Photochemistry of α-(o-tolyl)acetone and some derivatives: Triplet α-cleavage and singlet δ-hydrogen abstraction

Article abstract of DOI:10.1021/ja00061a008

Photolysis of α-(o-tolyl)acetone (TA) in 2-propanol was reported not to produce the indanol product expected from δ-hydrogen abstraction and cyclization of the resulting 1,5-biradical. A reinvestigation of this reaction reveals that the photolysis of solutions of TA does produce an indanol, albeit as a minor product. Similarly, photolysis of benzene solutions of o-tolylmethyl benzyl ketone (TBK) and o-tolylmethyl cyclohexyl ketone (TCK) results in the formation of indanols as minor products (ca. 5-10%). However, the photolysis of mesitylmethyl benzyl ketone (MBK) yields an indanol in significant yield (ca. 40%). In all cases, the diphenylethanes (DPEs) expected from free-radical recombination of benzylic radicals produced by α-cleavage are produced as dominant products. In order to determine the synthetic limitations of indanol formation from the photolysis of α-(o-tolyl)acetones, the mechanism of these photolyses was investigated. Sensitization with triplet acetone generated from the thermolysis of tetramethyl-1,2-dioxetane at 70 °C, quenching of the disappearance of ketone by isoprene, and isoprene quenching of the formation of indanol and fluorescence demonstrate that α-cleavage occurs dominantly from the triplet state and that δ-hydrogen abstraction occurs exclusively from the singlet state. Rates of quenching of acetone phosphorescence by dibenzyl ketone (DBK) and TBK in acetonitrile are found to be about 1 order of magnitude less than the rate of diffusion control. The yield of indanol can be enhanced by the introduction of methyl groups to the aryl ring, an increase in the reaction temperature, the addition of a triplet quencher to the reaction mixture, and, as previously reported, the use of microheterogeneous media. The quantum yield measurements for product formation show that the efficiency of α-cleavage drops by half from DBK to TBK and by a factor of 3 from TBK to MBK. We interpret this inefficiency to result from a radiationless deactivation of the singlet state which occurs when the o-tolyl group is attached to the α-position of a dialkyl ketone and this radiationless transition is induced by an incipient but incomplete δ-hydrogen abstraction, as previously proposed for γ-hydrogen abstraction.