Recent Progress in Homogeneous Catalytic Hydrogenation of Esters

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 gamma-valerolactone. A significant break-through has also been made in the catalytic asymmetric hydrogenation of esters to chiral primary alcohols. The asymmetric hydrogenations of ketoesters, racemic delta-hydroxyesters, and racemic alpha-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.