Diphosphine-rhodium, chiral catalyst

Borner, A., The effect of internal hydroxy groups in chiral diphosphine rhodium(I) catalysts on the asym metric hydrogenation of functionalized olefins, Eur. J. Inorg. Chem., 2001, 327. 

Finally, Sinou et al. have shown that rhodium(I) catalysts formed with various chiral sulfonated diphosphines were efficient catalysts for the... 

Monsanto (2) A catalytic process for synthesizing the drag L-DOPA. The catalyst is a chiral diphosphine-rhodium complex. Invented in the early 1970s. 

The development of chiral catalysts for use in enantioselective rhodium-catalyzed hydroborations was pioneered by Burgess9, Suzuki,77 and Hayashi.78 The chiral diphosphine ligands employed in their preliminary investigations 23-26 (Figures 2(a) and 2(b)), had previously been successfully applied in other catalytic asymmetric transformations. 

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. 

Asymmetric Hydrogenation. Rhodium complexes of the type Rh(diene)(diphos )+, where diphos is a chiral bidentate diphosphine ligand, are catalyst precursors for the asymmetric hydrogenation of certain prochiral olefins (15). Asymmetric hydrogenation of a-acylaminoacrylates, for example, affords chiral amino acid derivatives, some of which have medicinal utility such as L-DOPA. 

N-acyldehydrodipeptides were readily prepared either by the condensation of N -acyldehydro-a-amino acids with a-amino acid esters or by the reaction of the azlactones of dehydro-a-amino acid with a-amino acid esters (eq. 1). Asymmetric hydrogenation of the N-acyldehydrodipeptides thus obtained (eq. 2) was carried out by using rhodium complexes with a variety of chiral diphosphines such as -Br-Phenyl-CAPP (3), Ph-CAPP (3), (-)BPPM (4), (+)BPPM (4), (-)DIOP ( ), (+)DIOP ( ), diPAMP (6), Chiraphos (7), Prophos (S), BPPFA (9) and CBZ-Phe-PPM (Fig. 1)(10). The chiral catalysts were prepared in situ from chiral diphosphine ligand with [Rh(NBD)2l -CIO4 (NBD = norbomadiene). Typical results are summarized in Tables I-V. 

Linear polystyrene has also been used to support asymmetric hydrogenation catalysts containing chiral diphosphine rhodium(I) complexes (50). Asymmetric hydrogenations of itaconic acid were carried out, forming (R)-2-raethylbutanedioic acid with e.e. s ranging from 20-37%. None of the polymer-bound catalysts were more effective than (-)-DIOP-RhCl and the observed e.e. s were found to be dependent on the molecular weight of the polymer chain, its raicrostructure and solubility. 

Rhodium-Diphosphine Catalysts. The mechanism of rhodium-catalyzed asymmetric hydrogenation is one of the most intensively investigated and best understood. Reaction pathways have been accurately studied both experimentally and theoretically (138,162,213-221). In early studies, Halpern (222) and Brown (214) established that the hydrogenation proceeds according to the reaction sequence presented in Figure 51 for the hydrogenation of a dehydroamino acid with a chiral diphosphine-rhodium complex. Many variants on both catalyst and reactant have been described. Stereoselectivity takes place via the difference in reactivity of the involved diastereomeric square-planar... 

The ultimate variant of this approach was obtained recently by G. M. Whitesides, from M.I.T. (16, 17). He constructed an asymmetric hydrogenation catalyst based on embedding an achiral diphosphine-rhodium(I) moiety at a specific site in a protein. In this case the protein tertiary structure provides the chirality required for enantioselective hydrogenation. 

Asymmetric Hydrogenation.—The asymmetric hydrogenation of a-acylamino-acrylates and cinnamates using chiral rhodium(i) diphosphine complexes as catalysts is now established as one of the best methods for obtaining optically pure a-amino-acids (see previous reviews in this series). In the past year, some new chiral diphosphines have been added to the already considerable number of such ligands. A bis(diphenylphosphino)-derivative of pyrrolidine in conjunction with Rh can be used to hydrogenate a-acetamidocinnamates and itaconic acid with chiral inductions of 90%, whereas an Rh -diphos complex derived from natural tartaric acid effects the reduction of some a-acylaminoacrylic acids to natural (5)-a-acylamino-acids with optical yields of between 80 and 100%. ... 

Boron Intramolecular, enantioselective hydroboration of the normally unreactive A -het-erocyclic carbene (NHC)-boranes, such as (84), has been attained by B-H activation in the presence of rhodium(I) catalyst and the chiral diphosphine ligand (86). The cyclic boranes (85) thus obtained were of <98% eeP... 

Similar heterogeneous chiral catalysts were prepared by the impregnation of mesoporous Al-MCM-41, Al-MCM-48, and Al-SBA-15 with rhodium diphosphine organometaUic complexes and were tested for the hydrogenation of dimethyl itaconate, methyl a-acetamidoacrylate, and methyl a-acetamidocinnamate [90]. The immobilized catalysts showed high activity and excellent chemo- and enantioselec-tivities, that is, up to >99% conversion, 99% selectivity, and 98% ee. 

The most effective catalysts for enantioselective amino acid synthesis are coordination complexes of rhodium(I) with 1,5-cyclooctadiene (COD) and a chiral diphosphine such as (JR,jR)-l,2-bis(o-anisylphenylphosphino)ethane, the so-called DiPAMP ligand. The complex owes its chirality to the presence of the trisubstituted phosphorus atoms (Section 9.12). 

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