Mechanistic studies of photochemical quinone methide formation via ESPT and formal long-range ESIPT

The photochemical generation of several novel quinone methide-type intermediates has been observed upon photolysis of pyridoxine (150-Vitamin B6) and its derivatives 151 and 152, hydroxybiphenyl alkenes 153 and 154, and hydroxybiphenyl alcohols 159 and 160. Mechanistic investigations, utilizing product, fluorescence and laser flash photolysis studies, have suggested two distinct pathways for the formation of these reactive intermediates, depending upon the functional groups present on the progenitor. Formal excited state intramolecular proton transfer (ESIPT) between the phenol and the alkene led to quinone methides upon irradiation of the hydroxybiphenyl alkenes, while excited state proton transfer (ESPT) to solvent followed by dehydroxylation was responsible for formation of these intermediates from the hydroxybiphenyl alcohols. The quinone methide-type intermediates obtained from the pyridoxine systems arise from formal loss of water, although it is not certain whether this is through ESIPT or ESPT from the phenol at neutral pH.; Studies of the photogeneration of quinone methide-type intermediates from the pyridoxine systems are important due to their biological relevance. Formation of such reactive intermediates in vivo may explain some of the toxicological properties associated with the intake of large doses of the vitamin.; Irradiation of 150 or 151 in 1:1 CH3OH/H 2O gave the corresponding methyl ethers (phip = 0.18 and 0.21, respectively), consistent with formation of quinone methide-type intermediates. Similarly, photolysis in aqueous CH3CN with ethyl vinyl ether resulted in the regioselective formation of the respective chroman products through [4+2] cycloaddition. LFP spectra pointed to formation of two quinone methide-type intermediates upon irradiation of both 151 and 152 in neutral aqueous solution, only one of which is present at pH 12.; Previous studies on m-hydroxystyrene have suggested that m-quinone methide formation occurs via formal ESIPT between the phenol and the alkene, mediated by a bridging water trimer. Studies on 153 and 154 were undertaken to determine whether this solvent-mediated ESIPT can occur over longer distances. The photochemistry of the related hydroxybiphenyl alcohols (159 and 160) was also investigated, as quinone methides have been observed upon photolysis of similar systems. (Abstract shortened by UMI.)...