Rhodium diphosphine

It is interesting to note that using the sol-gel procedure (I) the pre-formation of the rhodium diphosphine complex suppressed the formation of ligand free rhodium-cations on the silica surface. This approach gave rise to a well-defined, very selective hydroformylation catalyst. All immobilised catalysts were 10 to 40 times slower than the homogeneous catalyst under the same conditions, the sol-gel procedure yielding the fastest catalyst of this series. 

Acetylenic esters react with arylboron reagents in the presence of rhodium diphosphine catalyst to give cyclic ketones.409 Equation (61) shows an example which may involve ortfe-metallation and ketone formation. A catalytic, enantioselective reaction was also achieved (Equation (62)). These processes presumably involve unprecedented addition of organorhodium species to the ester carbonyl group. 

Rhodium diphosphine catalysts can be easily prepared from [Rh(nbd)Cl]2 and a chiral diphosphine, and are suitable for the hydrogenation of imines and N-acyl hydrazones. However, with most imine substrates they exhibit lower activities than the analogous Ir catalysts. The most selective diphosphine ligand is bdppsuif, which is not easily available. Rh-duphos is very selective for the hydrogenation of N-acyl hydrazones and with TOFs up to 1000 h-1 would be active enough for a technical application. Rh-josiphos complexes are the catalysts of choice for the hydrogenation of phosphinyl imines. Recently developed (penta-methylcyclopentyl) Rh-tosylated diamine or amino alcohol complexes are active for the transfer hydrogenation for a variety of C = N functions, and can be an attractive alternative for specific applications. 

In summary, the asymmetric hydrogenation of olefins or functionalized ketones catalysed by chiral transition metal complexes is one of the most practical methods for preparing optically active organic compounds. Ruthenium and rhodium-diphosphine complexes, using molecular hydrogen or hydrogen transfer, are the most common catalysts in this area. The hydrogenation of simple ketones has proved to be difficult with metallic catalysts. However,... 

The rhodium-diphosphine catalysts are generally sensitive to oxygen, hence the reactions have to be carried out under strictly inert atmospheric conditions. A decrease in the yield or the enantiomeric excess can be due to a lack of sufficient precaution during the procedure or to the inactivation of the catalyst when exposed to oxygen. However, the reactions using rhodium complexes as catalysts give very good results which correlate well with the published material. 

In accord with the Dewar-Chatt-Duncanson model, we find that the dominant interaction is donation from the C C n bond into the rhodium LUMO. This interaction is enhanced when the double bond lies in the rhodium-diphosphine plane and with electron donating substituents which raise the energy of 7ito more closey match the LUMO Charge Decomposition Analysis (CDA) [81] shows that the amount of donation is... 

Abstract A review of theoretical progress in the modeling of rhodium diphosphine... 

Figure 9. Proposed free energy profile for rhodium diphosphine (ref 22)...

Electronic effects. To study the nature of the electronic effect in the rhodium diphosphine catalysed hydroformylation, a series of thixantphos 18 ligands with varying basicity was synthesized 25-30 (Figure 8.11). In this series of ligands, steric differences are minimal so purely electronic effects could be investigated. 

Thus the two plausible catalytic cycles have been considered, one via an Ir dihydride complex A and the other via an IrH2(ri -H2) complex B (Fig. 3). The first is analogous to the well-established mechanism for rhodium diphosphine-catalyzed hydrogenation of olefins going through Ir(I) and Ir(III) intermediates [26-29]. 

Figure 19. Immobilization of chiral rhodium diphosphine catalysts (5) to A1-SBA-15/A1-MCM-41 [85].

A special case is the test of immobilization of rhodium-diphosphine complexes on all-silica materials. After the first immobilization step in dichloromethane, the solid material had a yellow color and a Rh content of 0.07 mmol g was found. After the extraction with methanol, the entire amount of organometallic complex was washed out and the final material had again the original white color. No rhodium was detected in ICP-AES analysis of this sample. However, in the case of aluminum-containing materials the orange color obtained after the immobilization of the rhodium complexes in dichloromethane is clearly maintained even after extraction in methanol. 

Enantioselective Hydrogenation over Immobilized Rhodium Diphosphine Complexes... 

To summarize, chiral heterogeneous catalysts were prepared from rhodium-diphosphine complexes and aluminum-containing mesoporous materials. The bonding occurred via an ionic interaction of the cationic complex with the host. These catalysts were suitable for asymmetric hydrogenation of functionalized olefins. The catalysts can be recycled easily by filtration or centrifugation with no significant loss of activity or enantioselectivity. 

The immobilization by ionic bonding on Al-MCM-41 was carried out similarly to the immobilization of rhodium-diphosphine complexes as described above. This catalyst was named MCMIHC. For immobihzation via the metal center and covalent bonding of salen, the all-silica MCM-41 was modified with (3-aminopro pyl)triethoxysilane (APTES) [52]. The catalyst obtained by the metal center immobilization was denoted MCM2HC [53], whereas the material obtained by covalent bonding of the salen ligand was named MCM3HC [54]. Detailed procedures are described extensively elsewhere [55]. 

L-Dopa was produced industrially by Hoffrnann-LaRoche, using a modification of the Erlenmeyer synthesis for amino acids. In the 1960s, research at Monsanto focused on increasing the L-Dopa form rather than producing the racemic mixture. A team led by William S. Knowles (1917—) was successful in producing a rhodium-diphosphine catalyst called DiPamp that resulted in a 97.5% yield of L-Dopa when used in the Hoffrnann-LaRoche process. Knowles s work produced the first industrial asymmetric synthesis of a compound. Knowles was awarded the 2001 Nobel Prize in chemistry for his work. Work in the last decade has led to green chemistry synthesis processes of L-Dopa using benzene and catechol. 

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