Iridium-Catalyzed Hydrogenation Using Phosphorus Ligands

Abstract

Due to the requirement of obtaining enantiomerically pure products through environmentally friendly technologies, the development of new methods in asymmetric catalysis is becoming an important approach to asymmetric synthesis. Asymmetric hydrogenation is one of the most applied catalytic reactions for the preparation of enantiomerically pure products. Iridium complexes containing phosphorus ligands have been less applied as catalysts in hydrogenation reaction than their corresponding rhodium complexes. On the other hand, most of the iridium complexes used as catalyst in hydrogenation reaction contain phosphorus–nitrogen ligands. However, iridium complexes containing a great variety of mono- and bidentate phosphorus ligands (phosphines, phosphinites, phosphites, phosphoramidites, and the combination of these functionalities) have been applied in hydrogenation reaction with different grades of success. In the early days, diphosphines were the most widely used phosphorus ligands. However, they are generally difficult to synthesize and prone to oxidation. In recent years, chiral phosphites, phosphinites, phosphonites, and phosphoroamidites have emerged as new types of ligands, which present several advantages and high catalytic activity and selectivity. Homogeneous and heterogeneous systems have been studied, allowing the recycling of catalysts. Slight modifications in the electronic and steric properties of the ligands influence the catalytic results, which indicates the potentiality of modular ligands. Imines, including unfunctionalized N–H imines and cyclic imines as quinolines and other substrates, have been transformed in the corresponding chiral amines with ee higher than 95% in many cases. Binol-derived phosphoroamidite constitute one of the most successful ligands in these asymmetric reaction achieving, in some cases, practically total enantioselectivity.