Hypervalent Iodine-Catalyzed Synthesis of 1,2,4-Oxadiazoles from Aldoximes and Nitriles

A convenient procedure for oxidative cycloaddition of aldoximes with nitriles using stoichiometric or catalytic amounts of hypervalent iodine(III) reagent IBA-OTf (2-iodosylbenzoic acid triflate) has been developed. This efficient procedure works well for the preparation of various 1,2,4-oxadiazoles in generally high yields under mild reaction conditions. The actual oxidant in this reaction, IBA-OTf, is prepared from 2-iodosylbenzoic acid (IBA) and trifluoromethanesulfonic acid (TfOH) or it can be generated in situ from 2-iodobenzoic acid and m-chloroperoxybenzoic acid (m-CPBA) in the presence of TfOH. The nature of hydroxy(aryl)iodonium species, generated in situ from 2-iodobenzoic acid, m-CPBA, and TfOH under catalytic reaction conditions, has been confirmed by ¹H NMR spectroscopy, ESI-mass spectrometry, and by X-ray diffraction.