Investigation of the Photo-Fries Rearrangements of Two 2-Naphthyl Alkanoates by Experiment and Theory. Comparison with the Acid-Catalyzed Reactions

A detailed investigation of the photochemistry of 2-naphthyl acetate ( 1a ) and 2-naphthyl myristate ( 1b ) has been conducted under a variety of conditions. Factors related to the reactions such as temperature and solvent type have been explored. The results, most easily interpreted by photo-Fries type processes, are contrasted with those from the Lewis-acid catalyzed (dark) Fries reactions of 1a . They are also compared to the predictions of semiempirical and ab initio calculations using 2-naphthyl propanoate ( 1c ) and species derived from it as models. Unsuccessful triplet sensitization experiments with benzophenone and calculations point to the excited singlet states of 1 as the immediate precursor to the acyl/2-naphthoxy radical pairs that recombine to form keto intermediates 2, reform 1, or diffuse apart and eventually yield 2-naphthol ( 4 ); enolization of 2 results in the isolated rearrangement products, n -acyl-2-naphthols ( n - 3 ). Static and dynamic fluorescence studies provide some insights into the nature of the lysis process. Irradiation of a mixture of appropriately labeled derivatives of 1 led to none of the expected “cross-over” products, indicating that the intermediates 2 arise from recombination of radical pairs from the same parent molecule. Irradiation of 1b in ethanol and 1-octanol provides no evidence for the intermediacy of dodecylketene and supports out-of-cage mechanisms as the exclusive source of 4 . There are indications of subtle solvent effects and a conformational dependence on the distribution of photoproducts.