Stereospecificity in the rearrangement reactions of an N-phosphinoyl-O-sulfonylhydroxylamine with methylamine and tert-butylamine: retention of configuration at phosphorus as evidence for the initial formation of a phosphonamidic sulfonic mixed anhydride

Abstract

Reaction of the N-phosphinoyl-O-sulfonylhydroxylamine PhMeCH(Ph)P(O)NHOMs 10 with RNH₂(R = Me or Bu^t) results in migration of the phenyl group from phosphorus to nitrogen which leads to the rearrangement product PhMeCHP(O)(NHPh)NHR. Using samples of 10 enriched in one diastereoisomer (80 : 20) or the other (3 : 97), the reaction with neat RNH₂ proceeds with a high degree of stereospecificity, thereby ruling out the possibility of a free metaphosphonimidate intermediate. For the MeNH₂ reaction, the relative configurations of substrate and product, deduced from their X-ray crystal structures, show the sense of the stereospecificity to be retention of configuration at phosphorus; for the Bu^tNH₂ reaction, indirect evidence leads to the same conclusion. Retention of configuration is thought to result from initial base-induced rearrangement to the phosphonamidic sulfonic mixed anhydride PhMeCHP(O)(NHPh)OMs, with inversion of configuration at phosphorus; this then undergoes nucleophilic substitution with RNH₂ to give the observed product. The nucleophilic substitution can have an associative S_N2(P) mechanism (inversion of configuration at phosphorus) but also a dissociative elimination–addition mechanism. The latter is responsible for departures from complete stereospecificity; these are small with MeNH₂ and neat Bu^tNH₂, but large with Bu^tNH₂ at high dilution.