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 (kH/kD 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.