PHOTOINDUCED ELECTRON-TRANSFER CARBON-CARBON BOND-CLEAVAGE OF RADICAL CATIONS OF CARBONYL-COMPOUNDS IN SOLUTION - 2,4,6-TRIPHENYLPYRYLIUM SALT-SENSITIZED OXYGENATION OF ARALKYL KETONES AND ALDEHYDES

Photosensitized electron transfer oxygenation of several aralkyl ketones and aldehydes such as 1,2,2-triphenylethanone and diphenyl- and triphenylethanal has been carried out with 2,4,6-triphenylpyrylium tetrafluoroborate (TPP+BF4-) in dichloromethane. The carbonyl compounds underwent C-C bond cleavage through their cation radicals, generated via electron transfer to the excited singlet state of the pyrylium salt. For example, diphenylethanal afforded benzophenone in 80% yield. Direct evidence for the electron transfer and subsequent formation of a carbocation through C-C bond cleavage was obtained by laser flash photolysis of tetrakis(4-methylphenyl)ethanone in dichloromethane. It is proposed that arylmethyl cations generated by the C-C bond cleavage of the cation radicals undergo electron transfer with pyryl radicals to give arylmethyl radicals, which react with molecular oxygen to afford the final oxygenation products.