A quantitative examination of the photoisomerization of some protonated phenols

The photoisomerization of a series of protonated, methyl substituted phenols to protonated bicyclo[3.1.0]hexenones has been examined. These reactions, which were carried out in CF 3 SO 3 H as a strong acid solvent at ambient temperatures, provide a convenient route to a variety of bicyclo[3.1.0]hexenones. The quantum yields for these photoisomerizations vary from 0.018 for protonated 3,5-dimethylphenol to 0.65 for protonated 2,6-dimethylphenol. This variation in efficiency can be understood in terms of a competition between ring opening, to regenerate the starting phenol, or cyclopropyl migration, to give product, of an initially formed intermediate.