Mechanistic studies of La3+ and Zn2+-catalyzed methanolysis of O-ethyl O-aryl methylphosphonate esters. An effective solvolytic method for the catalytic destruction of phosphonate CW simulants

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Abstract
The kinetics of methanolysis of six O-ethyl O-aryl methylphosphonates (6a–f) promoted by methoxide, La3+ and 1,5,9-triazacyclododecane complex of Zn2+(−OCH3) (5:Zn2+(−OCH3)) were studied as simulants for chemical warfare (CW) agents, and analyzed through the use of Brønsted plots. The βlg values are, respectively, −0.76, −1.26 and −1.06, pointing to significant weakening of the P–OAr bond in the transition state. For the metal-catalyzed reactions the data are consistent with a concerted process where the P–OAr bond rupture has progressed to the extent of 84% in the La3+ reaction and ca. 70% in the Zn2+ catalyzed reaction. The catalysis afforded by the metal ions is remarkable, being about 106-fold and 108-fold for poor and good leaving groups, respectively, relative to the background reactions at s spH 9.1. Solvent deuterium kinetic isotope studies for two of the substrates promoted by 5:Zn2+(−OCH3) give kH/kD = 1.0 ± 0.1, consistent with a nucleophilic mechanism. A unified mechanism for the metal-catalyzed reactions is presented which involves pre-equilibrium coordination of the substrate to the metal ion followed by intramolecular delivery of a coordinated methoxide.

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