Acid-controlled photoreactions of aryl alkanoates: competition of transesterification, decarboxylation, Fries-rearrangement and/or transposition.

Abstract

2,4,6-Trimethylphenyl (mesityl) cyclohexanecarboxylate 1c and related mesityl esters 1a,b and d-f were photodecarboxylated upon irradiation at 254 nm in neutral solvents to give alkylmesitylenes 15 in good yields. In contrast, in the presence of a catalytic amount of acid and a sufficient amount of alcohol, the same compounds underwent facile phototransesterification to afford the corresponding ester 9 and phenol 10 in almost quantitative yields. Photolyses of less-substituted aryl alkanoates such as 2,6-dimethylphenyl (xylenyl), phenyl, 4-methoxyphenyl (4-anisyl) and 4-methoxynaphthyl alkanoates 2-4 and 8 were also investigated to elucidate the reaction mechanism of acid-catalyzed phototransesterification. Irradiation of these esters afforded the corresponding photo-Fries rearrangement products 20-23 both in neutral and acidic acetonitrile with significant acceleration of the processes in the presence of acid. It was elucidated that the photolysis of the esters affords the geminate radical pair 28, which in turn recombines to cyclohexadienone intermediates 29 and 30. Added acid accelerates not only the nucleophilic attack of alcohol to 29 and 30 but also other processes. Prolonged irradiation of the esters in neutral solution led to skeletal rearrangements of the initial product, affording isomeric alkylbenzenes. The phototransposition of cyclohexylmesitylene 15c via benzvalene intermediates was, on the contrary, retarded under acidic conditions. These acid-controlled competitive photoreactions are representative examples, in which a catalytic amount of acid can alter the fate of reactive intermediates on both ground-state and excited-state surfaces.