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

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 β_lg 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 k_H/k_D = 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.