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

The multiple methoxy-substituted benzyl acetates (3g-i) and benzyl piv alates (4g-i) have been photolyzed in methanol solution. The products of these reactions are derived from two critical intermediates; the be nzyl radical/acyloxy radical pair and the benzyl cation/carboxylate an ion pair. As predicted by the meta effect, the yield of ion-derived pr oduct, the methyl ether in this case, was enhanced by the presence of the m-methoxy groups. The yield of ether, for the acetate esters, vari ed from 2% for the 4-methoxy-substituted ester to 66% for the 3,4,5-tr imethoxy-substituted ester. In contrast, the yield of ether, for the p ivalate esters, varied from <1% for the 4-methoxy-substituted ester to 20% for the 3,4,5-trimethoxy-substituted one. The meta effect does no t explain these differences; electron transfer converting the radical pair to the ion pair is still an important pathway in the mechanism fo r ion formation. A quantitative analysis of the yield of the ethers wa s done in order to obtain the electron-transfer rate constants. This a nalysis revealed that the yield of the ethers was higher than expected based on previous results for other substituted benzyl acetates. A po ssible explanation for this discrepancy is that internal return of the radical pair to starting material for the acetate esters is more effi cient than for the pivalate esters. Also, the esters 3k and 3l, were p repared to study the effect of electron-withdrawing groups in the meta position. For these esters, the benzylic cleavage reactions were inef ficient and an isomerization reaction, the benzvalene rearrangement, w as competitive.