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-trimethylsilylmethyl) amidoalkyl]phthalimides (alkyl = ethyl, n-propyl, n-pentyl and n-hexyl) and N-[(N-mesyl-N-trimethylsilylmethyl)amidoethyl]phthalimide. Photocyclizations occur on irradation of these substances in methanol in modest 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 photocyclization reactions involving intramolecular SET from nitrogen in the alpha-silylamido group to the singlet excited state of the phthalimide followed by desilylation of the intermediate alpha-silylamido cation radical and cyclization by radical coupling is proposed. Furthermore, photoreactivity of N-[(N-methyl-N-trimethylsilylmethyl)aminoethyl] phthalimide differs from the other members of this series. Here a route involving tripler hydrogen atom abstraction predominates over that involving sequential singlet SET-desilylation. A relationship between the quantum efficiencies for reactions of the phthalimides with various alpha-silyl-n-electron donors and the oxidation potentials of the electron donors has been noted. The results suggest that the rate of desilylation of the cation radical intermediate is an important factor in determining the quantum efficiency of the SET-induced photoreaction and that the desilylation rates are directly proportional to the oxidation potentials of the donors, The efficient and regioselective cyclization reactions observed for photolyses in methanol represent synthetically useful processes for construction of medium and large ring heterocyclic compounds. (C) 1997 Elsevier Science S.A.