Phenacyl derivatives baring leaving group (L) at its α-position to carbonyl exhibit C-L cleavage upon photoirradiation, followed by neighboring phenyl group migration to result corresponding phenylacetic acid derivatives. The reactions are known as Photo-Favorskii rearrangement, and the reactions of phenacyl compounds including chlorides and phosphates have been studied as synthetic method or photoremoval protecting group. The rearrangements are considered to proceed through some ionic intermediate, since the reactions are favored in polar solvents such as alcoholic or aqueous media, although the existence of reduction products indicates the possibility of the involvement of phenacyl radical intermediate in the mechanism. Thus, the reaction mechanisms are to be subjected to further investigation. To test whether the Photo-Favorskii rearrangement undergoes through an ionic or a radical intermediate, the photosolvolysis of phenacyl mesylate (1) were investigated in 2,2,2-trifluoroethanol (TFE) or in acetonitile (MeCN).
The photosolvolysis of 1 in TFE afforded phenacyl trifluoroethyl ether (2) and phenylacetic acid trifluoroethyl ester (3) in 16.2 and 83.8 % yields, respectively. The former product is derived by the nucleophilic trapping of phenacyl carbocation with solvent, suggesting the photochemical generation of phenacyl carbocation in TFE. The photosolvolysis of 1 in MeCN afforded slight amount (1.4 %) of phenacyl alcohol (4) which is formed by the nucleophilic trap of phenacyl carbocation with a trace of water in MeCN. The major products were phenylacetic acid (5, 58.8 %), and its methanesulfonyl mixed anhydride (6, 33.5 %), which were obtained by the Photo-Favorskii rearrangement, followed by the addition of water or methanesulfonate leaving group.
When 0.01M of N₃^- was added to the photosolvolysis in TFE, the yield of 2 was reduced to 4.8 %, while that of 3 was little affected (84.0 %). The addition of 0.01M (t-Bu)₂NO as radical trapping reagent, on the other hand, exhibited the considerable decrease in the yield of 3 to 60.5 % and slight decrease in the yield of 2 to 13.2%. The result indicates that the Photo-Favorskii rearrangement proceeds through phenacyl radical intermediate. The effect of added trapping reagent was more remarkable in the photosolvolysis in MeCN, where the addition of (t-Bu)₂NO completely prohibited the formation of nucleophilic substitution product 4 as well as the Photo-Favorskii rearrangement products 5 and 6. This remarkable effect of (t-Bu)₂NO addition indicate that the phenacyl carbocation is formed by the electron transfer within the radical pair generated by the C-L bond homolysis in the photolysis.

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