Substituent Effects on Hydrogen Abstraction by Phenyl Ketone Triplets

Article abstract of DOI:10.1021/ja00310a056

Triplet lifetimes in deaerated cyclopentane have been measured for a variety of ring-substituted benzophenones, acetophenones, and α-α-α-trifluoroacetophenones.The ketones undergo photoreduction under these conditions to mixtures of products formed from cyclopentyl and hemipinacol radicals.That triplet lifetimes are determined by rates of hydrogen abstraction from solvent is indicated by lifetimes being three times in cyclohexane-d12 than in cyclohexane-h12.For the benzophenones, reciprocal lifetimes correlate comparably well with Hammett ? or ?⁺ constants, with ρ values of 0.55 and 0.43, respectively.The effect of two substituentsis best fitted to the sum of both ? values.These weak inductive effects are in accord with expectations for a reactive n,?^* triplet.Rates for the acetophenones show the same substituent effects as previously observed for valerophenones; conjugating and electron-donating substituents stabilize the ?,?^* triplet and sharply reduce reactivity.Substituent effects are largest for the trifluoroacetophenones, consistent with their all having ?,?^* lowest triplets and reacting from weakly populated but highly reactive (k>10⁷ M^-1 s^-1) n,?^* states.Fluorine substitution, both on the ring and at the α-carbon, produces large rate enhancements, decafluorobenzophenone triplet being too short-lived to measure.