The Reduction of β-Iodo Azides. A Stereospecific Synthesis of Aziridines

A new synthesis of aziridines has been developed and the synthetic potential examined in detail. The substrates for the synthesis, β-iodo azides, were readily obtained from the highly stereoselective and regioselective addition of iodine azide to substituted ethylenes. Aziridine formation requires selective reduction of the azide function followed by base-catalyzed ring closure; competing side reactions are elimination of the elements of iodine azide and hydrogenolysis of the iodo group. The extent of reductive elimination is dependent on the stability of conformers with coplanar iodo and azido groups; aryl groups further enhance elimination in these conformers. Elimination specifically occurred in the LAH reduction of Δ²-cholestene, trans-stilbene, and cinnamyl alcohol adducts. Hydrogenolysis of the iodo function becomes competitive with aziridine formation in the sterically less hindered primary iodides. A study of reducing agents by means of product analysis suggests that only reducing agents which may possess Lewis acid properties favor reduction over elimination, i.e., LAH, LAH • A1Cl₃, or B₂H₆, but not NaBH₄, LiBH₄, or Zn. The optimum reagent, LAH, was applied to the mild synthesis of mono-, di-, tri-, and tetrasubstituted aziridines.