Mechanism of photosolvolytic rearrangement of p-hydroxyphenacyl esters: Evidence for excited-state intramolecular proton transfer as the primary photochemical step

The photosolvolytic rearrangement of a variety of p-hydroxyphenacyl esters and related compounds 7-16 has been studied in solutions with up to 50% aqu eous content, using product studies, triplet quenchers, and nanosecond lase r flash photolysis. The p-hydroxyphenacyl moiety has recently been proposed as a new and efficient photoactivated protecting group in aqueous solution . Practical applications have been demonstrated, but much less is known abo ut the mechanism of photoreaction. Our data support a novel mechanism in wh ich the primary photochemical step from the singlet excited state is formal intramolecular proton transfer from the phenolic proton to the carbonyl ox ygen of the dietal ketone, mediated by solvent water, to generate the corre sponding p-quinone methide phototautomer. This reactive intermediate (most likely in its excited state) subsequently expels the carboxylic acid with c oncerted rearrangement to a spiroketone intermediate, which subsequently le ads to the final observed product, p-hydroxyphenylacetic acid. An alternati ve mechanism is deprotonation of the phenolic proton, loss of the carboxyla te, and rearrangement to the spiroketone, all in one concerted primary phot ochemical step from S-1.