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ortho and para-Quinone methides (2-methylene-3,5-cyclohexadien-I-one and 4-methylene-2,5-cyclohexadien-1-one, respectively) are intermediates in a variety of important chemical systems. In particular, o-quinone methides are useful in synthesis for the construction of chroman ring systems. A brief account of the relevance of quinone methide chemistry will be provided. This is followed by a review of recent studies from our laboratory on efficient methods for the photogeneration of quinone methides, concentrating on the use of hydroxy-substituted benzyl alcohols in aqueous media. It is shown that this method is general since it provides access too-, 171-, andp-quinone methide isomers. When appropriately substituted, all of these quinone methide isomers have been spectroscopically characterized by laser flash photolysis, making this technique the one of choice for studying the dynamics of these reactive intermediates. The mechanism of photochemical generation from hydroxybenzyl alcohols and extensions of the reaction to photogeneration of fluorenyl and biphenyl quinone methides will also be presented.Key words: quinone methide, biphenyl quinone methide, carbocation, photosolvolysis, photodehydroxylation, hetero-DielsAlder reaction.R6surnC : Les mCthides d'o-et p-quinones (respectivement 2-mCthylknecyclohexa-3,5-dikn-I-one et 4-mCthylknecyclohexa-2,5-dikn-1-one) sont des intermediaires dans une variCtC de systkmes chimiques importants. En particulier, les mCthides d'oquinones sont utiles en synthkse pour la construction de systkmes incorporant le noyau chromane. On fournit un bref rCsumC de l'importance de la chimie des methides de quinone. On prCsente ensuite une revue des Ctudes rCcentes rCalisCes dans notre laboratoire sur des mCthodes efficaces de photogCnCration des mCthides de quinone, en se concentrant sur l'utilisation, en milieux aqueux, d'alcools benzyliques substituCs par des groupes hydroxyles. On montre que cette mCthode est gCnCrale puisqu'elle permet d'obtenir facilement les isomkres ortho, mCta et para des methides de quinone. Lorsqu'ils sont substituts d'une faqon adtquate, tous ces isomkres de methides de quinone ont pu Etre caractCrisCs d'un faqon spectroscopique par photolyse Cclair au laser qui en fait une technique de choix pour I'ttude dynamique de ces intermkdiaires rCactifs. On prCsente aussi le mecanisme de la gCnCration photochimique B partir des alcools hydroxybenzyliques et des extensions de la rCaction B la photogtntration des mCthides de quinones dCrivCs du fluortnyle et du biphinyle.Mots clCs : mtthide de quinone, mCthide de quinone du biphinyle, carbocation, photosolvolyse, photodCshydroxylation, riaction d' hCtCro-Diels-Alder.[Traduit par la ridaction]

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The enhanced photochemical reactivity of o-substituted phenols in its propensity to give o-quinone methide (o-QM) intermediates via excited state intramolecular proton transfer (ESIPT) was uncovered by Keith Yates as part of his now classic studies of photohydration of aromatic alkenes, alkynes, and related compounds. Photogeneration of QMs and the study of their chemistry along with potential biological applications are the focus of many groups. In this work, photochemical precursors to o-, m-, and p-QMs based on substituted phenols (hydroxybenzyl alcohols) and related compounds have been studied in aqueous solution as a function of pH and water content. The focus will be on QMs that are stabilized by an α-phenyl substituent, which enhances quantum yields for their formation, with the resulting QMs having longer lifetimes and easier to detect. Noteworthy is that all QM isomers can be photogenerated with the o and m isomers being the most efficient, consistent with the Zimmerman “ortho-meta” effect. m-QMs have formal non-Kekulé structures, and although they can be routinely photogenerated, are found to be most reactive. One m-QM was found to undergo a photocondensation reaction at high pH giving rise to m-substituted oligomers. The mechanism of QM formation in aqueous solution is believed to involve singlet excited phenols that undergo adiabatic deprotonation to give the corresponding photoexcited phenolate ion, which subsequently expels the hydroxide ion (photodehydroxylation). A pathway involving direct loss of water for the o-isomers is also possible in organic solvents.Key words: quinone methides, phenols, excited state acidity, solvolysis, carbocations, meta effect, photopolymerization, non-Kekulé intermediates.

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