Dinuclear Ru(II) Schiff Base Complex Catalyzed One-Pot Synthesis of Quinolines through Acceptorless Dehydrogenative Coupling of Secondary Alcohols with 2-Nitrobenzyl Alcohol
Gopal Deshmukh, Santosh J. Gharpure, Ramaswamy Murugavel
Abstract
Dinuclear Ru(II) complexes [( p -cymene) 2 (RuCl) 2 L 1 ]2X (X = BF 4 ( Ru1 ); X = PF 6 ( Ru2 )) and mononuclear [( p -cymene)(RuCl)L 2 ]BF 4 ( Ru3 ) (where L 1 = N, N ′-(3,3′,5,5′-tetraisopropyl-[1,1′-biphenyl]-4,4′-diyl)bis(1-(pyridin-2-yl)methanimine); L 2 = 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.