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

Abstract

The thermodynamics, pH dependency and solvent effects of the fragmentation reaction of a series of α-oxyiminobenzylphosphonate monomethyl esters [(E)-1a–f] were examined in water and other hydroxylic solvents by UV and by ³¹P 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 constants k₁ and k₂. The ρ 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 probably enhance the protonation of the oxime OH group. Activation parameters for k₁ and k₂ reactions were also calculated. The near- zero values of the entropies of activation obtained are consistent with a dissociative transition 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 alcohol’s acidity and not hampered by the steric requirements of the alcohol molecule. This rules out in our opinion, the likelihood for nucleophilic solvent assistance in the rate-determining step. On the other hand, product studies show that both the nucleophilicity and the steric requirements of the alcohol are of importance in determining the product formed in the fragmentation of (E)-1a. The highest selectivity (S) value was found for MeOH, while S values of <1 were observed for 2,2,2-trifluoroethanol and the sterically 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 that the rate limiting step and the product determining step do not share a common transition state and that the reaction coordinate includes at least one reactive intermediate, probably methyl metaphosphate. The results are compatible 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 hydration shell, while the solvated phosphonic group becomes a metaphosphate without nucleophilic assistance. The fragmentation of oxyiminobenzylphosphonates to metaphosphate is perceived as a special case of the “abnormal” Beckmann reaction.