Coupling of Aromatic N-Heterocycles Mediated by Group 3 Complexes

Group 3 η2-N,C-pyridyl complexes supported by a ferrocene-diamide ligand have been known to mediate the coupling of η2-N,C-pyridyl and coordinated-pyridine ligands with a concomitant dearomatization of the pyridine ligand. Examples reported previously by us were limited to a few cases. In order to investigate the scope of the coupling reaction, various η2-N,C-pyridyl scandium complexes were isolated and characterized. Their reactivity toward other aromatic N-heterocycles is presented along with the characterization of the subsequent reaction products. The coupling reaction is favored by ortho substitution, the presence of fused aromatic rings on the pyridine ligand, and chelating substrates. In one instance, the product of the coupling reaction between a scandium η2-N,C-pyridyl complex and 7,8-benzoquinoline could not be isolated because a subsequent isomerization reaction was favored. The coupling reaction is not restricted to η2-N,C-pyridyl fragments, and it proceeds also from CH2 groups bound to the metal center and connected to a pyridine ligand. The reaction between η2-N,C-imidazolyl scandium complexes and 2,2′-bipyridine is also discussed.