Major mechanistic differences between the reactions of hydroxylamine with phosphate di- and tri-esters

Hydroxylamine reacts as an oxygen nucleophile, most likely via its ammonia oxide tautomer, towards both phosphate di- and triesters of 2-hydroxypyridine. But the reactions are very different. The product of the two-step reaction with the triester TPP is trapped by the NH₂OH present in solution to generate diimide, identified from its expected disproportionation and trapping products. The reaction with H₃N⁺–O⁻ shows general base catalysis, which calculations show is involved in the breakdown of the phosphorane addition-intermediate of a two-step reaction. The reactivity of the diester anion DPP⁻ is controlled by its more basic pyridyl N. Hydroxylamine reacts preferentially with the substrate zwitterion DPP^± to displace first one then a second 2-pyridone, in concerted S_N2(P) reactions, forming O-phosphorylated products which are readily hydrolysed to inorganic phosphate. The suggested mechanisms are tested and supported by extensive theoretical calculations.