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Observation of a Stable Hemiortho Ester Anion. Acidity Constant for a Tetrahedral Intermediate
McClelland, Robert A.,Seaman, N. Esther,Cramm, David
, p. 4511 - 4515 (2007/10/02)
Pinacol mono-p-nitrobenzoate (POH) undergoes a rapid, reversible cyclization in base producing an anionic tetrahedral intermediate (TO-), the conjugate base of 2-(p-nitrophenyl)-2-hydroxy-4,4,5,4-tetramethyl-1,3-dioxolane (TOH).Presence of the anion is established by changes in UV spectra and by the rate behavior exhibited in the hydrolysis of the ester to pinacol and p-nitrobenzoate.Rate constants in base do not rise as rapidly as expected as the ester form is converted to the unreactive TO-.Interconversion of POH and TO- occurs in a typical acid:base manner, and UV spectral analysis in base provides Kb= ->/->=0.7.The equilibrium constant K0=/ for cyclization of the pinacol ester to the neutral hemiortho ester TOH is 8*10-4, this has been measured as the ratio of forward and reverse rate constants.Combination of Kb and K0 provides a value Ka=->+>/ for the acidity constant of the tetrahedral intermediate of 10-10.4.This number is compared with estimates of tetrahedral intermediate acidity based on substituent effects and free energy correlations, and excellent agreement is noted.
