Photogeneration and chemistry of biphenyl quinone methides from hydroxybiphenyl methanols.

The photosolvolysis of several biphenyl methanols (Ph-PhCH[Ph]OH) substituted with hydroxy or methoxy groups on the benzene ring not containing the -CH(Ph)OH moiety has been studied in aqueous solution. This work is a continuation of our studies of photosolvolysis of hydroxy-substituted arylmethanols that generate quinone methide intermediates, some of which are known to be relevant intermediates in toxicology and in biological and organic chemistry in general. In this study, we further probe the ability of the biphenyl ring system to transmit charge from the ring substituted with a potential electron-donating group (hydroxy and methoxy) to the adjacent benzene ring that contains a labile benzyl alcohol moiety. We show that in systems with a hydroxy substituent, biphenyl quinone methides (BQM) are the first formed intermediates that are detectable by nanosecond laser flash photolysis, and are responsible for the observed overall photosolvolysis reaction of these compounds. The highly conjugated BQM are found to absorb at long wavelengths (lambda(max) 580 and approximately 750 nm for the p,p' and o,p'-isomers, respectively) with relatively long lifetimes in neutral aqueous solution (500 and 30 micros, respectively). The BQM from the o,p'-isomer was found to undergo a competing intramolecular Friedel-Crafts alkylation, to give a fluorene derivative.