The Photochemical Reactivity of Some Benzoylthiophenes. III. The Effect of an Adjacent Methyl Group on the Excited State Reactivity of 2-Benzoylthiophene

Abstract

The ultraviolet absorption and phosphorescence emission spectra of 2-(2-methylbenzoyl) thiophene (1), 2-benzoyl-3-methylthiophene (2), 2-benzoyl-4-methylthiophene (3), and 2-benzoyl-5-methylthiophene (4) are reported and analyzed. Partial energy diagrams are constructed. The lowest triplet is assigned as a π,π* state of the 2-thienoyl chromophore in every case. Photocycloaddition of isobutylene to the carbonyl group was observed with 1 and 2. The oxetanes are thermally unstable; olefins 1-(2-thienyl)-1-(2-methylphenyl)-2-methylpropene (from 1) and 1-phenyl-1-(2-(3-methylthienyl))-2-methylpropene (from 2) were formed and characterized. Irradiation of 1 and 2 in O-deuteromethanol did not lead to incorporation of deuterium; no reaction was observed. These results are discussed in terms of the existing generalizations potentially useful for predicting photochemical reactivity.