Rhodium-ferrocenyl-diphosphine

A broad screening of ligands and ionic liquids was carried out by Feng et al. [104]. For rhodium-catalyzed hydrogenation of enamides the best catalysts were found to be the rhodium-ferrocenyl-diphosphine complexes with taniaphos, josiphos, walphos and mandyphos as ligands (Fig. 41.9). 

The first example of asymmetric rhodium-catalyzed hydrogenation of prochi-ral olefins in dendrimer catalysis was reported by Togni et al., who immobilized the chiral ferrocenyl diphosphine Josiphos at the end groups of dendrimers, thus obtaining systems of up to 24 chiral metal centres in the periphery (Fig. 2) [12-14]. The fact that the catalytic properties of the dendrimer catalysts were almost identical to those of the mononuclear catalysts was interpreted as a manifestation of the independence of the individual catalytic sites in the macromolecular systems. 

Valuable intermediates 78 for a novel synthesis for (+)-biotin (79), which is referred to as vitamin H, are obtained effectively by use of a rhodium catalyst with the ferrocenyl diphosphine 70 (Scheme 24) [80]. The optimized process is suitable for industrial production. 

The carbon-carbon double bond of an enamine is also applicable for asymmetric hydrogenation leading to chiral amino acids. For example, hydrogenation of 13 by rhodium catalyst with ferrocenyl diphosphine 15 as a ligand was successful for the synthesis of methyl 3-amino-4-polyfluorophenylbutanoate 14 with excellent stereoselectivity (see Scheme 9.5) [15]. 

Many chiral ferrocenyl diphosphines (Fig. 45) are effective for the rhodium-, iridium-, or ruthenium-catalyzed hydrogenation of dehydroamino acids and related compounds, as well as imines (145,166-172). 

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