Rhodium catalyzed asymmetric imine hydrogenation catalysts

Alkyl and aryl substituted imines have received the most attention as substrates for asymmetric hydrogenation, and the development of the field can therefore be outlined by examining their reductions. These are usually catalyzed by chiral complexes of titanium, ruthenium, rhodium, or iridium, though gold catalysts have also recently proven useful for this purpose [31]. New catalysts are generally tested for the reductions of substrates A-D (Scheme 6.1). 

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. 

The creation of an asymmetric center by C-H bond formation is a very common process which can involve several types of reactions. Hydrogenation of prochiral olefins is often used with the rhodium catalysts of the Wilkinson type (5). These catalysts were shown to be inactive for ketone or imine reduction except in some cases (15), It was then interesting to develop an alternate method for asymmetric synthesis of chiral alcohols or amines. Since it was found that RhCl(PPh3)3 was able to catalyze silane additions to ketones (16,17) or imines (18), preparation of chiral alcohols or amines by asymmetric hydrosilylation could be envisaged (Figure 2). The 1,4-addition of silanes to conjugated... 

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