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.

Synthesis of 2-substituted tetraphenylenes via transition-metal-catalyzed derivatization of tetraphenylene

“…The acetoxy group is an important functional group because it can be transformed into a variety of other functionalities [ 46 – 47 ], thus making the acetoxylation a highly interesting reaction. The Sanford and Wang group, respectively, developed a highly efficient palladium and gold-catalyzed direct acetoxylation of arenes with iodobenzene diacetate [ 48 – 49 ].…”

Supramolecular photochirogenesis

“…37 Photolysis of chiral aryl ester 16 (Scheme 7) affords decarboxylation product 17 through concerted elimination of carbon dioxide via a spiro-lactonic transition state. [38][39][40][41][42] In accordance with this concerted mechanism, the photodecarboxylation of enantiopure 16 gives 17 in 499% ee in the solution phase. However, the photoirradiation of racemic 16 in the presence of b-CD yields (R)-17 in 14.1% ee, suggesting preferential complexation and/or faster photodecomposition of one of the enantiomers of 16 when mediated by b-CD.…”

Gold-catalyzed ethylene cyclopropanation

“…Ethyl cyclopropanecarboxylate has been prepared in several ways, alternative to the direct carbene addition to ethylene (Scheme 2): ring contraction of 2-halocyclobutanone [5], cyclization of alkyl 4-halobutanoates [6], electroreductive dehalogenation [7] and decarboxylation of diethyl 1,1-cyclopropyldicarboxylate [8]. Other methods include the transesterification of other alkyl cyclopropanecarboxylates [9] and the esterification with ethanol of the cyclopropanecarboxylic acid [10]. This product finds applications as lubricant additives [11], alkylating reagent in the Friedel–Crafts synthesis of indanones [12] or as synthon toward the introduction of cyclopropyl moieties in compounds with medicinal or biological interest [13–14].…”