Benzyl alcohol oxidation mechanisms by one- and two-electron oxidized species of Cu(II)-salen complexes with para-R-substituents, [Cu(R-salen)]n+ (R = MeO, MeS; n = 1, 2)

Abstract

Benzyl alcohol oxidation by one- and two-electron oxidized copper(II)-salen complexes with the para-substituted phenolate moieties, [Cu(R-salen)]n+ (H2R-salen = N,N’-bis(3-tert-butyl-5-R-salicylidene)ethylenediamine; R = methoxy (MeO), methylthio (MeS) groups; n = 1, 2) was kinetically investigated under the conditions with large excess of the substrate in comparison to the copper complex concentration (CBnOH ≫ CCu). The one- and two-electron oxidized complexes [Cu(R-salen)]n+ (n = 1 and 2) oxidized benzyl alcohol irrespective of R-groups to generate benzaldehyde and the Cu(R-salen) species in stoichiometries of complex : substrate = 2 : 1 and 1 : 1, respectively. Kinetic analyses for benzyl alcohol oxidation by the one-electron oxidized complexes [Cu(R-salen)]+ revealed significantly different kinetics depending on R-groups: one-step pseudo second-order kinetics was assigned for R = MeO, while two-step pseudo first-order kinetics was suitable for R = MeS. The reaction kinetics by two-electron oxidized complexes [Cu(R-salen)]2+ were assigned to two- and three-step first-order processes for R = MeO and MeS, respectively, in which the first step consists of parallel pathways of complex : substrate = 1:1 and 1:2 reactions to generate the one-electron oxidized complex intermediates irrespective of the R-group. However, the following steps were different depending on R-groups. Associating with the results of kinetic isotope effect (KIE) measurements, the reaction mechanisms were discussed and proposed.