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Flash photolysis of 2-diazo-4,4,6,6-tetramethylcyclohexane-1,3-dione in aqueous solution produced 2-oxo-3,3,5,5-tetramethylcyclopentylideneketene, which underwent hydration to the enol of 2-oxo-3,3,5,5-tetramethylcyclopentanecarboxylic acid; the enol then isomerized to the keto form of the acid. Rates of hydration of the ketene and rates of ketonization of the enol were measured in perchloric acid, sodium hydroxide, and buffer solutions, and rate profiles were constructed. Rates of enolization of 2-oxo-3,3,5,5-tetramethylcyclopentanecarboyxlic acid were also measured, using bromine to scavenge the enol as it formed, and rates of enolization and ketonization were then combined to give the keto-enol equilibrium constant p[K.sub.E] = 1.65. This and other results are discussed in comparison with the behavior of the unmethylated 2-oxocyclopentanecarboxylic acid system. Key words: flash photolysis, photo-Wolff reaction, ketene hydration, enolization, ketonization, keto-enol equilibria, [beta]-oxocarboxylic acids.

Flash photolysis of benzocyclobutenone in aqueous solution produced a transient species with a microsecond lifetime whose rates of decay were measured in perchloric acid, sodium hydroxide, and buffer solutions over the acidity range [[H.sup.+]] 1 x [10.sup.-13] - [10.sup.O] M. This produced a rate profile, isotope effects, and buffer behaviour typical of ketene reactions, and that, together with product identification, served to identify this transient as 6-methylene-2,4-cyclohexadienylideneketene, formed by electrocyclic opening of the four-membered ring of benzocyclobutenone. Comparison of rates of reaction of this ketene with those of its saturated analog, pentamethyleneketene, produced some expected as well as some unexpected results. Key words: cyclobutenone chemistry, electrocyclic ring opening, ketene hydration, rate profile, solvent isotope effects.

Rates of acid-catalyzed hydrolysis of 3-diazobenzofuran-2-one, measured in concentrated aqueous perchloric acid and hydrochloric acid solutions, were found to correlate well with the Cox-Yates [X.sub.o] excess acidity function, giving [[kappa].sub.[ETA]+] = 1.66 x [10.sup.-4][M.sup.-1][s.sup.-1], m[double dagger] = 0.86 and [[kappa].sub.H+]/[[kappa].sub.D] = 2.04. The normal direction [[kappa].sub.H]/[[kappa].sub.D] > 1) of this isotope effect indicates that hydrolysis occurs by rate-determining protonation of the substrate on its diazois-carbon atom. It was found previously that the next higher homolog of the present substrate, 4-diazoisochroman-3-one, also undergoes hydrolysis by this reaction mechanism but with a rate constant 15 times greater than that for the present substrate; this difference in reactivity can be understood in terms of the various resonance forms that contribute to the structures of these substrates. The product of the present hydrolysis reaction is 3-hydroxybenzofuran-2-one, which itself quickly undergoes subsequent acid-catalyzed hydrolysis to 2-hydroxymandelic acid. The acidity dependence of this subsequent hydrolysis is much shallower than that of the diazo compound precursor, and rates of reaction correlate as well with [[H.sup.+]] as with [X.sub.o]. This is due in part to incursion of a nonproductive protonation on the hydroxy group of 3-hydroxybenzofuran-2-one that impedes hydrolysis and produces saturation of acid catalysis. Rates of hydrolysis of the hydroxy compound were also measured in dilute HCI[O.sub.4] and NaOH solutions as well as in C[H.sub.3]C[O.sub.2]H, [H.sub.2]P[O.sup.-.sub.4], [(C[H.sub.2]OH).sub.3]CN[H.sup.+.sub.3], and N[H.sup.+.sub.4] buffers, and the rate profile constructed from these data showed the presence of uncatalyzed and hydroxide ion-catalyzed reactions. This hydroxide-ion catalysis became saturated at [NaOH] [congruent to] 0.05 M, implying occurrence of yet another nonproductive substrate ionization. Key words: diazo compound hydrolysis, lactone hydrolysis, Cox-Yates excess acidity, acid catalysis, alcohol protonation.