Catalyst, Chiral rhodium

Efficient enantioselective asymmetric hydrogenation of prochiral ketones and olefins has been accompHshed under mild reaction conditions at low (0.01— 0.001 mol %) catalyst concentrations using rhodium catalysts containing chiral ligands (140,141). Practical synthesis of several optically active natural... 

A. Rhodium Catalysts with Chiral Phosphines 1. Olefinic Substrates... 

The review of Morrison et al. (10) traces the development of the use of rhodium-chiral phosphine catalysts to about the end of 1974. This field was initiated by the suggested incorporation (216) of chiral phosphines, instead of triphenylphosphine, into the so-called Wilkinson catalyst, RhCl(PPh3)3 (Section II,A), or into closely related systems. Horner s group (217, 218) used such catalysts, formed in situ in benzene... 

Besides the major thrust using chiral phosphine catalysts, other chiral ligands have been used with rhodium and other metals for asymmetric hydrogenation. 

Asymmetric cyclization-hydrosilylation of 1,6-enyne 91 has been reported with a cationic rhodium catalyst of chiral bisphosphine ligand, biphemp (Scheme 30).85 The reaction gave silylated alkylidenecyclopentanes with up to 92% ee. A mechanism involving silylrhodation of alkyne followed by insertion of alkene into the resulting alkenyl-rhodium bond was proposed for this cyclization. 

Some neutral rhodium catalysts with chiral ligands, such as MCCPM 9 (see Scheme 33.3) [20c], Cy,Cy-oxoProNOP 15, and Cp,Cp-IndoNOP 18, demonstrate excellent enantioselectivities and reactivities in the hydrogenation of a-ketoesters and ketoamides indeed, they compare well with ruthenium-based catalysts (Table 33.2). Togni et al. have successfully used the Josiphos 47 ligand for the hydrogenation of ethyl acetoacetate [27], while the use of MannOPs has led to somewhat high enantioselectivities [18]. 

Other hydrogenation methods are less chemoselective. Use of Raney nickel provides hydroxylamines in low yield °. Hydrogenation of 1-acetonaphthone oxime over rhodium-chiral phosphine catalysts was found to proceed under harsh conditions and provided low... 

An efficient asymmetric hydrogenation of a-acetamidocin-namic acids has been achieved using a rhodium-chiral phosphine catalyst. This paper describes the preparation of the catalyst and the hydrogenation procedure as well as studies on structure vs. activity. 

II. Homogeneous Rhodium-Chiral Phosphine Catalyst Systems. 85... 

FIG. 20. Synthesis of rhodium catalysts containing chiral amide ligands. 

If some of the ligands bonded to the metal atom in a homogeneous catalyst are chiral, then the hydrogenation can, in theory, produce an excess of one enantiomer of the reduction product. One catalyst that has been found to be effective in such an asymmetric hydrogenation reaction is this chiral rhodium complex ... 

Chiral hydrogenation catalysts. Rhodium and ruthenium phosphines are effective homogeneous catalysts for hydrogenation. Chiral ligands can be attached to accomplish asymmetric induction, the creation of a new asymmetric carbon as mostly one enantiomer. 

The catalyst is a cationic complex of rhodium with another diphosphine, DIPAMP. DIPAMP s chirality resides in the two stereogcnic phosphorus atoms unlike amines, phosphines are configurationally stable, rather like sulfoxides (which we will discuss in the next chapter). The catalyst imposes chirality on the hydrogenation by coordinating to both the amide group and the double bond of the substrate. Two diastereoisomeric complexes result, since the chiral catalyst can coordinate to either of the enantiotopic faces of the double bond. 

Successively a rhodium chiral biphosphine catalyst ([Rh(l,5-COD)bisphosphine] + BF allowed better selectivity in reducing a-acylaminoacrylic acids (95-96% e.e.)193. 

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