Hybrid with rhodium complexes

These examples are part of a broader design scheme to combine catalytic metal complexes with a protein as chiral scaffold to obtain a hybrid catalyst combining the catalytic potential of the metal complex with the enantioselectivity and evolvability of the protein host [11]. One of the first examples of such systems combined a biotinylated rhodium complex with avidin to obtain an enantioselective hydrogenation catalyst [28]. Most significantly, it has been shovm that mutation-based improvements of enantioselectivity are possible in these hybrid catalysts as for enzymes (Figure 3.7) [29]. 

Complexation of (124) and (125) with [ Rh(COD)Cl 2] in the presence of Si(OEt)4, followed by sol-gel hydrolysis condensation, afforded new catalytic chiral hybrid material. The catalytic activities and selectivities of these solid materials have been studied in the asymmetric hydro-gen-transfer reduction of prochiral ketones and compared to that of the homogeneous rhodium complexes containing the same ligands (124) and (125) 307... 

Phosphoramidites, a ligand class that has only recently been introduced into asymmetric hydrogenation, in the form of hybrid chelate ligands [29], induce excellent enantioselectivity as monodentate ligands. Thus de Vries, Feringa, and co-workers could reduce standard substrates in >96% ee with a rhodium complex based upon the binaphtholphosphoramidite 3d, once the solvent and reaction temperature had been optimized [30],... 

On treatment with proton sponge 1, cationic rhodium complexes of type 262 have been shown to undergo double intramolecular dehydrofluorinative/C—C coupling to produce quantitatively rhodium complexes of hybrid cyclopentadienyl-phosphine ligands 263 (equation 27)240. 

Comparisons may be made between the above two transition metal o-bonded metal-loporphyrins. In the rhodium complex the metal atom is almost in the plane of the macrocycle and this is not exactly the case for the ferric complex. It is not clear if this conformation difference is due only to a difference between the metal atoms. One explanation is that the o-bonded ligands are not the same (C Hs or CH3 with two different hybridization schemes). As shown above, this leads to different magnetic behaviour and in this case the out-of-plane distance could be different. One would expect a slightly larger out-of-plane distance for the methyl species because of its spin state. However, the steric interactions of a methyl group are far less than those of the phenyl ring and Fe(TPP)(CH3) could have the iron atom more in the porphyrin plane, thus favoring hexacoordination. ... 

The hydrogen atom of the central cyclopentadiene ring can be displaced by potassium with t-BuOK to form the K(CgoMe5) complex in which the potassium atom can then be displaced to form iridium complexes such as Ir(ri5-C6oMe5)(CO)2 [Matsuo, Iwashita and Nakamura Organometallics 24 89 2005], and with rhodium to form Rh(ri5-C6oMe5)(CO)2 [Sawamuia, Kuninobu and Nakamura Chem Soc 122 12407 2000], structures which are supported by X-ray structure analyses. The same cyclopentadiene can complex with Fe and cyclopentadiene (Cp) to form a hybrid of buckyferrocene Fe(C6oMe5)Cp [Sawamura et al. J Am Chem Soc 124 9354 2002, Nakamura Pure Appl Chem 75 427 2003],... 

With these hgands enantioselectivihes in excess of 95% were obtained in the rhodium-catalyzed addition of phenylboronic acid to cyclohexenone. Furthermore, the results of kinetic experiments suggested that catalyst turnover occurred much more rapidly with these mixed rhodium complexes of hybrid ligands compared to the corresponding rhodium complexes of BINAP or cod [76]. 

Lindner E., Auer F., Baumann A., Wegner P., Mayer H.A., BertagnoUi H., Reinohl U., Ertel T.S., Weber A. Supported organometallic complexes. Part XX. Hydroformylation of olefins with rhodium(I) hybrid catalysts. J. Mol. Catal. A Chem. 2000 157 97-109 Lopez T., Herrera A., Gomez R., Zou W., Robinson K., Gonzalez R.D. Improved mechanical stability of supported Ru catalysts preparation by the sol el method. J. Catal. 1992 136 621 625... 

It is also important to note that the hybrid calixphyrin-palladium and -rhodium complexes catalyze the Heck reaction (p-bromobenzaldehyde with n-butyl acrylate in A, A -dimethylacetamide at 100 °C) and hydrosilylations (acetophenone with Ph2SiH2 and of phenylacetylene with PhMe2SiH), respectively, with high efficiency. The present results demonstrate the potential utility of the phosphole-containing hybrid calixphyrins as highly promising hemilabile macrocyclic ligands for the construction of efficient transition-metal catalysts [160, 162]. 

An especially important case is the enantioselective hydrogenation of a-amidoacrylic acids, which leads to a-amino acids.14 A particularly detailed study has been carried out on the mechanism of reduction of methyl Z-a-acetamidocinnamate by a rhodium catalyst with a chiral disphosphine ligand.15 It has been concluded that the reactant can bind reversibly to the catalysts to give either of two complexes. Addition of hydrogen at rhodium then leads to a reactive rhodium hybride and eventually to product. Interestingly, the addition of hydrogen occurs most rapidly in the minor isomeric complex, and the enantioselectivity is... 

Abstract The applications of hybrid DFT/molecular mechanics (DFT/MM) methods to the study of reactions catalyzed by transition metal complexes are reviewed. Special attention is given to the processes that have been studied in more detail, such as olefin polymerization, rhodium hydrogenation of alkenes, osmium dihydroxylation of alkenes and hydroformylation by rhodium catalysts. DFT/MM methods are shown, by comparison with experiment and with full quantum mechanics calculations, to allow a reasonably accurate computational study of experimentally relevant problems which otherwise would be out of reach for theoretical chemistry. 

Carbon-based sorbents are relatively new materials for the analysis of noble metal samples of different origin [78-84]. The separation and enrichment of palladium from water, fly ash, and road dust samples on oxidized carbon nanotubes (preconcentration factor of 165) [83] palladium from road dust samples on dithiocarbamate-coated fullerene Cso (sorption efficiency of 99.2 %) [78], and rhodium on multiwalled carbon nanotubes modified with polyacrylonitrile (preconcentration factor of 120) [80] are examples of the application of various carbon-based sorbents for extraction of noble metals from environmental samples. Sorption of Au(III) and Pd(ll) on hybrid material of multiwalled carbon nanotubes grafted with polypropylene amine dendrimers prior to their determination in food and environmental samples has recently been described [84]. Recent application of ion-imprinted polymers using various chelate complexes for SPE of noble metals such as Pt [85] and Pd [86] from environmental samples can be mentioned. Hydrophobic noble metal complexes undergo separation by extraction under cloud point extraction systems, for example, extraction of Pt, Pd, and Au with N, A-dihexyl-A -benzylthiourea-Triton X-114 from sea water and dust samples [87]. 

The first bisphosphine calixarenes that have been used in catalysis are di(amide)-phosphine hybrids calix[4]ar-ene. Reaction of [RhCl(norbornadiene)]2 with these calixarene derivatives gave an organometallic complex whose norbornadiene-rhodium moiety lies above the cavity defined by the four substituents of the calixarene and between the two amide functionalities. This complex was applied in the hydroformylation reaction of styrene. The rather low reaction rate observed (7.5 turnovers per Rh per hour) has been attributed to a partial encapsulation of the metal center preventing the approach of the substrate. Indeed, the metal center may be viewed as located in a hemispherical ligand environment. 

Two recent examples of papain bioconjugation with formation of potential hybrid catalysts are shown in Figs. 6 and 7, featuring a manganese salen catalyst and dipyridyl complexes of copper, palladium, and rhodium, respectively [53, 58]. 

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