Electron-transfer photochemistry of iminium salts. Olefin photoadditions to 2-phenyl-1-pyrrolinium perchlorate

Article abstract of DOI:10.1021/ja00401a036

The photoaddition reactions of 2-phenyl-1-pyrrolinium perchlorate (2) and a variety of olefins have been investigated. Irradiation of methanolic solutions of 2 in the presence of the electron-rich olefins isobutylene, cyclohexene, butadiene, isopropenylcyclopropane, and methyl β,β-dimethylacrylate leads to formation of two types of addition products. One of these results from anti-Markovnikov addition of the components of methanol and the pyrrolinium salt across the olefinic π bond, generating a carbon-carbon bond to the pyrrolidine 2-position. The other corresponds to addition of the 2-phenylpyrrolidine unit across allylic C-H bonds of the olefins. Both reactions are explained by using electron-transfer mechanisms. Support for these mechanisms, which rationalize the nature of the reactions, their regiochemistry, and their stereochemistry, has come from studies with the electron-poor olefins methyl acrylate, acrylonitrile, and methyl methacrylate. Irradiation of 2 in the presence of these olefins leads to generation of 9-substituted 1-aza-6,7-benzospiro[4.4]non-6-ene products, results of [2 + 2] cycloadditions of the olefins to the C-1,C-2 π bond of the pyrrolinium salt phenyl ring. Fluorescence quenching studies have been conducted to gain further evidence in support of the mechanistic postulates.