A kinetic study of competing fragmentation and hydrolyses of phenyl hydrogen alpha-hydroxyiminobenzylphosphonate - a case of acid mediated inhibitionof acid catalysis

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.