Temperature dependent product distribution in photolysis of o-alkylphenacyl benzoates

Temperature effect on photochemical reactions of ortho-alkylphenacyl benzoates, one of the recently developed photoremovable protecting groups (PPG), giving indanones (IN) and benzocyclobutenols (CB) was examined. As temperature was lowered, CB was formed preferentially over IN and the amount of IN increased as temperature was elevated. Arrhenius plot of ln(IN/CB) versus 1/T gave a straight line from which EaIN–EaCB and AIN/ACB were obtained. The least sterically hindered 2-methylphenacyl benzoate (1) gave the highest EaIN–EaCB and AIN/ACB among the ortho-alkylphenacyl benzoates tested in this research including 2,4,6-trimethylphenacyl benzoate (2) and 2,4,6-triisopropylphenacyl benzoate (3).