Reversible formation of aryloxenium ions from the corresponding quinols under acidic conditions

Quinols, 1, are products of the hydration of O-aryloxenium ions, 2, and N-arylnitrenium ions, 3, and they are being investigated for medical uses. Under acidic conditions (pH 1–3) kinetics and products of Br– trapping demonstrate that 1a, 4-phenyl-4-hydroxy-2,5-cyclohexadienone, and 1b, 4-p-tolyl-4-hydroxy-2,5-cyclohexadienone, generate the corresponding oxenium ions 2a and 2b, respectively, as steady-state intermediates. Formation and trapping of the oxenium ions occurs in competition with the acid catalyzed dienone–phenol rearrangement. Because oxenium ion formation is reversible, the ion can only be detected by trapping with a nucleophile. Br– is an efficient trap under acidic conditions because, unlike N3–, it is not protonated under those conditions. Attempts to detect the oxenium ions 2a and 2b at pH 4.6 and 7.1 with N3– were unsuccessful indicating that oxenium ion formation only occurs under acidic conditions. The oxenium ion 2c could not be detected under acidic conditions from the quinol 1c, 4-(benzothiazol-2-yl)-4-hydroxy-2,5-cyclohexadienone, by Br– trapping methods, even though this ion can be detected during hydrolysis of the corresponding ester, 4c. Although the benzothiazol-2-yl group is a resonance electron donor that is capable of stabilizing an O-aryloxenium ion, it is also a strong inductive electron withdrawing group that hinders the formation of 2c from 1c by decreasing the extent of protonation of 1c to generate 1cH+ and by destabilizing the transition state for ionization of 1cH+. Generation of an oxenium ion from the corresponding quinol is feasible under acidic conditions as long as the 4-substituent of the quinol is both a resonance and inductive electron donor. Copyright © 2012 John Wiley & Sons, Ltd.