Fragmentations and Rearrangements of α-Hydroxyiminoalkylphosphonates

This article reviews the unique reactivities of α-hydroxyiminoalkylphosphonates, which result from the proximity and the interaction between the hydroxyimino and the phosphonic groups. The reaction course of α-hydroxyiminoalkylphosphonates depends markedly on the type of groups linked to the phosphorus and on the stereochemistry of the oxime function. (Z)- α-Hydroxyiminoalkylphosphonates and phosphinates undergo, upon heating, fragmentation to a nitrile and a phosphate or phosphonate, while (E)- α-hydroxyiminoalkylphosphonates and -phosphinates undergo Beckmann rearrangement to N-acylphosphoramidates or N-acylphosphonamidates, respectively. In contrast to the diesters, α-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 -α hydroxyiminobenzylphosphonates to metaphosphoric acid and esters are reviewed. The fragmentation of α-hydroxyiminobenzylphosphonic acid monoesters to metaphosphate esters requires acid catalysis and slows down at about pH 3. In contrast, α-hydroxyiminobenzylphosphonic acid undergoes fragmentation in aqueous solutions at a wide range of pH, from strongly acidic up to about pH 9. The α-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 α-hydroxyiminophosphonates that can be induced to fragment thermally, photochemically or by base catalysis is described. (E)-α-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)-α-hydroxyiminobenzylphosphonates to metaphosphates is considered a special case of the Beckmann fragmentation.