| Abstract
| - 6-Alkylidenecyclohexa-2,4-dienones (o-quinone methides II) have been generated by photolysis of2-(2‘-cycloalkenyl)phenols 1 and trapped by methanol to give the ring-opened products 2. The bestresults have been obtained with the cyclohexenyl derivatives 1a, 1e, and 1f. In the case of thecyclopentenyl derivative 1b, photoproduct 2b was not observed, whereas only small amounts of 2cand 2d were formed from the seven- and eight-membered ring analogues 1c and 1d. Thus, ringsize appears to be a key factor in the formation of o-quinone methides. This experimental resulthas been rationalized by means of density-functional theory (DFT) calculations. On the other hand,phenol substitution also appears to play a role in the process. Thus, electron-withdrawing groupssuch as CF3 (1f) accelerate the reaction, whereas the opposite is true for electron-donating groupssuch as OCH3 (1e). This is explained by an excited-state intramolecular proton transfer (ESIPT)mechanism, as the above results are consistent with the excited-state acidities of the differentphenols. The lack of reactivity in the case of ketone 1g, where the intersystem crossing quantumyield is close to unity, allows us to rule out a mechanism involving the triplet state.
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