The reduction of esters to alcohols is one of the most important chemical transformations in the production of fine chemicals, such as pharmaceuticals, agricultural chemicals, fragrances, and biofuels. Homogeneous catalytic hydrogenation of esters represents a green, atom-economic, and sustainable alternative to conventional stoichiometric approaches, avoiding the generation of large amount of wastes and the difficulties arose in work-up procedure by using metal hydride reductants. Although challenges still exist, significant progress has been made in catalytic hydrogenation of esters over the last ten years. Numerous transition metal catalysts including noble metal (such as ruthenium, osmium, and iridium) complexes and base metal catalysts (such as iron, cobalt, and manganese) have been developed for the hydrogenation of esters. The ligands of the catalysts have been well studied. A wide range of bidentate ligands including diamines, amino-phosphines, pyridine-amines, N-heterocyclic carbene-amines, and bipyridines, tridentate pincer ligands containing diethylamine and pyridine skeletons, tetradentate ligands containing pyridine and bipyridine skeletons have been applied in the hydrogenation of esters. The efficiency of hydrogenation of esters has been significantly improved, and the highest turnover number (TON) reached 90000 for the hydrogenation of benchmark substrates such as ethyl acetate, ethyl benzoate, and γ-valerolactone. A significant breakthrough has also been made in the catalytic asymmetric hydrogenation of esters to chiral primary alcohols. The asymmetric hydrogenations of ketoesters, racemic δ-hydroxyesters, and racemic α-aryl/alkyl substituted lactones provided efficient methods for the asymmetric synthesis of optically active chiral diols including chiral 1,5-diols and 1,4-diols. The significant progress achieved in recent years in the area of homogeneous catalytic hydrogenation of esters to alcohols is presented in this review. The focus of this review are the development of ligands and catalysts, and the advances in the catalytic asymmetric hydrogenation of esters and lactones.

The hydrogenation of esters catalyzed by a manganese complex of phosphine‐aminopyridine ligand was developed. Using this protocol, a variety of (hetero)aromatic and aliphatic carboxylates including biomass‐derived esters and lactones were hydrogenated to primary alcohols with 63–98% yields. The manganese catalyst was found to be active for the hydrogenation of methyl benzoate, providing benzyl alcohol with turnover numbers (TON) as high as 45,000. Investigation of catalyst intermediates indicated that the amido manganese complex was the active catalyst species for the reaction.

Manganese complexes with tridentate PNN ligands have been synthesized as catalysts for hydrogenative cross-coupling reaction of nitriles and amines to form secondary imines. This reaction afforded a variety of unsymmetrical secondary imines in good yields with excellent selectivity. Investigation of catalyst intermediates indicated that an amido manganese complex may be the active catalyst species for this reaction.

Abstract:The redox-active ligand bis(2-isopropylaminophenyl)amide (NNN) was used to prepare ionic yttrium complex+ (2; sq = semiquinonate form). A solidstate magnetic measurement on 2 (μ eff = 2.40 μ B ) indicated the presence of two unpaired electrons on two separate NNN ligands. The paramagnetic compound 2 was characterized by EPR spectroscopy in a dilute frozen solution of thf, which sug-