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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