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.
Dedicated to Professor Andre M. Braun on the occasion of his 60th birthday a-Methylstyrene (1) was photo-oxidized in the presence of a series of alkylated dimethoxybenzenes as sensitizers in an oxygen-saturated MeCN solution to afford the cleaved ketone 2, epoxide 3, as well as a small amount of the ene product 4 in ca. 1 : 1 : 0.04 ratio. The relative rate of conversion was well-correlated with the fluorescence quantum yield of sensitizers. Thus, a non-singlet-oxygen mechanism is proposed, in which an excited sensitizer is quenched by (ground-state) molecular oxygen to produce a sensitizer radical cation and a superoxide ion (O À.2 ), the former of which oxidizes the substrate, while the latter reacts with the resulting olefin radical cation (1 . ) to give the major oxidation products. Photodurability of such electron-donating sensitizers is dramatically improved by substituting four aromatic H-atoms in 1,4-dimethoxybenzene with Me or fused alkyl groups, which provides us with an environmentally friendly, clean method of photochemical functionalization with molecular oxygen, alternative to the ene reaction via singlet oxygenation.
Labile cyclohexadienones were isolated for the first time in good yields in the photo-Fries rearrangement of partially blocked naphthyl esters. Upon direct excitation at 313 nm, 1,4-dimethylnaphth-2-yl and 2,4-dimethylnaphth-1-yl 2,4,6-trimethylbenzoates afforded 1-acyl-2-naphthalenone and 2- and 4-acyl-1-naphthalenones, respectively. This isolation is of particular importance as a direct mechanistic proof and also as a convenient route to these thermally less-accessible compounds.
Successful isolation of “ortho” and “para”-acylcyclohexadienones allowed us to comparatively study their ground- and excited-state behavior under a variety of conditions. In neutral solutions, the two isomeric cyclohexdienones showed completely different reactivities for photochemical and thermal reactions, while in acidic methanol both quantitatively afforded the corresponding transesterification product and naphthol. These studies help us understand the detailed photo-Fries rearrangement mechanism, which involves several crucial photochemical and thermal steps.
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