Migration of a benzyl group in the Lossen-like rearrangement of an N-phosphinoyl-O-sulphonylhydoxylamine

Abstract

Migration of a group other than aryl has been observed for the first time in the Lossen-like rearrangement of an N-phosphinoyl-O-sulphonylhydroxylamine: the benzylic substrate (ArCH₂)₂P(O)-NHOMs (Ar =p-tolyl) gives the phosphonamidate ArCH₂P(O)(OR)NHCH₂Ar (14) on treatment with alkoxide. The benzyl group seems to migrate less readily than aryl, so rearrangement is relatively slow and competing reactions can be of major importance. Thus, while with isopropoxide or t-butoxide the phosphonamidate (14; R = Prⁱ or Bu^t) is formed in 70–80% yield, with methoxide or ethoxide the rearrangement accounts for only 25–35% and major amounts of (ArCH₂)₂P(O)OR (R = Me or Et) and (ArCH₂)₂P(O)NH₂ are formed. Rearrangement is even less important with t-butylamine, the hydrazide (ArCH₂)₂P(O)NHNHBu^t being much the most abundant product. The fact that a benzyl group can migrate is mechanistically significant; it shows that the migration centre can be a saturated (sp³) carbon atom, and that the new bond to nitrogen can be formed without direct involvement of π electrons.