Stereochemical studies. Part 112. Geometrical dependence of intramolecular catalysis in the hydrolysis and aminolysis of aryl esters

Abstract

The spontaneous hydrolyses of a series of p-nitrophenyl esters of 2-aminocarboxylic acids show rate enhancements of ca. 10³. These can be attributed to intramolecular general base catalysis by the neighbouring amino group, or to hydrogen-bonding/electrostatic stabilisation of the tetrahedral intermediate formed in the kinetically equivalent mechanism of hydroxide ion attack on the ammonium ion ester. The hydrolysis of the esters catalysed by phosphate and carbonate ions shows rate enhancements of 10³ and 10², respectively, attributable to electrostatic/hydrogen-bonding catalysis. Conversely, the aminolysis of the esters with trifluoroethylamine shows a rate enhancement of 10² attributable to intramolecular general base catalysis. In all these reactions there is little dependence of the rate enhancement upon the geometrical relationship between the amino and ester functions. These observations support the suggestion that proton transfer/hydrogen bonding requires relatively little loss of entropy.