The preparation of N-tert-butyloxycarbonyl-(Boc-)protected sulfoximines and sulfimines by an iron(II)-mediated nitrene transfer from BocN(3) to sulfoxides and sulfides

The imidation of sulfides and sulfoxides to the corresponding sulfimides an d sulfoximides was carried out with N-tertbutyloxycarbonyl azide (BocN(3)) in the presence of FeCl2. Sulfoxides 1 reacted at room temperature in CH2Cl 2 to give the corresponding sulfoximides 3 in 40-95% yield. The imidation o f the sterically congested substrate tert-butyl methyl sulfoxide (1f) proce eded sluggishly (10% yield). The sterospecificity of the reaction was demon strated with the enantiomerically enriched substrates (R)-(+)-1b and (S)-(- )Id which yielded the sulfoximides (R)-(+)-3b and (S)-(-)-3d with retention of configuration. Mechanistically, an intermediate (nitrene)Fe-IV complex is postulated as the reactive nitrene transfer reagent which is formed from FeCl2 and BocN(3). The more nucleophilic sulfides 2 reacted more readily i n the imidation than sulfoxides. Their conversion to the corresponding sulf imides 4 was conducted with BocN(3) and a substoichiometric amount of FeCl2 (0.25 equiv.). Yields ranged between 44 and 92%. In an alternative reactio n mode, BocN(3) was utilized at 0 degrees C in the presence of FeCl2 and ac etyl acetone. The sulfimidation, which did not otherwise occur at this temp erature, was accelerated by the ligand (36-90% yield).