114142-42-6Relevant articles and documents
Fragmentation of methyl hydrogen α-hydroxyiminobenzyl-phosphonates-kinetics, mechanism and the question of metaphosphate formation
Katzhendler, Jehoshua,Schneider, Hava,Ta-Shma, Rachel,Breuer, Eli
, p. 1961 - 1968 (2007/10/03)
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 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 (DN*AN or DN + AN), 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.
Fragmentation of Methyl Hydrogen α-Hydroxyiminophosphonates to Monomeric Methyl Metaphosphate: Stereochemistry and Mechanism
Katzhendler, Jehoshua,Karaman, Rafik,Gibson, Dan,Breuer, Eli,Leader, Haim
, p. 589 - 594 (2007/10/02)
Methyl α-(hydroxyimino)benzylphosphates undergo fragmentation in alcohols to benzonitrile and a (mixed) phosphodiester.Kinetic studies of the behaviour of (E)-(2) and (Z)-(2) in a series of alcohols indicate that (Z)-(2) undergoes acid catalysed isomerization to (E)-(2) which subsequently undergoes fragmentation via concerted, dissociative bond cleavage to benzonitrile and monomeric methyl metaphosphate (7).The latter is trapped by the solvent in the second step of the reaction to give the final phosphodiester product.
