Imines rhodium catalyzed asymmetric transfer

Table 6.11 Rhodium catalyzed asymmetric transfer hydrogenation of imines.

Compared to the rhodium-catalyzed stereoselective reactions, studies on the iridium-catalyzed reactions have been limited until recently. Usually lower selectivities have been observed in the Ir(i)-catalyzed reactions.459,460 The asymmetric hydrosilylation of imines affords optically active secondary amines. These are very valuable compounds, but the studies on that reaction are quite limited.461 Close examinations of these reactions revealed that they proceed via a transfer hydrogenation. Other conditions such as the 2-propanol/base system in the presence of an appropriate metal complex have been employed as well, but only low selectivities were obtained.462... 

There have been multiple efforts toward supported catalysts for asymmetric transfer hydrogenation, and the 4 position on the aryl sulfonate group of 26 has proven a convenient site for functionalization. Thus far, this ligand has been supported on dendrimers [181,182], polystyrenes [183], silica gel [184], mesoporous siliceous foam [185], and mesoporous siliceous foam modified with magnetic particles [186]. The resulting modified ligands have been used in combination with ruthenium, rhodium, and iridium to catalyze the asymmetric transfer of imines and, more commonly, ketones. 

Asymmetric Transfer Hydrogenation of Ketones. The first reports on asymmetric transfer hydrogenation (ATH) reactions catalyzed by chiral metallic compounds were published at the end of the seventies. Prochiral ketones were reduced using alcohols as the hydrogen source, and Ru (274,275) or Ir (276) complexes were used as catalysts. Since then, many chiral catalytic systems for homogeneous ATH of ketones, imines, and olefins have been developed (37,38,256,257,277-289). The catalytic systems are usually based on ruthenium, rhodium, or iridium, and the ATH of aryl ketones is by far the most studied. Because of the reversibility of this reaction, at high conversions, a gradual erosion of the ee of the product has been frequently reported. An azeotropic 5 2 mixture of formic acid/triethylamine can be used to overcome this limitation. 

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