Hydrolysis of 2-pyridylphosphonic acid mono-and di-esters: electrophilic catalysis by transition metal ions and the irrelevance of intramolecular participation by the pyridyl group

Abstract

The hydrolysis of O-(4-nitrophenyl) 2-pyridylphosphonate possesses kinetic terms in hydroxide and hydrogen ion concentration and zwitterion and anion concentration. Comparison with suitable models where the proton is localised shows that the ‘anion’ term arises from hydroxide ion attack on the zwitterion rather than water attack on the monoanion; ‘zwitterion’ term involves attack of hydroxide ion on the cationic species. The hydrolysis of O-(4-nitrophenyl)(N-methyl-2-pyridinio)phosphonate possesses a ‘neutral’ term from hydroxide ion attack on the cationic species. 2-Pyridylphosphonate diesters hydrolyse via attack by water on the neutral form of the ester; there is no detectable anchimeric assistance by the pyridine nitrogen possibly due to the very low basicity of the base. The hydrolysis of O-(4-nitrophenyl) phenylphosphonate at neutral pH involves decomposition of the monoanion presumably by water attack rather than the equivalent reaction of hydroxide ion with the neutral form of the ester. O-(4-Nitrophenyl) 2-pyridyl)phosphonate is shown to complex with transition metal ions which also catalyse the hydrolysis of the ester possibly via electrophilic participation.