Flash photolytic generation and study of p‐quinone α‐phenylmethide and p‐quinone α,α‐diphenylmethide in aqueous solution

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Abstract

Abstract p ‐Quinone α‐phenylmethide ( 7 ) and p ‐quinone α,α‐diphenylmethide ( 8 ) were generated by flash photolytic dehydration of p ‐hydroxy‐α‐phenylbenzyl alcohol ( 9 , R 1 = Ph, R 2 = H) and p ‐hydroxy‐α,α‐diphenylbenzyl alcohol ( 9 , R 1 = R 2 = Ph) respectively, and rates of their decay were measured in aqueous perchloric acid and sodium hydroxide solutions and also as in acetic acid, biphosphate ion, tris(hydroxymethyl)methylammonium ion, ammonium ion and bicarbonate ion buffers. The rate profiles for the hydration of these quinone methides that these data provide show hydronium ion and hydroxide ion‐catalyzed regions and also ‘uncatalyzed’ water reactions, with saturation of hydronium ion catalysis in dilute solution for the diphenyl‐substituted substrate but only in concentrated acid solution for the less basic monophenyl substrate. Both substrates also give inverse hydronium ion isotope effects ( k H + / k D + < 1). These results show that the hydronium ion‐catalyzed reactions occur by rapid pre‐equilibrium protonation of the quinone methide on its carbonyl oxygen atom followed by rate‐determining capture of the benzyl‐type carbocation thus formed by water. Phenyl substitution retards the rate of hydration and also reduces the acidity constant of the quinone methide conjugate acid. The molecular basis of these substituent effects is discussed. Copyright © 2004 John Wiley & Sons, Ltd.

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