Photochemical Generation and Characterization of Quinone Methides

Abstract

Quinone methides (QMs) are reactive intermediates that are commonly encountered in many areas of chemistry and biology. Quinone methides contain a cyclohexadiene core with a carbonyl and a methylene unit attached and are related to quinones, which have two carbonyl groups, and quinone dimethides, which have two methylene groups. Quinone methides are highly polar and are much more reactive than their relatives. Ortho (1) and para (2) quinone methides are the most commonly encountered isomers, although meta-quinone methides (3), which are non-Kekul e and must be drawn as zwitterionic (3a) or biradical (3b) structures, are known as well. Both orthoand para-quinone methides can also have zwitterionic resonance structures that give these species both cationic and anionic centers, emphasizing their polarized nature and indicating that they may react with both nucleophiles (similar to carbocations) and electrophiles (similar to phenolates). Quinone methides are much more reactive than simple enones (such as a,b-unsaturated ketones) since nucleophilic attack on a quinone methide produces an aromatic alcohol, with aromatization of the ring being a significant driving force (Scheme 1.1). Ortho-quinone methides can also readily engage in [4 þ 2] cycloadditions with electron-rich dieneophiles to give chroman derivatives, again leading to rearomatization of the ring (Scheme 1.1). The reaction rates exhibited by quinone methides are highly