Rearrangement reactions of N-phosphinoyl-O-sulphonylhydroxylamines with amines under competitive conditions. Possible involvement of monomeric metaphosphonimidates and phosphonamidic sulphonic mixed anhydrides

Abstract

The O-methylsulphonyl-N-phosphinoylhydroxylamines, RPhP(O)NHOMs (R = Ph, Me, Et, Prⁱ), react with an excess of an equimolar mixture of isopropylamine and t-butylamine to give the rearrangement products RP(O)(NHPh)(NHPrⁱ) and RP(O)(NHPh)(NHBu^t). In the absence of solvent, there is little discrimination between the competing amines (NHPrⁱ/NHBu^t product ratio 1.45–1.65), as would be expected if rearrangement gave initially a reactive monomeric metaphosphonimidate. However, use of a diluent (CH₂Cl₂) greatly increases the selectivity, implying that much of the initial metaphosphonimidate does not go directly to product. Instead, it seems to recombine with the sulphonate anion to form a phosphonamidic sulphonic mixed anhydride. The observed rearrangement products could then result from reaction of the mixed anhydride with the amines. This may proceed with high selectivity by an S_N2(P) mechanism, or with low selectivity by a preassociative elimination–addition mechanism.