Hydrogenation Involving Two Different Proton- and Hydride-Transferring Reagents through Metal–Ligand Cooperation: Mechanism and Scope

Abstract

Metal–ligand cooperation (MLC) allows cooperative action between active sites at both the metal and the ligand for transferring hydrogen to the substrate using hydride and proton transfer. Despite their utility in the development of green and sustainable synthetic transformations from a single source, these transfers using two different sources remain limited. Herein, we presented a bench-stable bifunctional 2,2′-bibenzimidazole (BiBzImH2)-based Ru­(II)-para-cymene for selective and efficient hydrogenation of unsaturated carbonyl/nitro to saturated carbonyl/nitro using hydride and proton transfer from two different sources (silane for hydride and methanol for proton) via substrate-mediated interconvertible coordination modes (imino N → Ru and amido N–Ru) of the active catalyst. Remarkably, the generation of Ru–H and the presence of N–H of the coordinated BiBzImH2 are important for the generation of interconvertible coordination modes, which in turn is not operative without the N–H. The initial rate kinetics under standard reaction conditions showed a broken positive order in the substrate, first order in the catalyst, and first order in the hydrogen donor (TES). Mechanistic studies, evaluated from spectroscopic, kinetic, Hammett study, kinetic isotope effects (KIEs), and a few other controlled experiments, further reveal that both solvent-mediated proton transfer via the interconvertible coordination mode and hydride transfer between the substrate-coordinated intermediate and Ru–H might be involved in two separate rate-determining steps. The catalyst demonstrated good efficiency, selectivity (>98%), and functional group tolerance and displayed a broad scope with unsaturated ketones and β nitrostyrenes, affording their saturated keto and nitro products with excellent selectivity and emphasizing its potential synthetic utility.

Ruthenium(II)-Catalyzed Hydrogenation and Tandem (De)Hydrogenation via Metal–Ligand Cooperation: Base- and Solvent-Assisted Switchable Selectivity

“…Synthesis and Characterization of Ligands. 30 1H,1′H-2,2′-Bibenzo [d]imidazole (L-1). L-1 was synthesized by the previously reported procedure with slight modification.…”

“…13 1,1′-Dimethyl-1H,1′H-2,2′-bibenzo [d]imidazole (L-2). 30 In a 50 mL round-bottom flask, 2 mM (470 mg) 1H,1′H-2,2′-bibenzo [d]imidazole (L-1) and 10 mM (240 mg) sodium hydride (60% dispersion in mineral oil) were stirred at ∼4−8 °C in 20 mL of DMF for 30 min. To this reaction mixture, 4 mM (250 μL) methyl iodide was added, and the reaction was continued for 01 h at ∼4−8 °C.…”

“…After that, the reaction mixture was stirred at room temperature (298 K) for 12 h. After completion of the reaction (via thin-layer chromatography (TLC) monitoring), water (500 mL) was added to the reaction mixture to quench the reaction, and the product was extracted with ethyl acetate (150 mL × 5), washed with water (100 mL × 3) and [d]imidazol-2-yl)thiazole (L-5). 30 The above-mentioned procedure was adopted for L-5 using the 4-(1Hbenzo [d]imidazol-2-yl)thiazole ligand (402 mg). Yield = 370 mg (92%), off-white powder, mp = 148−150 °C.…”

“…13 General Procedure for the Synthesis of Complexes. 30 A mixture of 0.25 mmol (150 mg) [Ru(para-cymene) Cl 2 ] 2 and 0.5 mM corresponding (L-1 to L-5) ligand was dissolved in 30 mL of methanol and refluxed in an oil bath at 80 °C for 3 h. After that, 1.0 mM (180 mg) KPF 6 was added to the reaction mixture, and the reaction was continued at 80 °C for an additional one hour. After that, the solvent was reduced to 5 mL, and diethyl ether was added to it to get the precipitate of the complexes.…”

“…L-1 was synthesized by the previously [d]imidazole (L-3). 30 The above-mentioned procedure was adopted for the synthesis of L-3 except using 5-methyl 1,2 diamino benzene (5.8 mM; 15.2 g) instead of 1,2 diamino benzene. Yield = 13.0 g (86%), Off White, mp = 348− 350 °C.…”