The reactivity of phosphate esters: reactions of monoesters with nucleophiles. Nucleophilicity independent of basicity in a bimolecular substitution reaction

Abstract

The reactions of amine nucleophiles at the phosphorus centre of mono- and di-anions of phosphate monoesters involve concerted displacement processes. The reactivity of the dianions depends strongly on the leaving group, but only weakly on the basicity of the nucleophile, and for a sufficiently good leaving group the Brönsted coefficient is zero. Thus, substituted pyridines differing in basicity by more than eight powers of ten attack the dianion of 2,4-dinitrophenyl phosphate at the same rate. These reactions must lie close to the borderline between unimolecular and bimolecular processes. The reactions with the monoanions are sensitive to the basicity of the nucelophile, and also depend strongly on the leaving group. The reactivity of phosphate monoester monoanions is similar to that of the anions of the diesters, and both react readily with fluoride anion.