PHOTOCHEMISTRY OF SUBSTITUTED BENZYL ACETATES AND BENZYL PIVALATES - A REINVESTIGATION OF SUBSTITUENT EFFECTS

The photosolvolysis reactions, in methanol, of six substituted benzyl acetates (7a-f) and benzyl pivalates (8a-f) were studied. Five major b enzylic products were formed from two critical intermediates. The ethe rs (9) were formed from the ion pair, 15, and all of the other product s (10-14) were formed from the radical pair, 16. Quenching studies sho wed that only excited singlet state reactivity was important. The prod uct yields were found to be highly substituent dependent. For instance , for the acetate esters, the yield of ether (9) varied from 2% for X = 4-OCH3 to 32% for X = 3-OCH3. Most of the differences in the yields could be attributed to ground state processes that occur after bond cl eavage. The important competition is between electron transfer, conver ting the radical pair to the ion pair, and decarboxylation of RCO(2)(. ). The rates of electron transfer are shown to fit Marcus theory in bo th the normal and inverted regions. Direct heterolytic cleavage to for m the ion pair is of minimal importance.