Syntheses and rearrangements of tris(hydroxymethyl)phosphine and tetrakis(hydroxymethyl)phosphonium salts

Tetrakis(hydroxymethyl)phosphonium (HOCH2)4P+ salts, particularly the chloride (THPC) and sulfate (THPS), are among the most accessible organophosphorus compounds that can be made quantitatively from PH3 and formaldehyde in aqueous media under ambient conditions. These phosphonium salts are air-stable, and are widely used as reactants in organic syntheses, as well as in textile or oil industries. Synthesis of tris(hydroxymethyl)phosphine (THP), an active component of the salts, is more complicated because direct synthesis from PH3 and CH2O requires either pressure and high temperature, at which THP rearranges into bis(hydroxymethyl)methylphosphine oxide, CH3P(O)(CH2OH)2, or a metal catalyst that is usually difficult or impossible to recover. Syntheses of THP based on neutralization of the salts are more convenient but require pH control and additional steps to separate THP from CH2O. This review summarizes literature data on syntheses and purification of the salts and THP, and side reactions such as thermal rearrangement and oxidation.