Competing sulfonylation and phosphonylation following rearrangement of an O-sulfonyl-N-phosphinoylhydroxylamine with tert-butylamine: demonstration of a phosphonamidic-sulfonic anhydride intermediate and 18O-labelling evidence on how it may be formed

Abstract

Reaction of R(Ph)P(O)NHOSO₂Bn (R = PhMeCH) with Bu^tNH₂ in CH₂Cl₂ gives a mixture of RP(O)(NHPh)NHBu^t and Bu^tNHSO₂Bn, the proportion of the sulfonamide increasing steadily (14.6% to 32.9%) as the concentration of amine is reduced (8.0 to 0.2 mol dm⁻³). Apparently the phenyl and sulfonate groups in the substrate become transposed, giving a phosphonamidic-sulfonic anhydride RP(O)(NHPh)OSO₂Bn which then reacts at the phosphorus or sulfur atom to give the final products; an authentic sample of the anhydride gives similar mixtures of products. Substrate labelled with ¹⁸O in the sulfonyl position (57 mol% one ¹⁸O atom) gives sulfonamide containing most but not all of the label (43.7 mol% one ¹⁸O atom with 2.0 mol dm⁻³ amine). This implies partial equilibration of the three sulfonate oxygen atoms during rearrangement, or after the anhydride intermediate has been formed.