O-nucleophilicity of hydroxamate ions in reactions with ethyl 4-nitrophenyl ethylphosphonate, diethyl 4-nitrophenyl phosphate, and 4-nitrophenyl 4-toluenesulfonate

Abstract

The nucleophilicity of hydroxamate ions toward ethyl 4-nitrophenyl ethylphosphonate, diethyl 4-nitrophenyl phosphate, and 4-nitrophenyl 4-toluenesulfonate in water (µ=1, KCl, 25°C) is described by the Bro nsted equation (βN=0.54, 0.70, and 0.59, respectively). In these reactions, hydroxamate ions act as typical α -nucleophiles; they are more reactive than phenoxide ions with the same basicity by a factor of 300 to 800. In the series of hydroxamate ions, an anomalously high nucleophilicity was revealed for the anions possessing catalytic centers (in terms of general base catalysis), which are capable of providing anchimeric assistance in the transition state. An equation has been proposed, which relates the efficiency of such assistance in anions derived from aminohydroxamic acids to the Δ pKa values characterizing their acidic and basic groups.