Fragmentation of methyl hydrogen alpha-hydroxyiminobenzylphosphonates - kinetics, mechanism and the question of metaphosphate formation

The thermodynamics, pH dependency and solvent effects of the fragmentation reaction of a series of alpha-oxyiminobenzylphosphonate monomethyl esters [ (E)-1a-f] were examined in water and other hydroxylic solvents by UV and by P-31 NMR spectroscopy at pH 0-3.1. The fragmentation of compounds (E)-1a-f was found to be a first-order reaction in substrate over the acidity range studied, while the dependence on the acidity is more complex, with rate co nstants k(1) and k(2). The rho values corresponding to the first and second order rate constants were -1.12 and -0.835, respectively, indicating that the reaction is facilitated by electron-donating substituents, which probab ly enhance the protonation of the oxime OH group. Activation parameters for k(1) and k(2) reactions were also calculated. The near- zero values of the entropies of activation obtained are consistent with a dissociative transi tion state with almost no bonding to a nucleophilic solvent. Monitoring the fragmentation reaction of (E)-1a in several binary alcohol-water mixtures at different acidities showed that the reaction rate is enhanced by the alc ohol's acidity and not hampered by the steric requirements of the alcohol m olecule. This rules out in our opinion, the likelihood for nucleophilic sol vent assistance in the rate-determining step. On the other hand, product st udies show that both the nucleophilicity and the steric requirements of the alcohol are of importance in determining the product formed in the fragmen tation of (E)-1a. The highest selectivity (S) value was found for MeOH, whi le S values of < 1 were observed for 2,2,2-trifluoroethanol and the sterica lly hindered alcohols. The divergence between the effects of the solvent on the rate, on the one hand, and on the products on the other, indicates tha t the rate limiting step and the product determining step do not share a co mmon transition state and that the reaction coordinate includes at least on e reactive intermediate, probably methyl metaphosphate. The results are com patible with a dissociative mechanism (D-N*A(N) or D-N+A(N)), in which the solvating water molecules pull the departing water molecule into the hydrat ion shell, while the solvated phosphonic group becomes a metaphosphate with out nucleophilic assistance. The fragmentation of oxyiminobenzylphosphonate s to metaphosphate is perceived as a special case of the "abnormal" Beckman n reaction.