Inorganic Chemistry · 2020 · 97 citations · 86 references
Both imidazol-2-ylidene (ImNHC) and 1,2,3-triazol-5-ylidene (tzNHC) have evolved to be elite groups of N-heterocyclic carbene (NHC) ligands for homogeneous catalysis. To develop efficient ruthenium(II)-based catalysts incorporating these ligands for C-N bond-forming reactions via hydrogen-borrowing methodology, we utilized chelating ligands integrated with ImNHC and mesoionic tzNHC donors connected via a CH<sub>2</sub> spacer with a diverse triazole backbone. The synthesized ruthenium(II) complexes <b>3</b> are found to be highly efficient for C-N bond formation across a wide range of primary amine and alcohol substrates under solvent-free conditions, and among all of the complexes studied here, catalyst <b>3a</b> with a mesityl substituent displayed maximum activity. To our delight, catalyst <b>3a</b> is also effective for the selective mono-N-methylation of various anilines utilizing methanol as a coupling partner, known to be relatively more difficult than other alcohols. Furthermore, complex <b>3a</b> also delivers various substituted quinolines successfully via the reaction of 2-aminobenzyl alcohol with several secondary alcohols. Importantly, catalyst <b>3a</b> exhibited the highest activity among the reported ruthenium(II) complexes for both the N-benzylation of aniline [achieving a turnover number (TON) of 50000] and the realization of quinoline <b>8a</b> by reacting 2-aminobenzyl alcohol with 2-phenylethanol (attaining a TON of 30000).
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