The preparation of N-tert-butyloxycarbonyl-(Boc-)protected sulfoximines and sulfimines by an iron(II)-mediated nitrene transfer from BocN₃ to sulfoxides and sulfides

The imidation of sulfides and sulfoxides to the corresponding sulfimides and sulfoximides was carried out with N-tert-butyloxycarbonyl azide (BocN₃) in the presence of FeCl₂. Sulfoxides 1 reacted at room temperature in CH₂Cl₂ to give the corresponding sulfoximides 3 in 40-95% yield. The imidation of the sterically congested substrate tert-butyl methyl sulfoxide (1f) proceeded sluggishly (10% yield). The sterospecificity of the reaction was demonstrated with the enantiomerically enriched substrates (R)-(+)-1b and (S)-(-)1d 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 BocN₃. The more nucleophilic sulfides 2 reacted more readily in the imidation than sulfoxides. Their conversion to the corresponding sulfimides 4 was conducted with BocN₃ and a substoichiometric amount of FeCl₂ (0.25 equiv.). Yields ranged between 44 and 92%. In an alternative reaction mode, BocN₃ was utilized at 0°C in the presence of FeCl₂ and acetyl acetone. The sulfimidation, which did not otherwise occur at this temperature, was accelerated by the ligand (36-90% yield).