Substrate specificity in the biomimetic catalytic aerobic oxidation of styrene and cyclohexanone by metalloporphyrins: kinetics and mechanistic study

Abstract

Substrate specificity is a hallmark of enzymatic catalysis. In this work, the biomimetic catalytic oxidation of styrene and cyclohexanone by iron (III) porphyrins and molecular oxygen was carried out, and remarkable differences in efficiency were observed. The specificity of the substrates for biomimetic catalytic oxidation was investigated by kinetics and mechanistic studies. Kinetics studies revealed that the oxidation of styrene followed Michaelis–Menten kinetics with KM ​= ​8.99 ​mol L-1, but the oxidation of cyclohexanone followed first-order kinetics with kobs ​= ​1.46 ​× ​10−4 ​s−1, indicating that the styrene epoxidation by metalloporphyrins exhibited characteristics of enzyme-like catalysis, while the oxidation of cyclohexanone was in agreement with the general rules of chemical catalysis. Different catalytic mechanisms for the two substrates were discussed by operando electron paramagnetic resonance spectroscopy, operando UV–vis spectroscopy, and KI/starch experiments. Substrate specificity was concluded to be attributed to the stability of high-valence species and oxygen transfer rate.

Keywords

• Substrate specificity
• Kinetics
• Oxidation
• Metalloporphyrins
• Mechanism