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.-amino alcohols to quinolines and pyridines. Ru(III) complexes [LRuCl3] (L= 6-(3-R1,5-R2-1H-pyrazol-1-yl)-N-(pyridin-2-yl) pyridin-2-amine; 1a: R1 = R2 = H (L1); 1b: R1= R2= Me (L2); 1c: R1= H, R2= CF3 (L3); 1d: R1= H, R2= Ph (L4); 1bMe: L= 6-(3,5-dimethyl-1H-pyrazol-1-yl)-N-methyl-N-(pyridin- 2-yl) pyridin-2-amine (L2Me)) were obtained by refluxing RuCl3 center dot xH2O with the corresponding ligand in EtOH. Five Ru(II) complexes [LRu(DMSO-kS) Cl2] (2a: L= L1; 2b: L= L2; 2c: L= L3; 2d: L= L4; 2bMe: L= L2Me) were formed by reducing the corresponding Ru(III) complex in refluxing EtOH. The latter complexes could also be prepared directly by refluxing Ru (DMSO) 4Cl2 with the corresponding ligand in EtOH. These Ru(III) and Ru(II) complexes, especially 1b/2b, exhibited high catalytic efficiency and broad functional group tolerance in ADC reactions of secondary alcohols with 2-aminobenzyl or.-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.