Dinuclear Ru(II) Schiff Base Complex Catalyzed One-Pot Synthesis of Quinolines through Acceptorless Dehydrogenative Coupling of Secondary Alcohols with 2-Nitrobenzyl Alcohol

Dinuclear Ru(II) complexes [(p-cymene)2(RuCl)2L1]2X (X = BF4 (Ru1); X = PF6 (Ru2)) and mononuclear [(p-cymene)(RuCl)L2]BF4 (Ru3) (where L1 = N,N′-(3,3′,5,5′-tetraisopropyl-[1,1′-biphenyl]-4,4′-diyl)bis(1-(pyridin-2-yl)methanimine); L2 = N-(2,6-diisopropyl-phenyl)-1-(pyridin-2-yl)-methanimine) have been synthesized and characterized by spectroscopic and analytical techniques. Dinuclear Ru1 and Ru2 orchestrate direct transformation of 2-nitrobenzyl alcohols to quinolines under mild conditions with significant efficiency even when employed at a minimal catalyst loading of 0.1 mol %. Proportional experiments carried out with the corresponding mononuclear complex Ru3 by keeping the Ru content the same (0.2 mol % of Ru3) reveal superior activity by the bimetallic system Ru1 for the one-pot quinoline synthesis. Late-stage functionalization of bioactive steroids and scale-up synthesis demonstrate the practical applicability of the present catalyst system. A probable mechanism of this conversion is proposed based on trapping of many of the intermediates by ESI-mass spectroscopy. These mechanistic studies have further been substantiated by ReactIR studies by monitoring the progress of the reaction in real time.