Imide-amide rearrangement of oxazaphosphorimidates: studies towards the application to the synthesis of chiral Lewis bases

The Lewis acid catalysed imide-amide rearrangement of oxazaphosphorimides to diazaphoshoramides is reported for the first time. In spite of the similarity to the previously reported Lewis acid catalysed imide-amide rearrangement of dioxaphosphorimides to oxazaphosphoramides we show that this rearrangement proceeds by a different mechanism, not involving the formation of an oligomeric intermediate. The oxazaphosphorimides are prepared in situ by the Staudinger reaction of the appropriate trivalent phosphorus compound with an azide and after the addition of BF₃·OEt₂, undergo rearrangement to the corresponding diazaphosphoramides. We have found that the rearrangement occurs with retention of configuration at the phosphorus atom and inversion of configuration at the rearranged carbon atom. When starting from chiral 1,2-aminoalcohol, substituted at the carbon atom that undergoes rearrangement, a mixture of diastereomers is obtained, but the diastereomeric ratio, initially obtained in the formation of the trivalent phosphorus compounds is maintained during the whole transformation. This implies that if the rearrangement is to be used for the preparation of chiral phosphoramides with defined stereochemistry at the phosphorus atom, a high diastereoselectivity during the preparation of the trivalent phosphorus precursors should be obtained.