Mechanism of photosolvolytic rearrangement of p-hydroxyphenacyl esters : evidence for excited state intramolecular proton transfer as the primary photochemical step
Date
1998
Authors
Zhang, Kai
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Abstract
The photosolvolytic rearrangement of p-hydroxyphenacyl alcohols 56, 57, p- hydroxyphenacyl esters 58a-d, 59a-d, and p-methoxyphenacyl derivative 60 has been studied in aqueous solution using product studies and nanosecond laser flash photolysis. The p-hydroxphenacyl moiety has recently been proposed as a new and efficient photoactivated protecting group in aqueous solution. However, although their practical applications have been amply demonstrated, much less is known about the mechanism of photoreaction. Our data support a novel mechanism in which the primary photochemical step from the singlet excited state is intramolecular proton transfer from the phenolic proton to the carbonyl oxygen of the distal ketone, to generate the corresponding p-quinone methide phototautomer, which subsequently expels the ester group with concerted rearrangement to a spiroketone intermediate leading to the final observed product, p-hydroxyphenylacetic acid.
Irradiation of these p-hydroxyphenacyl derivatives in 1: 1 (v/v) H20-CH3CN produced the corresponding p-hydroxyphenyl acetic acid (31) or its di-tert-butyl derivative 61 as the only photoproduct. Conversion to 31 or 61 can be taken to quantitative yield upon prolonged photolysis for all of these compounds without the formation of significant by-products. These results rule out the involvement of C-OCOR bond homolysis in the mechanism. All of these p-hydroxyphenacyl derivatives were unreactive in neat CH3CN indicating that H20 is necessary for the reaction. p-Methoxyphenacyl acetate (60) failed to give any observable reaction under the same conditions, which shows that the phenol HO group is required. Quantum yield measurements showed that neither acid nor base catalyzed the reaction. All of the above results are indicative of the involvement of excited state intramolecular proton transfer (ESiraPT) in the mechanism. Such a mechanism would be expected to generate p quinone methide (p-QM) intermediates. Direct evidence was provided by laser flash photolysis (LFP), which gave observable transients at 330 and 360 nm assignable to p QM 67a and 67b, respectively.