J. Katzhendler et al., Fragmentation of methyl hydrogen alpha-hydroxyiminobenzylphosphonates - kinetics, mechanism and the question of metaphosphate formation, J CHEM S P2, (9), 2000, pp. 1961-1968
Citations number
64
Categorie Soggetti
Physical Chemistry/Chemical Physics
Journal title
JOURNAL OF THE CHEMICAL SOCIETY-PERKIN TRANSACTIONS 2
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.