Phosphine Ligand-Free Ruthenium Complexes as Efficient Catalysts for the Synthesis of Quinolines and Pyridines by Acceptorless Dehydrogenative Coupling Reactions

A series of phosphine-free Ru(III)/Ru(II) complexes of NH functionalized N ^˄ N ^˄ N pincer ligands exhibit excellent activity for acceptorless dehydrogenative coupling (ADC) of secondary alcohols with 2-aminobenzyl or γ-amino alcohols to quinolines and pyridines. Ru(III) complexes [LRuCl ₃ ] (L=6-(3-R ¹ ,5-R ² -1H-pyrazol-1-yl)-N-(pyridin-2-yl)pyridin-2-amine; 1 a: R ¹ =R ² =H (L1); 1 b: R ¹ =R ² =Me (L2); 1 c: R ¹ =H, R ² =CF ₃ (L3); 1 d: R ¹ =H, R ² =Ph (L4); 1b ^Me : L=6-(3,5-dimethyl-1H-pyrazol-1-yl)-N-methyl-N-(pyridin-2-yl)pyridin-2-amine (L2 ^Me )) were obtained by refluxing RuCl ₃ ⋅ xH ₂ O with the corresponding ligand in EtOH. Five Ru(II) complexes [LRu(DMSO-κS)Cl ₂ ] (2 a: L=L1; 2 b: L=L2; 2 c: L=L3; 2 d: L=L4; 2b ^Me : L=L2 ^Me ) were formed by reducing the corresponding Ru(III) complex in refluxing EtOH. The latter complexes could also be prepared directly by refluxing Ru(DMSO) ₄ Cl ₂ with the corresponding ligand in EtOH. These Ru(III) and Ru(II) complexes, especially 1 b/2 b, exhibited high catalytic efficiency and broad functional group tolerance in ADC reactions of secondary alcohols with 2-aminobenzyl or γ-amino alcohols to quinolines and pyridines. A detail mechanistic study indicated the Ru(III) complex was reduced into the Ru(II) species, which is the active catalytic center for ADC via a Ru−H/N−H bifunctional outer-sphere mechanism. This protocol provides a reliable, atom-economical and environmentally benign procedure for C−N and C−C bond formation.