Copper-catalyzed aerobic autoxidation of N-hydroxycarbamates probed by mass spectrometry

We present herein a mechanistic investigation by nanoelectrospray ionization mass spectrometry of copper-catalyzed aerobic oxidative processes involved in the N-nitrosocarbonyl aldol reaction of N-hydroxycarbamates. Protonated amine and copper as charge-tags aided the detection of reaction intermediates, which verified the enamine mechanism together with a competing enol process. Our experimental results reveal that the copper-catalyzed aerobic oxidation of N-hydroxycarbamates may proceed through an autoxidation catalytic mechanism in which a CbzNHO(.) radical abstracts a hydrogen from the bound N-hydroxycarbamate to release the nitroso intermediate through a bimolecular hydrogen-atom transfer. In this process, the chiral diamine also works as a ligand for copper to facilitate the aerobic oxidative step. The dual role of the chiral vicinal diamine as both an aminocatalyst and a bidentate ligand was finally uncovered.