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

Reaction of the N-phosphinoyl-O-sulfonyl hydroxylamine PhMeCH(Ph)P(O)NHOMs 10 with RNH(2) (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 RNH(2) proceeds with a high degree of stereospecificity, thereby ruling out the possibility of a free metaphosphonimidate intermediate. For the MeNH(2) 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(t)NH(2) 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 RNH(2) to give the observed product. The nucleophilic substitution can have an associative S(N)2(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 MeNH(2) and neat Bu(t)NH(2), but large with Bu(t)NH(2) at high dilution.