Computational Exploration of the Mechanism of Alcohol Oxidation by Dioxygen Activated with Biquinolyl-Containing Cu Complexes

Abstract

The mechanism of benzyl alcohol oxidation with dioxygen activated by Cu complexes with biquinolyl (biQ)-containing ligands was investigated using theoretical DFT calculations to identify the active species and to reveal similarities with naturally occurring oxidations performed by Cu-containing enzymes. The detailed potential energy profiles for two possible reaction paths with the alpha-H atom abstraction key step (via direct benzaldehyde formation and via hydroxylation of benzyl alcohol followed by the expulsion of a water molecule) were calculated for the both triplet and singlet states of a Cu(II)-superoxo complex and a Cu(III)-oxo complex. The DFT estimated activation barrier for the alpha-H-atom abstraction (which is involved in the both pathways) from a molecule of benzyl alcohol by the triplet biquinolyl-containing Cu(II)-superoxo complex is 16.4 kcal/mol. This value is reasonable for an enzymatic C-H bond activation and is very close to that calculated for benzylic C-H bond activation in dopamine and formylglycine. Thus, both reaction pathways are reasonable, and the existence of the parallel pathways cannot be ruled out. However, the first path (via direct benzaldehyde formation) is less complicated and the entropy factor should facilitate this route. (biQ)Cu(III)-oxo species formed in the catalytic cycle can also be involved in the further catalytic oxidation of the benzyl alcohol. DFT exploration performed for the triplet and singlet (biQ)Cu(III)-oxo complexes revealed that this process is barrierless and much more exothermic and, consequently, might be rather fast. It also includes the alpha-H-atom abstraction step and results in the formation of benzaldehyde. Thus, the primary catalytic cycle in which the Cu-superoxo complex performs the process can be coupled with the second cycle which is governed by the Cu-oxo complex formed in the first cycle.