Effect of neighboring pyridinium groups on the basic hydrolysis of arylbenzoate esters

Abstract

Arylbenzoate esters with a quaternary pyridinium group ortho to the ester function exhibit enhanced reactivity towards basic hydrolysis relative to their para analogs in the order pyridinium < 4-aminocarbonylpyridinium < 3-aminocarbonylpyridinium < 3-aminocarbonylquinolium. In contrast, a trimethylammonium group in the ortho position shows a decelerating effect relative to the para analog. It is concluded that the catalytic effect of a neighboring pyridinium group is based upon interaction of the negatively charged transition state of ester hydrolysis with the electron deficient π-system of the pyridinium ring. For the p -methoxy-phenolate ester containing the 3-aminocarbonylpyridinium group in the ortho position this interaction leads to a change in mechanism from rate limiting hydroxide ion attack to hydroxide ion catalyzed expulsion of the leaving group as became apparent from deviation of the Hammett plot and second-order dependence on OH - concentration. For the p -nitrophenolate ester it was observed that decrease of the solvent polarity by addition of dioxane results in an increase of the rate of hydrolysis and a marked increase of the neighboring group effect.