EXPLORATORY-STUDY OF PHOTOCYCLIZATION REACTIONS OF N-(TRIMETHYLSILYLMETHYLTHIOALKYL)PHTHALIMIDES

Studies have been conducted to explore single electron transfer (SET) induced photocyclization reactions of N-(trimethylsilylmethylthioalkyl )phthalimides (alkyl=ethyl, n-propyl, n-butyl, n-pentyl, and n-hexyl). Photocyclizations occur in methanol in modest to high yields to produ ce cyclized products in which phthalimide carbonyl carbon is bonded to the carbon of side chain in place of the trimethylsilyl group. Mechan ism for these photocyclizations involving intramolecular SET from sulf ur in the alpha-silylmethylthioalkyl groups to the singlet excited sta te phthalimide moieties followed by desilylation of the intermediate a lpha-silylmethylthio cation radicals and cyclization by radical coupli ng is proposed. In contrast, photoreactions of N-(trimethylsilylmethyl thioalkyl)phthalimides in acetone follow different reaction routes to produce another cyclized products in which carbon-carbon bond formatio n takes place between the phthalimide carbonyl carbon and the carbon a to silicon and sulfur atoms via triplet carbonyl hydrogen abstraction pathway. The normal singlet SET pathway dominates this triplet proces s for photoreactions of these substances in methanol while the triplet process dominates the singlet SET pathway for those in acetone. The e fficient and regioselective cyclization reactions observed for photoly ses in methanol represent synthetically useful processes for construct ion of medium and large ring heterocyclic compounds.