OXIDATION OF METHYL AROMATIC-COMPOUNDS BY REDOX PHOTOSENSITIZATION

Redox photosensitization was used to oxidize substituted methyl aromatic substrates. The electron acceptor photosensitizers were 1,4-dicyanobenzene DCNB, 9,10-dicyanoanthracene DCA and chloranil CIA. With Me, MeO, Cl, Br substituents the main product formed was the corresponding aromatic aldehyde. It formed with a 50-80% yield and usually 5-10% aromatic acid was obtained as a by-product. p-Cresol was scarcely oxidized but dimerized. N,N-dimethyl o- and p-toluidines led to the mixture of aldehydes resulting from the oxidation of the methyl groups. Cyano- and nitro-alkylaromatics did not react with oxygen under our experimental conditions. The data concerning the donor-acceptor couple show that the oxidative electron transfer from the substrate to the excited photosensitizer is thermodynamically feasible. The experimental observations were consistent with the formation of an aromatic radical cation. Such a radical is then deprotonated by a Bronsted base such as O2{minus sign, dot below} or the reduced photosensitizer and a benzyl radical is formed. Then, this radical may be oxidized by O2 leading to the aromatic aldehyde. © 1990.