Kinetic and mechanistic studies of the reaction of a range of bases and metal-hydroxo complexes with the phosphonate ester 2,4-dinitrophenyl ethyl methylphosphonate in aqueous solution |
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Authors: | Hay Robert W Govan Norman Norman Paul R |
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Institution: | (1) School of Chemistry, University of St Andrews, St Andrews, UK;(2) Chemical and Biological Defence Establishment, Porton Down, Salisbury Wilts, SP4 0JQ, UK |
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Abstract: | The base hydrolysis and spontaneous water hydrolysis of the phosphonate ester 2,4-dinitrophenyl ethyl methylphosphonate (DNPEMP) has been studied in detail and the activation parameters for these processes determined. The catalytic effect of a series of 19 oxygen and nitrogen bases and metal hydroxo complexes has been studied. The Brønsted plot of pK versus logkN has a slope (β) of 0.47, which is fairly typical for phosphate and phosphonate esters. alpha-Effect nucleophiles such as 2-iodosobenzoate, hypochlorite and the hydroperoxide ion (HO-2) exhibit enhanced reactivity, as does fluoride ion and the bifunctional metal complex Cu(tmen) (OH2)(OH)]+ (tmen = N,N,N',N',-tetramethylethylenediamine). Reactions involving these species appear to take place by a nucleophilic pathway, while the other oxygen and nitrogen bases act as general base catalysts. The solvent deuterium isotope effect for Et3N catalysis, kH2O/kD2O=1.3, is consistent with general base catalysis. The mechanisms of these reactions are discussed in detail. |
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