Efficient Organoruthenium Catalysts for α‐Alkylation of Ketones and Amide with Alcohols: Synthesis of Quinolines via Hydrogen Borrowing Strategy and their Mechanistic Studies

Abstract

AbstractA new family of phosphine free organometallic ruthenium(II) catalysts (Ru1–Ru4) supported by bidentate NN Schiff base ligands (L1–L4 where L1=N,N‐dimethyl‐4‐((2‐phenyl‐2‐(pyridin‐2‐ylmethyl)hydrazineylidene)methyl) aniline, L2=N,N‐diethyl‐4‐((2‐phenyl‐2‐(pyridin‐2‐ylmethyl)hydrazineylidene)methyl)aniline, L3=N,N‐dimethyl‐4‐((2‐phenyl‐2‐(pyridin‐2‐yl)hydrazineylidene)methyl)‐ aniline and L4=N,N‐diethyl‐4‐((2‐phenyl‐2‐(pyridin‐2‐yl)hydrazineylidene)methyl) aniline) was prepared and characterized. These half‐sandwich complexes acted as catalysts for C−C bond formation and exhibited excellent performance in the dehydrogenative coupling of ketones and amides. In the synthesis of C–C bonds, alcohols were utilized as the alkylating agent. A broad range of substrates, including sterically hindered ketones and alcohols, were well tolerated under the optimized conditions (TON up to 47000 and TOF up to 11750 h−1). This ruthenium (II) catalysts were also active towards the dehydrogenative cyclization of o‐amino benzyl alcohol for the formation of quinolines derivatives. Various polysubstituted quinolines were synthesized in moderate to excellent yields (TON up to 71000 and TOF up to 11830 h−1). Control experiments were carried out and the ruthenium hydride intermediate was characterized to support the reaction mechanism and a probable reaction pathway of dehydrogenative coupling for the C−C bond formation has been proposed.