Molecular modeling studies of the reactions of phenoxy radical dimers: Pathways to dibenzofurans

The B3LYP hybrid density functional computational technique was applied to describe the sequence of phenoxy radicals coupling reactions leading to the formation of dibenzofurans. Reaction kinetic parameters were estimated for key reactions. Aromatization of bis-keto dimers of phenoxy radicals followed by intermediate dehydration or dehydroxylation was demonstrated to be a strongly stereoselective process. While the S,S-diastereomer of the ortho-C//ortho-C keto dimer forms (o,o′)-dihydroxybiphenyl, a known dibenzofuran intermediate, via inter-ring hydrogen transfer reaction, the less stable R,S-stereoisomer can easily be transformed into another 5-hydroxyl-4,5-cyclohexadiene-2,3-benzofuran intermediate that provides an energetically more favorable pathway for formation of dibenzofuran. The possible channels of radical-chain processes that convert these intermediates to dibenzofuran and polychlorinated dibenzofurans are discussed.