The photochemical reactivity of some benzoylthiophenes. IV. The effect of an adjacent methyl group on the excited state reactivity of 3-benzoylthiophene

Abstract

The electronic absorption and phosphorescence emission spectra and the photochemical reactivity of several methyl-3-benzoylthiophenes (2- and 4-methyl-3-benzoylthiophene (1, 2), 2,5-dimethyl-3-benzoylthiophene (3), and 3-(2-methylbenzoyl)thiophene (4)) have been studied. Partial state diagrams have been constructed. The lowest energy absorption in hexane solution in every case is the carbonyl n → π* transition. The two lowest triplet states of these ketones are close in energy and, in fact, the nature of the emitting triplet (n,π* or π,π*) depends upon the position of methyl substitution and upon the solvent. The photochemical reactions studied include intramolecular hydrogen abstraction (revealed by deuterium exchange in the adjacent methyl group upon irradiation in perdeuteriomethanol solution), photocycloaddition of dimethyl acetylenedicarboxylate to the thiophene ring, and photocycloaddition of isobutylene to the carbonyl group. Generalizations, potentially useful for predicting photochemical reactivity of these and other aromatic ketones are summarized.