Radical ions in photochemistry. 18. The photosensitized (electron transfer) tautomerization of alkenes; the 1,1-diphenyl alkene system

Abstract

The photosensitized (electron transfer) irradiation of several conjugated 1,1-diphenyl alkenes, in acetonitrile with 1,4-dicyanobenzene or 1-cyanonapthalene as electron accepting sensitizer and 2,6-lutidine as base, leads essentially quantitatively to tautomerization to the less stable unconjugated isomer(s). The proposed mechanism for this reaction involves formation of the alkene radical cation and sensitizer radical anion followed by deprotonation of the radical cation, reduction of the resulting radical to the ambident anion by back electron transfer from the radical anion, and reprotonation. There are several steps in this mechanism that could control the ratio of isomers. Evidence is provided that, at least in some cases, it is the relative rate of deprotonation from the isomeric radical cations that is the determining factor. This rate is influenced by the conformation of the radical cation; the carbon–hydrogen bond involved in the deprotonation step must overlap with the singly occupied molecular orbital.