Photoisomerization of Some N-Aryl α,β-Unsaturated Iminium Salts

Article abstract of DOI:10.1021/jo00249a026

A series of N- and C-diaryl-substituted α,β-unsaturated iminium salts have been prepared in which a systematic change in the substituents on the aromatic rings was made.The photochemically induced isomerizations of these salts were examined in trifluoroacetic acid, a medium in which thermally induced isomerizations of these cations were generally slow.Photoisomerization about both the C=C and C=N partial double bonds of these iminium salts was observed, leading eventually to the formation of a photostationary state consisting of four isomers.Apart from cations containing a nitro substituent, the ratios of the quantum efficiencies for C=C vs C=N isomerization remained essentially constant as substituents on the two aryl rings were changed, suggesting that the prependicular excited singlet states involved in these reactions have "biradical" character.The reduction potentials of the iminium salts were measured.Isomerization about the C=N bond of one of the salts ahown to occur by a photoinitiated electron transfer from tris (2,2'-bipyridine)ruthenium(II) dichloride.