Mechanistic Studies of Ullmann-Type C-N Coupling Reactions: Carbonate-Ligated Copper(III) Intermediates

In Ullmann type C-N coupling reactions involvement of Cu(III) species was proposed many times considering the oxidative addition-reductive elimination (OA-RE) path for these reactions but actual species could not be traced out in experimental studies. In the C-N coupling reactions carbonate and phosphate were widely considered as base. In the present study, copper-mediated C-N coupling reactions of aryl halides and NuNH (amide and imide) were extensively investigated and present studies provide direct spectroscopic evidences of actual Cu(III) species. First time, these studies have also revealed that carbonate and phosphate ions act as bidentate ligand as well as base in the catalytic cycle and thus actual intermediate species is carbonate or phosphate ligated distorted octahedral Cu(III) complex. Present experimental and computational studies have strongly strengthened the hypothesis that these reactions follow OA-RE path.