SINGLE-ELECTRON TRANSFER-INDUCED PHOTOCYCLIZATION REACTIONS OF N-[(N-TRIMETHYLSILYLMETHYL)AMINOALKYL] PHTHALIMIDES

Studies have been conducted to explore single electron transfer (SET) induced photocyclization reactions of N-[(N-acetyl-N-trimethylsilylmet hyl) amidoalkyl]phthalimides (alkyl = ethyl, n-propyl, n-pentyl and n- hexyl) and syl-N-trimethylsilylmethyl)amidoethyl]phthalimide. Photocyc lizations occur on irradation of these substances in methanol in modes t to high yields to produce cyclized products in which the phthalimide carbonyl carbon has become bonded to the carbon of the side chain in place of the trimethylsilyl group, A mechanism for these photocyclizat ion reactions involving intramolecular SET from nitrogen in the alpha- silylamido group to the singlet excited state of the phthalimide follo wed by desilylation of the intermediate alpha-silylamido cation radica l and cyclization by radical coupling is proposed. Furthermore, photor eactivity of N-[(N-methyl-N-trimethylsilylmethyl)aminoethyl] phthalimi de differs from the other members of this series. Here a route involvi ng tripler hydrogen atom abstraction predominates over that involving sequential singlet SET-desilylation. A relationship between the quantu m efficiencies for reactions of the phthalimides with various alpha-si lyl-n-electron donors and the oxidation potentials of the electron don ors has been noted. The results suggest that the rate of desilylation of the cation radical intermediate is an important factor in determini ng the quantum efficiency of the SET-induced photoreaction and that th e desilylation rates are directly proportional to the oxidation potent ials of the donors, The efficient and regioselective cyclization react ions observed for photolyses in methanol represent synthetically usefu l processes for construction of medium and large ring heterocyclic com pounds. (C) 1997 Elsevier Science S.A.