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Pure Appl. Chem. Vol. 72, No. 12, pp. 2299-2308 (2000)

Pure and Applied Chemistry

Vol. 72, Issue 12

Reactive intermediates. Some chemistry of quinone methides*

Y. Chiang, A. J. Kresge**, and Y. Zhu

Department of Chemistry, University of Toronto, Toronto, Ontario M5S 3H6, Canada

Abstract: Quinone methides were produced in aqueous solution by photochemical dehydration 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 thiocyanate ions. These reactions are acid-catalyzed and show inverse isotope effects (kH+/kD+ < 1), indicating that they occur through preequilibrium protonation of the quinone methide on its carbonyl carbon atom followed by rate-determining capture of the benzyl carbocations so formed by H2O, Br-, or SCN-. With some quinone methides (R = C6H5 and 4-CH3OC6H4) this acid catalysis could be saturated, and analysis of the data obtained in the region of saturation for the example with R = 4-CH3OC6H4 produced both the equilibrium constant for the substrate protonation step and the rate constant for the rate-determining step. Energy relationships comparing the quinone methides with their benzyl alcohol precursors are derived.

*Plenary lecture presented at the 15th International Conference on Physical Organic Chemistry (ICPOC 15), Göteborg, Sweden, 8 –13 July 2000. Other presentations are published in this issue, pp. 2219 –2358.
**Corresponding author

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