Hydrogenation of Benzonitrile to Benzylamine Catalyzed by Ruthenium Hydride Complexes with P−NH−NH−P Tetradentate Ligands:  Evidence for a Hydridic−Protonic Outer Sphere Mechanism

Abstract

The reaction of RuHCl(PPh3)3 with the tetradentate ligand [PPh2((ortho-C6H4)CH2NHCH2 - )]2 {ethP2(NH)2} in THF produces the new complex trans-RuHCl{ethP2(NH)2} (1) as a mixture of two isomers. The complex RuHCl{ethP2(NH)2} (1) when activated with KOtBu/KH is a very active catalyst for the hydrogenation of benzonitrile to benzylamine in toluene, more active than the known catalyst Ru(H2)2H2(PCy3)2 (2). A mixture of 1 and 2 and base also results in efficient conversion of benzonitrile to benzylamine. The complex RuHCl{tmeP2(NH)2} (3) where tmeP2(NH)2 is [PPh2((ortho-C6H4)CH2NHCMe2 - )]2 is a less active catalyst for this reaction. These catalyst systems are air sensitive and extremely moisture sensitive. Experimental and theoretical (DFT) evidence is presented for a new mechanism for nitrile hydrogenation: the successive hydrogenation of the CN triple bond and then the CN double bond of the intermediate imine by H+/H- transfer from a trans dihydride active catalyst. The amido complex RuH{tmeP2N(NH)} (4) has similar activity to 3/base for the base-free hydrogenation of benzonitrile and is moderately active for the catalytic hydration of benzonitrile. The new complex RuH(BH4){ethP2(NH)2} (7) was prepared by reacting 1 with NaBH4 but is found to be a poor catalyst for nitrile hydration.

Alkene Metathesis Catalysis: A Key for Transformations of Unsaturated Plant Oils and Renewable Derivatives

“…Takemoto et al [58] and Li et al [59] reported examples of homogeneous hydrogenation of nitriles into primary amines when alkoxide were added to ruthenium catalyst. Das et al [60] and Enthaler et al [61,62] also showed that addition of t BuOK and phosphine to simple ruthenium complex led to the fast hydrogenation of various nitriles and by replacing the phosphine ligand by a N-heterocyclic carbene, milder conditions could be operating [63].…”

Metal-catalyzed nitrile hydration reactions: The specific contribution of ruthenium

“…There was no noticeable decrease in the rate of the hydration when Hg(0) was added to the reaction mixtures, which suggested to the authors -10 -that, despite decomposition of the cluster occurs with electron-rich nitriles, the catalytically active species remains homogeneous. Concerning the mononuclear complex 9, it was exclusively applied to the transformation of benzonitrile into benzamide [21]. Compared to cluster 8, the activity of 9 was remarkably lower (TOF < 1 h -1 ) under similar reaction conditions (acetone, 80 ºC, 0.5 mol% of Ru, 10 equiv.…”

A Versatile Ru Catalyst for the Asymmetric Transfer Hydrogenation of Both Aromatic and Aliphatic Sulfinylimines

“…[23] [Ru(h 6 -arA C H T U N G T R E N N U N G ene)-A C H T U N G T R E N N U N G H(NH 2 CHPhCHPhNHTs)] was found to hydrogenate ketones through the bifunctional metal-ligand outer-sphere mechanism. [24] The reaction mechanism for the Shvo catalyst was up for debate. [25,26] A non-concerted mechanism has also been proposed in the literature.…”