Immobilization of rhodium-diphosphine

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. 

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]. 

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. 

Immobilization of homogeneous catalysts for hydrogenation reactions concerns essentially enan-tioselective hydrogenations, important for the synthesis of fine chemicals (see Chapter 9.2). The pioneering work of Pugin et al.131 concerns the synthesis of a rhodium-based catalyst, with a diphosphine-pyrrolidine-based ligand for the hydrogenation of methylacetamide cinnamate (Equation(8)). 

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

Diphosphine-rhodium catalysts bound to inorganic supports via isocyana-to-alkyl-trialkoxysilane linkers meet all requirements to be of practical use i. the preparation method is probably the most simple, general and efficient known so far, ii. immobilized cationic rhodium complexes with "molecular weights" as low as 5 kD exhibit a comparable performance in enantioselective catalytic... 

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. 

Rhodium complex contents varied between 0.02 and 0.07 mmol (g catalyst), depending on the type of carrier and diphosphine used, whereas the theoretical content was 0.15 mmol g . The color transfer during the immobilization in di-... 

Several diphosphine ligands have been applied and the corresponding complexes have been tested for the immobilization (Fig. 2.1.6.3). The activity of different free and immobilized complexes in the enantioselective hydrogenation of dimethyl itaconate and methyl a-acetamidoacrylate was investigated. In blank reactions over pure mesoporous materials no reaction took place. When rhodium supported on carriers was used as catalyst, no enantiomeric excess was observed. 

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. 

Four generations of dendrimers beating (72-symmetrical diphosphine rhodium complexes have been prepared and used in asymmetric hydrogenation (Scheme Acid-functionalized analog 146 of the easily immobilized... 

---