Ab Initio Calculations on 2,6-Dimethylphenol and 4-(2,6-Dimethylphenoxy)-2,6-dimethylphenol. Evidence of an Important Role for the Phenoxonium Cation in the Copper-Catalyzed Oxidative Phenol Coupling Reaction

Ab initio unrestricted Hartree−Fock calculations with a 6-31G* basis set were performed on 2,6-dimethylphenol (DMP or monomer) and 4-(2,6-dimethylphenoxy)-2,6-dimethylphenol (dimer) to gain more insight into the mechanism of the copper-catalyzed oxidative phenol coupling reaction. Atomic charges were determined for the phenol, phenoxyl radical, phenolate anion and phenoxonium cation (both the singlet and triplet state) of both species. Calculations for the monomer show that the only aromatic carbon atom bearing a partial positive atomic charge is the para carbon of the cation in the singlet state. In the case of the dimer, the presence of a p-phenoxy substituent results in a partial positive charge for the para carbon of the first aromatic ring in all states, but this charge is significantly higher for the singlet cation. In the singlet cationic state, the para carbon of the first aromatic ring is therefore the site most susceptible to nucleophilic aromatic substitution. This result strongly supports a proposal for the mechanism of the copper-catalyzed oxidative phenol coupling, in which dinuclear phenolate-bridged copper(II) species act as intermediates affording phenoxonium cations after a double one-electron transfer. Furthermore, the charges in the second aromatic ring of the dimer are hardly, -if at all, influenced by the overall electronic state of the molecule. The charge in the first ring cannot pass the ether bond. This result, combined with the observation that the para carbon of the first aromatic ring of the singlet cation bears a large positive charge, favors a mechanism where quinone ketals are formed as intermediates during the coupling of oligomeric phenols.