Photoenolization of ortho-substituted benzophenones by flash photolysis
The photoenolizations of 2,4-dimethylbenzophenone, 2-isopropylbenzophenone, and 2-benzylbenzophenone have been investigated by conventional and laser flash photolysis. The hydrogen-abstraction step occurs from the triplet state, the lifetime of which is 30 ns or less. The second transient, formed at the same rate as the triplet disappears, has not been assigned unambiguously but, while the decay rates vary with compound and solvent, they are not affected by oxygen. Each of the ketones produces two isomeric enols. The rate constants for decay (return to ketone), as well as reaction with oxygen and dimethyl acetylenedicarboxylate, have been measured and illustrate the high degree of reactivity of these molecules. Photolysis of the enols result in the formation of another transient which has been assigned a dihydroanthrone structure. Three transients are observed in the photolysis of 2,6-dimethylbenzophenone but these are not similar to those detected in the other ketones and no assignments are suggested.