Fragmentations and Rearrangements of α-Hydroxyiminoalkylphosphonates

Abstract

This article reviews the unique reactivities of � -hydroxyiminoalkylphosphonates, which result from the proximity and the interaction between the hydroxyimino and the phosphonic groups. The reaction course of � -hydroxy- iminoalkylphosphonates depends markedly on the type of groups linked to the phosphorus and on the stereochemistry of the oxime function. (Z)-� -Hydroxyiminoalkylphosphonates and phosphinates undergo, upon heating, fragmentation to a nitrile and a phosphate or phosphonate, while (E)-� -hydroxyiminoalkylphosphonates and -phosphinates undergo Beckmann rearrangement to N-acylphosphoramidates or N-acylphosphonamidates, respectively. In contrast to the diesters, � -hydroxyiminoalkylphosphonic acids (and monoesters) are unstable and fragment to metaphosphates, which can perform in situ phosphorylation of appropriate groups. Kinetic and mechanistic studies of the fragmentations of � - hydroxyiminobenzylphosphonates to metaphosphoric acid and esters are reviewed. The fragmentation of � -hydroxyimino- benzylphosphonic acid monoesters to metaphosphate esters requires acid catalysis and slows down at about pH 3. In contrast, � -hydroxyiminobenzylphosphonic acid undergoes fragmentation in aqueous solutions at a wide range of pH, from strongly acidic up to about pH 9. The � -hydroxyiminophosphonate fragmentation methodology was applied to phosphorylation of silica gel, which can potentially be useful as chromatographic stationary phase. A variety of stable precursors of � - hydroxyiminophosphonates that can be induced to fragment thermally, photochemically or by base catalysis is described. (E)-� -Hydroxyiminobenzylphosphonamidates of various types have been found to undergo Beckmann rearrangement to yield N-acylphosphordiamidates upon heating in nonpolar solvents such as toluene, or fragmentation to metaphosphonamidate by heating in polar solvents. The fragmentation of (E)-� -hydroxyiminobenzyl- phosphonates to metaphosphates is considered a special case of the Beckmann fragmentation.