by p-benzoquinone in the presence of [PPh₄]₂[Fe₄S₄(tipbt)₄](tipbt = 2,4,6-triisopropylbenzenethiolate) obeys first-order kinetics in the concentrations of benzoin and the catalyst. The observed rate constant was found to depend on the bulkiness of the thiolate or the peptide ligand of [Fe₄S₄(SR)₄]^2–. A complex with a bulky peptide, [Fe₄S₄(Z-CysS-Pro-Val-OMe)₄]^2–(Z = benzyloxycarbonyl, CysS = S-deprotonated cysteinate), had an activity five times higher than that of [Fe₄S₄(Z-CysS-Pro-Gly-OMe)₄]^2–. para-Substitution of benzoin indicated a trend in the oxidation rate: 4-Cl > H, 4-OMe. It is proposed that the methine hydrogen of benzoin is released as a proton in the rate-determining step. This is supported by the isotope effect (k_H/k_D 2.5:1) on the oxidation rate of α-C-deuteriated benzoin. An increase in the relative permittivity of the solvent results in an increase in the rate constant, indicating a polar transition state.