The Stereochemical Course of Migration from Phosphorus to Nitrogen in the Photo-Curtius Rearrangement of Phosphinic Azides (Harger Reaction)

The homochiral phosphinic azides (R,R) -1 and (S,S)-1 have been prepared in enantiomerically pure form by resolution of diastereomeric phosphinamides derived from (S)-l-phenylethylamine and (R)-phenylglycine. Irradiation of the azides in methanol induced a photo-Curtius rearrangement of phosphonamidates in which the stereogenic carbon unit migrated to a nitrogen atom. Hydrolysis of the phosphonamidates produced 1-phenylethylamine, which was 99.0% ee and of the same configuration as the carbon unit in the starting azide (99.0% net retention). The implication for asymmetric synthetic methodology is discussed.