Observation of a Stable Hemiortho Ester Anion. Acidity Constant for a Tetrahedral Intermediate

Article abstract of DOI:10.1021/ja00328a034

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 K_b= ->/->=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 K_b and K₀ provides a value K_a=->+>/ 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.