Binuclear O2 activation and hydrogen transfer mechanism for aerobic oxidation of alcohols

A density functional theory study of the aerobic oxidation of 1-phenylethanol into acetophenone catalysed by phenanthroline copper (phenCu) complexes reveals a binuclear O-2 activation and hydrogen transfer mechanism with multiple spin-crossover steps. When di-tert-butyl azodicarboxylate (DBAD) exists, it acts as a stoichiometric oxidant and forms DBADH(2) through successive transfers of the proton and hydride from 1-phenylethanol to DBAD in one transition state with a free energy barrier of 21.8 kcal mol(-1). After the consumption of DBAD, DBADH(2) acts as a co-catalyst assisting O-2 activation and acetophenone formation through binuclear transition states for the cleavages of O-O and C-H bonds with a total free energy barrier of 24.6 kcal mol(-1). Without the presence of DBAD or DBADH(2), the total free energy barrier for the aerobic oxidation of 1-phenylethanol with the participation of two phenCu complexes is 26.1 kcal mol(-1). In all the above three situations, the rate-determining step is the activation of the C-H bond in 1-phenylethanol. The formation of HOO radical and the breaking of the O-O bond in hydrogen peroxide for the formation of a Cu(ii)-hydroxyl dimer are also key steps in the reaction and need the participation of two phenCu complexes.