Quinone methide intermediates in organic photochemistry

Abstract

Quinone methides are widely encountered reactive intermediates in the chemistry of phenols and related compounds. This paper summarizes our recent progress in uncovering new and general photochemical methods for forming quinone methides of various structural types in aqueous solution. Their mechanism of formation and subsequent chemistry are also discussed. New examples of excited-state intramolecular proton transfer (ESIPT) have been uncovered in these studies. We have also discovered that appropriately designed biphenyls and terphenyls display photochemistry that is best rationalized by highly polarized and planar excited states of these ring systems, which can efficiently lead to the corresponding extended quinone methides.

Dissociation of chloride from ortho-, meta- and para-chloromethylphenoxides: The enthalpy of formation of meta-quinomethane

“…In all cases, the ground states for the Kekule isomers are predicted to be singlets, whereas those for the non-Kekule, meta-isomers are predicted to be triplets. The singlet ground states of the ortho- [25] and para-isomers [28] are well established for 1-3 [1]. Triplet ground states have been confirmed for 1b and 3b, and the spectroscopic data and reactivity of 2b are consistent with the presence of a triplet ground state, as well [29,32].…”

“…Quinones (1), quinomethanes (2) and quinodimethanes (3) are substituted aromatic molecules that differ by the presence of oxygen atoms or methylene groups [1].…”

A spectroelectrochemical and chemical study on oxidation of hydroxycinnamic acids in aprotic medium

“…3d-f). This kind of absorption could be due to an extended conjugation of dimerization products involving the side-chain, possible when neutral phenoxyl radicals are initially formed [37][38][39], as shown in Scheme 2. Formation of dimerization products is further supported by HPLC-MS data, evidencing in all cases the presence of the corresponding molecular ione [Dim-H] − , respectively, m/z 357 for CA, m/z 325 for CUA, m/z 385 for FA and m/z 445 for SA, as shown in Fig.…”

The influence of reducing substances on the oxygen consumption in model solutions.

“…Polyfenoly oxidací poskytují semichony a chinony. Za reaktivní meziprodukty při oxidaci na chinony se považují methidy (Wan a Brousmiche 2001).…”

“…There is little information about radical or nonradical redox reactions of polyphenols. Polyphenols can provide reactive semiquinones and quinones and methides are considered to be reactive intermediates in the oxidation of the quinones (Wan and Brousmiche 2001).…”