Tandem energy transfer electron transfer in the photosensitized alkylation of alpha,beta-unsaturated ketones

alpha,beta-Unsaturated ketones are not conveniently alkylated by radicals generated from tetraalkylstannanes via photoinduced electron transfer (PET), either by direct irradiation or when a singlet sensitizer (an aromatic nitrile, a pyrilium salt) is used, However, the procedure is successful with tetramethyl pyromellitate (TMPM) as the sensitizer, TMPM is promoted to the triplet state by energy transfer from the unsaturated ketones and then sensitizes the cleavage of alkylstannanes by electron transfer, The alkyl radicals thus formed finally add to the unsaturated ketones, giving the beta-alkyl derivatives. (C) 1997 John Wiley & Sons, Ltd.