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

Dedicated to Professor P. Selvam on the occasion of 65th Birthday.

Abstract

We report an investigation of the 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) and L² = N-(2,6-diisopropylphenyl)-1-(pyridin-2-yl)methanimine). At a low catalyst loading of 0.01 mol% (Cat2/Cat3), a broad range of substrates, comprising aromatic as well as aliphatic ketones and aldehydes, undergo the TH reaction in a short reaction time of just 10 minutes. Additionally, chemoselective hydrogenation of nitroaromatic compounds is achieved under microwave irradiation in the presence of Cat2 within 5 minutes. Control experiments demonstrate that microwave heating conditions outperform conventional heating in terms of improved catalytic activity and reaction efficiency. The bimetallic catalyst Cat2 is used at a very low loading of 0.001 mol% to achieve high TONs and TOFs of 7.7 × 10⁴ and 2.3 × 10⁵ h^–1, respectively, for the TH reaction. Spectrometry experiments involving trapping of intermediates are used to propose a mechanism for the TH of the carbonyl compounds.

Key words

bimetallic - microwave - transfer hydrogenation - ruthenium - mechanism

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