THE PHOTOCHEMISTRY OF CONFORMATIONALLY RIGID BENZYLIC ESTERS - 2,2-DIMETHYL-1-INDANYL ACETATES AND PIVALATES

The photochemistry, in methanol, of substituted 2,2-dimethyl-1-indanyl acetates 9a-c and pivalates 10a-c has been studied. In agreement with previous studies on benzylic esters, the results show that the substi tuents change the yield of products derived from the ion pair. The mec hanistic conclusion reached is that the substituents change the oxidat ion potential of the indanyl radicals and thus the rate constant of el ectron transfer for converting the radical pair to the ion pair. The r esults also reveal two other substituent effects. First, substituents can increase the overall efficiency of the photoreaction by enhancing homolytic cleavage, The second effect is conformational. In compounds where the bond that is cleaving is conformationally mobile, such as th e C-O bond in benzylic esters, substituents on the ring can change the population of the reactive conformer and thus the overall efficiency of the reaction. For the indanyl acetate esters, the difference in exc ited-state reaction rate between the m- and p-methoxy substituted este r is 15:1. For the m- and p-methoxy substituted benzyl acetates, this difference in reaction rate is 48:1. The larger difference in reaction rate for the conformationally mobile benzylic eaters is attributed to a higher population of the unreactive conformer for the p-methoxy sub stituted ester.