Theoretical studies on the reaction mechanism of oxidation of primary alcohols by Zn/Cu(ii)-phenoxyl radical catalyst

Abstract

The catalytic mechanism for the oxidation of primary alcohols catalyzed by the two functional models of galactose oxidase (GOase), M^II L (M = Cu, Zn; L = N,N′-bis(3,5-di-tert-butyl-2-hydroxyphenyl)-1-2-diiminoquinone)), has been studied by use of the density functional method B3LYP. The catalytic cycle of Cu- and Zn-catalysts consists of two parts, namely, substrate oxidation (primary alcohol oxidation) and O₂reduction (catalyst regeneration). The catalytic mechanisms have been studied for the two reaction pathways (route 1 and route 2). The calculations indicate that the hydrogen atom transfer within the substrate oxidation part is the rate-determining step for both catalysts, in agreement with the experimental observation. The calculated overall reaction barrier for Cu-catalyst is 16.74 kcal mol⁻¹, smaller than 22.96 kcal mol⁻¹ for the Zn-catalyst. This is consistent with the experimental result that the Zn-catalyst is less efficient when compared with the Cu-catalyst. The importance of the solvent effect is demonstrated. Insights into the hydrogen atom transfer process are discussed.