The hydrolysis of 4-acyloxy-4-substituted-2,5-cyclohexadienones: limitations of aryloxenium ion chemistry

The title compounds serve as potential precursors to aryloxenium ions, often proposed, but primarily uncharacterized intermediates in phenol oxidations. The uncatalyzed and acid-catalyzed decomposition of 4-acetoxy-4-phenyl-2,5-cyclohexadienone, 2a, generates the quinol, 3a. (18)O-Labeling studies performed in (18)O-H(2)O, and monitored by LC/MS and (13)C NMR spectroscopy that can detect (18)O-induced chemical shifts on (13)C resonances, show that 3a was generated in both the uncatalyzed and acid-catalyzed reactions by C(alkyl)-O bond cleavage consistent with formation of an aryloxenium ion. Trapping with N(3)(-) and Br(-) confirms that both uncatalyzed and acid-catalyzed decompositions occur by rate-limiting ionization to form the 4-biphenylyloxenium ion, 1a. This ion has a shorter lifetime in H(2)O than the corresponding nitrenium ion, 7a (12 ns for 1a, 300 ns for 7a at 30 degrees C). Similar analyses of the product, 3b, of acid- and base-catalyzed decomposition of 4-acetoxy-4-methyl-2,5-cyclohexadienone, 2b, in (18)O-H(2)O show that these reactions are ester hydrolyses that proceed by C(acyl)-O bond cleavage processes not involving the p-tolyloxenium ion, 1b. Uncatalyzed decomposition of the more reactive 4-dichloroacetoxy-4-methyl-2,5-cyclohexadienone, 2b', is also an ester hydrolysis, but 2b' undergoes a kinetically second-order reaction with N(3)(-) that generates an oxenium ion-like substitution product by an apparent S(N)2'mechanism. Estimates based on the lifetimes of 1a, 7a, and the p-tolylnitrenium ion, 7b, and the calculated relative stabilities of these ions toward hydration indicate that the aqueous solution lifetime of 1b is ca. 3-5 ps. Simple 4-alkyl substituted aryloxenium ions are apparently not stable enough in aqueous solution to be competitively trapped by nonsolvent nucleophiles.