Development of selective intramolecular C–H amidation reactions via novel cobalt catalyst system

Abstract

Transition metal-catalyzed C–H amidation has developed as useful tool to synthesize various nitrogen-prevalent molecules. Recently, earth-abundant and less expensive first-row transition metal catalyst such as cobalt has drawn attention in C–H functionalization instead of precious second- and third-row homologues. Important challenge is introduced by distinct physicochemical properties of cobalt. In this thesis, Cp*Co(III)LX catalyst system having piano-stool ligand demonstrated versatile intramolecular C–H amidation. The cobalt complexes were easy to prepare and bench-stable, thus offering a convenient reaction protocol. The catalytic reactivity was significantly improved by the electronic tuning of the bidentate LX ligands. The optimal catalyst system catalyzed both C(sp$^2$)–H and C(sp$^3$)–H bond to synthesize various 5-membered and 6-membered products. The comparatively short ionic radius of cobalt played an important role in regioselective C–H amidation induced by steric effect of substituents and ligands. The observation was rationalized by the conformational analysis and DFT calculations of the transition states.