Microwave-Assisted Transfer Hydrogenation of Carbonyl and ­Nitro Compounds Using Bimetallic Ru(II) Cymene Complexes

Abstract

We report on the investigations of microwave-assisted catalytic transfer hydrogenation (TH) of carbonyl and nitro compounds by employing Ru(II) complexes bimetallic [(p-cymene)2(RuCl)2L1]2X (X = BF4 (Cat2); X = PF6 (Cat3)) and mononuclear [(p-cymene)(RuCl)L2]BF4 (Cat4) (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). At a low catalyst loading of 0.01 mol% (Cat2/Cat3), a broad substrate scope has been achieved for aromatic as well as aliphatic ketones and aldehydes, with a short reaction time of just 10 minutes. Additionally, chemoselective hydrogenation of nitroaromatic compounds has been achieved under microwave irradiation by Cat2 within 5 minutes. Control experiments demonstrate that microwave heating conditions outperform conventional heating method in terms of improved catalytic activity and reaction efficiency. The bimetallic Cat2 catalyst can be used at a very low loading of 0.001 mol% to achieve the high TONs and TOFs of 7.7 × 104 and 2.3 × 105 h-1, respectively, for TH reaction. Spectrometry experiments for intermediate trapping have been used to propose a probable mechanism for TH of carbonyl compounds.