Photochemical reactions of aromatic compounds. Part XVIII. Photoaddition of benzophenone to aromatic amines

Abstract

Photoreactions of benzophenone with diphenylamine or 2,6-dimethylaniline in t-butyl alcohol or acetonitrile solution result in the formation of triarylmethanol (I). Irradiation of dilute benzene solutions gave mainly brown intractable materials, and, in the case of diphenylamine, some benzopinacol (1,1,2,2-tetraphenylethane-1,2-diol). With aniline, NN-dimethylaniline, the three isomeric toluidines, N-methyldiphenylamine, triphenylamine, and 2,3- and 2,5-dimethylanilines, the formation of a triarylmethanol (I) was not observed. From the solvent-dependence of the reaction, the structural limitation of aromatic amines for the formation of (I), and kinetic results, the following mechanism is suggested: in polar solvents, an exciplex formed between triplet benzophenone and diphenylamine or 2,6-dimethylaniline effectively becomes an ion-pair, and this is followed by proton transfer in an ion-pair giving a radical-pair in a solvent-cage (IV) which couples to afford (I).