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 O(2) reduction (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.

Aerobic Copper-Catalyzed Organic Reactions

“…Studies on the galactose oxidase model systems have shown a large kinetic isotope effect for the abstraction of the hydrogen atom. ,, Oxidation to the alcohol is concurrent with reduction of the copper center and is followed by the oxidation of the copper and ligand by molecular oxygen to regenerate the active catalyst. This proposed mechanism is supported by theoretical calculations …”

C—H Bond Activation in Transition Metal Species from a Computational Perspective

“…The oxidation of primary alcohols with O 2 catalyzed by these two complexes was studied by Wu at the DFT(B3LYP) level, including solvent effects. 560 The quinone acts as a tetradentate ONNO chelating ligand and its oxygen atoms have radical phenoxyl character. In the first step of the mechanism, one of these oxygen atoms abstracts a proton from the alcohol substrate by breaking the OsH bond.…”

“…The complexes [M(L)] (M = Zn(II), Cu(II); L = N , N ′-bis(3,5-di- tert -butyl-2-hydroxyphenyl)-1,2-diiminoquinone) are functional models of galactose oxidase. The oxidation of primary alcohols with O 2 catalyzed by these two complexes was studied by Wu at the DFT(B3LYP) level, including solvent effects . The quinone acts as a tetradentate ONNO chelating ligand and its oxygen atoms have radical phenoxyl character.…”

Aerobic oxidation of vanillyl alcohol to vanillin catalyzed by air-stable and recyclable copper complex and TEMPO under base-free conditions

“…A very similar role of the oxy-arms in similar Cu (II)-ONNO salen complexes was reported before. [85][86][87] In this regard, our intention was to design a tridentate Schiff base ligand with a labile pendent thioether moiety. Such a labile arm can easily leave the metal and thus creates a vacant coordination site for substrate binding during catalysis.…”