THE PHOTOCHEMISTRY OF METHOXY-SUBSTITUTED BENZYL ACETATES AND BENZYL PIVALATES - HOMOLYTIC VS HETEROLYTIC CLEAVAGE

The multiple methoxy-substituted benzyl acetates (3g-i) and benzyl pivalates (4g-i) have been photolyzed in methanol solution. The products of these reactions are derived from two critical intermediates; the benzyl radical/acyloxy radical pair and the benzyl cation/carboxylate anion pair. As predicted by the meta effect, the yield of ion-derived product, the methyl ether in this case, was enhanced by the presence of the m-methoxy groups. The yield of ether, for the acetate esters, varied from 2% for the 4-methoxy-substituted ester to 66% for the 3,4,5-trimethoxy-substituted ester. In contrast, the yield of ether, for the pivalate esters, varied from <1% for the 4-methoxy-substituted ester to 20% for the 3,4,5-trimethoxy-substituted one. The meta effect does not explain these differences; electron transfer converting the radical pair to the ion pair is still an important pathway in the mechanism for ion formation. A quantitative analysis of the yield of the ethers was done in order to obtain the electron-transfer rate constants. This analysis revealed that the yield of the ethers was higher than expected based on previous results for other substituted benzyl acetates. A possible explanation for this discrepancy is that internal return of the radical pair to starting material for the acetate esters is more efficient than for the pivalate esters. Also, the esters 3k and 3l, were prepared to study the effect of electron-withdrawing groups in the meta position. For these esters, the benzylic cleavage reactions were inefficient and an isomerization reaction, the benzvalene rearrangement, was competitive.