Photochemistry of α-aryl carboxylic anhydrides. Part 4. Photoreactions of some asymmetrical anhydrides derived from o- and p-methoxyphenylacetic acid, and of some of the phenylacetate ester photoproducts

Abstract

Irradiation of the two asymmetrical carboxylic anhydrides R¹C₆H₄CH₂·CO·O·CO·CH₂C₆H₅(1a and b) at 254 nm in acetonitrile leads to the formation of esters and bibenzyls. Among the esters produced, the asymmetrical ones with structure R¹C₆H₄CH₂·O(CO)CH₂C₆H₅ are the most abundant. These are for the greater part formed intra-molecularly by decarbonylation of one excited anhydride molecule. This result is in line with a mechanism involving electron transfer after excitation of the anhydride molecule. The bibenzyls are formed by the recombination of benzyl radicals. In the initial stage of the reaction of the anhydrides, the three bibenzyls are formed with the asymmetrical one predominating [(1a) gives the three bibenzyls (3a, c, and e) in yields of 20, 35, and 10%, respectively]. In the later stages, when substantial amounts of esters are present, the formation of the bibenzyls also originates in part from the photodecomposition of the esters. The photochemistry of the various (symmetrical and asymmetrical) esters which are photoproducts from the anhydrides (1a and b) has also been studied. The symmetrical esters (2a and b) afforded quantitatively the bibenzyls (3a and b), respectively. The asymmetrical esters gave variable yields of the three (possible) bibenzyls, depending on the type of ester considered. For the asymmetrical esters for which bibenzyl formation could be quenched by (Z)-piperylene, the three bibenzyls were formed in a close to statistical ratio, e.g.(2c) yielded the bibenzyls (3a, c, and e) in a ratio of 1 : 2.5 : 1. With the esters for which the photodecarboxylation was not quenchable by (Z)-piperylene, a relatively higher yield of the asymmetrical bibenzyl was found, e.g.(2d) yielded the bibenzyls (3b, d, and e) in a ratio of 1 : 6 : 1. These results are rationalized in terms of a recombination of free radicals resulting from the triplet excited esters after spin inversion in the former case, and a recombination of a radical pair in the solvent cage resulting from the singlet excited or short-lived triplet excited ester in the latter case.