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.
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 beta(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 10(6)-fold and 10(8)-fold for poor and good leaving groups, respectively, relative to the background reactions at pH 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.
Lewis
Roxanne E
RE
Department of Chemistry, Queen's University, Kingston, Ontario, Canada K7L 3N6.
Neverov
Alexei A
AA
Stan Brown
R
R
D002384:Catalysis; D002412:Cations / Q000737:chemistry; D002618:Chemical Warfare; D007811:Lanthanum / Q000737:chemistry*; D008697:Methane / Q000737:chemistry*; D008745:Methylation; D015394:Molecular Structure; D010755:Organophosphates / Q000138:chemical synthesis / Q000737:chemistry*; D063065:Organophosphonates / Q000737:chemistry*; D012997:Solvents / Q000737:chemistry; D015032:Zinc / Q000737:chemistry*
D002412:Cations; D010755:Organophosphates; D063065:Organophosphonates; D012997:Solvents; D007811:Lanthanum; D015032:Zinc; D008697:Methane