A kinetic study of competing fragmentation and hydrolyses of phenyl hydrogen alpha-hydroxyiminobenzylphosphonate - a case of acid mediated inhibitionof acid catalysis
R. Ta-shma et al., A kinetic study of competing fragmentation and hydrolyses of phenyl hydrogen alpha-hydroxyiminobenzylphosphonate - a case of acid mediated inhibitionof acid catalysis, J CHEM S P2, (8), 2001, pp. 1404-1407
Citations number
25
Categorie Soggetti
Physical Chemistry/Chemical Physics
Journal title
JOURNAL OF THE CHEMICAL SOCIETY-PERKIN TRANSACTIONS 2
The behavior of phenyl hydrogen alpha -hydroxyiminobenzylphosphonate (E)-2
in aqueous hydrochloric acid solution was examined by P-31 NMR spectroscopy
and by HPLC. Compound (E)-2 was found to undergo two competing acid-cataly
zed reactions. 1) Fragmentation to phenyl phosphate (6) and benzonitrile, s
imilar to the fragmentation of other hydroxyiminophosphonates to metaphosph
ate examined previously. The fragmentation of (E)-2 was found to be slower
by a factor of 4 than that of hydrogen methyl alpha -hydroxyiminobenzylphos
phonate ((E)-1). This phenomenon is interpreted in terms of inductive effec
ts on the suggested metaphosphate intermediate. 2) Compound (E)-2 was found
to undergohydrolytic cleavage of the oxime group giving NH2OH and hydrogen
phenyl benzoylphosphonate (4), which was found to hydrolyze to phenol and
benzoylphosphonic acid (5). The latter reacted with the NH2OH liberated in
the previous step to give alpha -hydroxyiminobenzylphosphonic acid ((E)-3),
which fragmented to benzonitrile and phosphoric acid. The rate of a possib
le hydrolysis of the phenol group in oxime (E)-2 was shown to be slower by
two orders of magnitude than that from ketone 4. This phenomenon is interpr
eted in terms of acid mediated retardation of acid catalyzed hydrolysis of
phenol due to initial protonation of the oxime nitrogen in (E)-2.