Research On Novel Transfer Hydrogenation Reactions And Their Applications

Transfer hydrogenation, which uses small molecules other than H2as hydrogen source for catalytic reduction, has the merit of operational simplicity and avoiding the use of hazardous hydrogen gas. However, the activities and selectivities of transfer hydrogenation systems are generally lower than hydrogenation. Since the1950s, great effort has been devoted to developing various transfer hydrogenation systems. Among the catalytic systems discovered, the Noyori metal-ligand bifunctional ruthenium catalysts have attracted the most attention. Many new transfer hydrogenation systems have been developed based on these catalysts or the metal-ligand bifuntion concept. Transfer hydrogenation catalysts without metal-ligand bifunction are also emerging, but fewer. Effects have also been made to develop aqueous transfer hydrogenation systems, which use the abundant, cheap and green water as solvent.Charpter II describes the use of cyclometalated iridium complexes for transfer hydrogenation of carbonyl groups in water. By controlling the solution pH, cyclometalated iridium complexes can be "switched on" to function as excellent catalysts for transfer hydrogenation of carbonyl compounds in water, with no need for organic solvents. This catalyst system is not only capable of the reduction of different ketones, but also aldehydes. These catalysts have simple and modular ligands, operate in a mechanism different from most of the current transfer hydrogenation catalysts, and offer new opportunities for developing more enabling and versatile catalysts for hydrogenation and other reactions in water or conventional solvents.Levulinic acid has been identified as a platform chemical from biomass-derived products, and can be transformed into pyrrolidinones via reductive amination. Charpter III reports a highly efficient transformation of levulinic acid into pyrrolidinones by iridium catalysed transfer hydrogenation. By controlling the pH of the HCOOH/HCOONa solution, a high conversion of reductive amination can be achieved in water. The system works for both aromatic and aliphatic amine, affording pyrrolidinones from levulinic acid.5-Oxohexanoic acid reacts with different amines to produce six membered heterocycles with this system. Mechanistic study was carried out to determine the rate-limiting step of the reductive amination reaction. This mild and green system provides a practical means for converting biomass derived chemicals into value added products.All the literature-reported examples of transformation of levulinic acid to pyrrolidinone, including the work described in Chapter III, use precious metals-based catalyst. Chapter IV showcases a catalyst-free transformation of levulinic acid into pyrrolidinones with formic acid. The solvent DMSO is critical for the reaction to proceed. Addition of Et3N can help to adjust the solution acidity, and improve the conversion. In this system, aromatic amines are less active than aliphatic ones. Mechanistic studies suggest that the rate-limiting step for the reaction is hydride transfer from formic acid to the iminium intermediate. The method described here is an extension to the classic Leuckart-Wallach reaction and provides a practical and economic way to convert LA to value-added chemicals.Among various methods for the preparation of aromatic amines, the reduction of nitroarenes is the mostly adopted method. Despite its development, homogeneous transfer hydrogenation of nitroarenes is still a challenge and successful examples are few. Chapter V develops a system for transfer hydrogenation of nitroarenes to corresponding amines or formanilides. Combining [Cp*RhCl2]2and KI and, using formic acid as hydrogen donor, nitro compounds can be reduced efficiently. By controlling the reaction conditions, amines and formanilides can been obtained separately in good yields. The protocol has a good tolerance and selectivity for the carbonyl and halogen groups. The iodide anion, I-helps to accelerate the reaction.