Nitrone or Oxaziridine? Further Insights into the Selectivity of Imine Oxidation Catalyzed by Methyltrioxorhenium

The oxidation of imines may give several products: oxaziridines, nitrones, amides, and other re-arranged compounds. Its selectivity is therefore a challenge which the various methods have to face. The controversial selectivity of the oxidation of imines with urea hydrogen peroxide (UHP) catalyzed by methyltrioxorhenium (MTO) is addressed, varying the solvent, temperature, reac-tion time, amount of oxidant and catalyst used. The reactivity and selectivity of the oxidation of imines proved to be particularly sensitive to the type of solvent. Use of methanol furnished the corresponding nitrones as the exclusive products, with the exception of very hindered N-tert-alkyl substituted substrates. Using the ionic liquid [bmim]BF4 as solvent resulted in a complete switch in reactivity and selectivity: N-methyl substituted imines gave the correspond-ing amides, while imines with bulkier substituents at nitrogen did not show any reactivity. A case apart was represented by C-phenyl,N-tert-butyl imine, the only substrate which was oxi-dized to the corresponding oxaziridine, albeit with low conversion. The results reported herein reaffirm the oxidation of imines with UHP/MTO in MeOH as the method of choice for their in-terconversion to nitrones.