Quantum chemical modelling of the C–H cleavage mechanism in oxidation of ethylbenzene and its derivates by ethylbenzene dehydrogenase

Abstract

Ethylbenzene dehydrogenase is an enzyme capable of oxygen-independent stereospecific oxidation of ethylbenzene to ( S)-1-phenylethanol. Moreover, it oxidises a wide range of other alkylaromatic and alkylheterocylic compounds. In oxidation processes the C–H bond cleavage is supposed to be a rate-limiting step that may proceed either via a radical or a carbocation intermediate. The reaction rate can also be under control of energy barrier of OH rebound to the activated hydrocarbon proceeding also according to radical or carbocation mechanism. In order to assess the probabilities of the two alternative mechanisms, the Gibbs free energies of formation of both radical and carbocation intermediates from various substrates are determined by quantum chemical calculations on DFT level. It is found that the obtained thermodynamic parameters Δ G radical and Δ G carbocation correlate with widely accepted molecular descriptors such as radical Yamamoto–Otsu E r ( R 2 = 0.70) and Hammett σ + values ( R 2 = 0.91). The effects of modification of substrate structures on the stabilization of radical/carbocation intermediate (as approximation of transition state) are correlated with enzyme kinetic results. None correlation is apparently satisfactory, but taking into account the distribution of scattered points, the carbocation intermediate seems to be (on average) more probable than a free radical intermediate. In addition, a simple QSAR model describing all characterized substrates is obtained taking into consideration only two parameters, ΔΔ G of carbocation formation and molecular refractivity (MR) as a measure of steric hindrance.