A Mixed-Carbene-Donor Strategy for Co(III)-Catalyzed Selective Reduction of Quinolines

The selective hydrogenation of aromatic N-heterocycles is an important transformation in synthetic chemistry, as the resulting partially saturated derivatives are widely found in pharmaceuticals and bioactive molecules. In this context, quinolines are especially important, with 1,2,3,4-tetrahydroquinolines serving as the privileged scaffolds in various drugs. To achieve such selective hydrogenation, we herein describe the synthesis and characterization of a series of NHC-derived heteroditopic bidentate ligands, including a combination of imidazole-based N-heterocyclic carbene (ImNHC) with a triazole-derived "mesoionic" NHC (TzNHC) donor, supported by well-defined Co(III)-complexes. Among them, the mixed-NHC-based Co(III) catalyst enables efficient, additive-free transfer hydrogenation (TH) of diverse quinoline derivatives with excellent selectivity and tolerance to diverse functional groups. Mechanistic studies indicate that a cobalt-hydride intermediate is the active species, with the mixed donor carbene environment enhancing the hydride donor capability, enabling selective reduction.