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

摘要

In Ullmann-type C-N coupling reactions, the involvement of Cu-III species has been proposed many times on the basis of the oxidative addition-reductive elimination (OA-RE) path for these reactions, but actual species could not be traced in experimental studies. In the C-N coupling reactions, carbonate and phosphate were considered widely as bases. In the present study, Cu-mediated C-N coupling reactions of aryl halides and NuNH (amide and imide) were investigated extensively, and we provide direct spectroscopic evidence of actual Cu-III species. For the first time, we reveal that carbonate and phosphate ions act as bidentate ligands as well as a base in the catalytic cycle, and thus the actual intermediate species is a carbonate- or phosphate-ligated, distorted octahedral Cu-III complex. Our experimental and computational studies have strengthened the hypothesis that these reactions follow an OA-RE path.