1995 Merck Frosst Award Lecture Quinone methides: relevant intermediates in organic chemistry

ortho and para-Quinone methides (2-methylene-3,5-cyclohexadien-1-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 to o-, m-, and p-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-Diels–Alder reaction.