Rhodium diphosphine complexes

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. 

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,... 

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]. 

Figure 1.13 Generation of rhodium-based supramolecularcatalysts by assembly of pyridine/hydroxypyridine pairs (a) Self-assembly modes of pyridine-based phosphines, (b) Alkene hydroformylation with supramolecular rhodium-diphosphine catalysts (c) CAChe minimized 3D structure ofthe rhodium-diphosphine complex (other ligands from the metal omitted for clarity).

Skander, M., Humbert, N., Collot, J., Gradinaru, J., Klein, G., Loosli, A., Sauser, J., Zocchi, A., Gilardoni, E. and Ward, TR. (2004) Artificial metaUoenzymes (Strept) avidin as host for enantioselective hydrogenation by achiral biotinylated rhodium-diphosphine complexes. J. Am. 

Wagner, H. H., Hausmann, H. and Holderich, W. F. Immobilization of rhodium diphosphine complexes on mesoporous Al-MCM-41 materials catalysts for enantioselective hydrogenation, J. Catal., 2001, 203, 150-156. 

Only very low catalyst concentrations down to 5 x 10-5 kmol/m3 are consumed that keeps also the catalyst inventory very small [266], Only 0.08 mg of Rh and about 0.2 mg-13 pg of the very expensive chiral ligands (about 300-1000 /g), depending on their molecular weight, are consumed. Finally, a performance comparison for three different reactors was made for the substrate methylacetamidocinnamate and the two rhodium diphosphine complexes Rh/Josiphos and Rh/Diop (see Figure 4.57). The first reactor was a commercial Caroussel reactor (Radleys... 

Table 2 The catalytic activity and enantioselectivity of immobilised rhodium diphosphine complexes in the hydrogenation of dimethylitaconate...

In conclusion, chiral heterogeneous catalysts are prepared from chiral Rhodium diphosphine complexes and Al-MCM-41. The bonding supposedly occurs via an ionic interaction of the cationic complex with the host. Also a slight reduction of weak acidic sites of Al-MCM-41 has been observed. These catalysts are suitable for the hydrogenation of functionalised olefins. The organometallic complexes remain stable within the mesopores of the carrier at reaction conditions. The catalyst can be recycled by filtration or centrifugation. 

The first example of catalytic asymmetric hydrogenation of N,N dialkyl enamines was reported by Buchwald and Lee in 1994. By using 5 mol% chiral ansa titanocene catalyst [(S,S,S) (EBTHI)TiO binaphtho] (EBTHI = ethylenebis(tetrahydroindenyl)), they achieved excellent enantioselectivities (up to 98% ee) in the hydrogenation of (1 arylvinyl)amines [50]. In 2000, Boner used chiral rhodium diphosphine complexes for the hydrogenation of 2 N piperidinylethylbenzene and 2 alkyl 1,3,3 trimethyle neindoline and obtained the tertiary amines in moderate enantiomeric excesses [51]. 

In summary, the subtle details that govern the asymmetric hydrogenation mechanism of olefins catalyzed by cationic rhodium-diphosphine complexes depend on the specific combination of the ligand and the substrate (228). Further studies are needed to achieve a deeper understanding of the relationship between the nature of the intermediates and the transition states that determine the stereoselectivity (229). 

A metal complex with a host protein using a combination of avidin as a protein and biotin with a rhodium diphosphine complex was prepared as a supramolecular asymmetric catalyst. Avidin shows a high affinity for biotin K = -10 M- ), indicating that the rhodium diphosphine complex quantitatively binds into the chiral space of avidin. 

Hydrogenation of A-acetamidoacrylate with the avidin-biotin and rhodium-diphosphine complexes shows a moderate enantioselectivity, yielding (S)-N-acetamidoalanine at41% ee (Figure 12.8). " Subsequently, an artificial metalloenzyme system using a combination of biotinylated diphosphine with mntated streptavidine was prepared. This catalytic system qnantitatively yielded highly enantioselective products ((R) = 94%). ... 

Rhodium diphosphine complexes display higher branched selectivities but could only achieve highest enan-tioselectivities of up to 60% ee. In the same year of the discovery of the Binaphos ligand. Union Carbide filed a... 

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