Reactive intermediates. Some chemistry of quinone methides

Quinone methides were produced in aqueous solution by photochemical dehydra tion of o-hydroxybenzyl alcohols (o-HOC6H4CHROH; R = H, C6H5. 4-CH3OC6H4), and flash photolytic techniques were used to examine their rehydration back to starting substrate as well as their interaction with bromide and thiocy anate ions. These reactions are acid-catalyzed and show inverse isotope eff ects (k(H+)/k(D+) < 1), indicating that they occur through preequilibrium p rotonation of the quinone methide on its carbonyl carbon atom followed by r ate-determining capture of the benzyl carbocations so formed by H2O, Br-, o r SCN-. With some quinone methides (R = C6H5 and 4-CH3OC6H4) this acid cata lysis could be saturated, and analysis of the data obtained in the region o f saturation for the example with R = 4-CH3OC6H4 produced both the equilibr ium constant for the substrate protonation step and the rate constant for t he rate-determining step. Energy relationships comparing the quinone methid es with their benzyl alcohol precursors are derived.