Flash photolysis production of the tetrahedral intermediate of the methanolysis of methyl benzoate. Direct measurement of the rate of breakdown of the conjugate base form

Abstract

. Can. J. Chem. 68, 375 (1990). Laser flash photolysis in aqueous basic solutions of the ortho acid derivatives 1-(phenyldimethoxymethyl)benzimidazole 2 and 4-bromo-1-(phenyldimethoxymethyl)imidazole 3 results in production of the phenyldimethoxymethyl cation, which has A, , at 260 nm. The cation decays in reactions with water (k = 9.9 X lo4 s-I) and hydroxide ion (2.5 X 10' M-' s-' )to finally yield methyl benzoate, whose formation was monitored at 234 nm. In solutions with pH 10-12, rate constants measured at this wavelength are the same as those,obtained at 260 nm, but with pH >13 and pH <9, rate constants at 234 nm are smaller. With pH 9-10 and pH 12-13, single exponential kinetics are not observed at 234 nm. This behavior is interpreted in terms of a scheme where at each pH there are two consecutive first-order reactions, cation -+ phenyldimethoxyhydroxymethane (5) + ester, and the pH dependencies of the rate constants are such that they cross twice over the pH range of this study. The intermediate 5 is the tetrahedral intermediate formed in the methanolysis of methyl benzoate, and the 234-nm buildup at pH > 13 and pH <9 directly measures its breakdown. At pH > 13 the rate constant is independent of pH with k = 9 X lo6 s-'. This represents the rapid expulsion of methoxide from the conjugate base of 5. At pH <9 the rate constants are proportional to hydroxide ion concentration, with koH-= 7.5 X 109M-I s-l. In these solutions the neutral intermediate predominates and the dependence on [OH-] of its rate of conversion to ester is interpreted in terms of breakdown of the anion and protonation of this species by water occurring at comparable rates. Thus, koH-represents a situation where there is partial rate-limiting deprotonation of the neutral intermediate by hydroxide. The intermediate of this study bears a close resemblance to the tetrahedral intermediate of the hydrolysis of methyl benzoate. The observation that the anionic forms of such intermediates undergo breakdown at rates similar to those associated with the establishment of proton transfer equilibrium explains why the ester undergoes carbonyl oxygen exchange in base at a rate slower than hydrolysis. , conduit a la formation du cation phCnyldimCthoxymCthyle qui prCsente un A, , , a260 nm. Le cation se dCcompose par rCaction avec l'eau (k = 9,9 x lo4 s-I) et l'ion hydroxyde (2,5 X 10' M-I s-I) pour donner finalement du benzoate de mCthyle dont on determine la formation en suivant la progression de la bande a 234 nm. Dans des solutions dont le pH varie de 10 a 12, les constantes de vitesse telles que mesurCes a cette longueur d'onde sont identiques celles que l'on obtient a 260 nm; toutefois, a des pH >13 ou <9, les constantes de vitesse CvaluCes h 234 nm sont plus faibles. A 234 nm et a des pH de 9 a 10 ou de 12 B 13, on n'observe pas de cinCtiques exponentielles uniques.On interprkte ce comportement en fonction d'un schCma selon lequel il existe, pour chaque pH, deux rCactions consCcutives du premier ordre : cation -+ phCnyldimCthoxyhydroxymCthane (5) ...

Expanding the Catalytic Triad in Epoxide Hydrolases and Related Enzymes

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Kinetics of breakdown of aryl hemiacetals of α-bromoacetophenone. Effect of phenol leaving groups on the lifetimes of tetrahedral intermediates

“…Studies of the base-catalyzed breakdown of cyclic hemiacetals with phenol (10) and carboxylate (1 1) leaving groups have shown that the class n mechanism is no longer followed. With hemiorthoester tetrahedral intermediates derived from benzoic acid, the basecatalyzed reaction was found to still follow a class n type of mechanism, but with the proton transfer step rate-limiting (12,13). This mechanism is enforced (2,14) by the short lifetime of the anionic intermediate in respect to its breakdown by expulsion of RO-, as in eq.…”

Kinetics and mechanism of the hydrolysis of a (5,6)-spirophosphorane. Thermodynamics of the hydrolysis of cyclic five-membered and six-membered phosphonium ions

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