Rhodium immobilized

Rhodium immobilized complexes were also found to be effective catalysts of the addition of HSiMe(OSiMe3)2 and HSi(OEt)3 to various allyl ethers. The data presented in Table 7.4 confirm a high catalytic activity of catalysts 1, 3 and 5 in the conversion of allyl ethers into the corresponding silyl derivahves, but, unfortunately, only in the case of allyl phenyl ether did the catalytic achvity remained unchanged up to 10 cycles. ICP analysis of the rhodium solid catalysts after hydrosilylation tests revealed a high concentration of rhodium. Therefore, the decrease in catalytic activity of 1 does not depend only on leaching of rhodium from the silica surface. 

Similar catalytic activity of rhodium-immobilized complexes was observed in the hydrosilylation of olefins with polyhydrosiloxane (Table 7.5). 

Supported aqueous phase catalysis can be used not only for hydrogenation and hydroformylation, but also for more specific transformations, as in the addition of diphenylacetylene to azobenzene catalyzed by TPPTS complex of rhodium immobilized on hydrated Si02 [196] ... 

To date, these functionalized ligands have been investigated on the laboratory scale, in batch operations to immobilize rhodium catalyst in hydroformylation. 

Probably the first non-covalent immobilization of a chiral complex with diazaligands was the adsorption of a rhodium-diphenylethylenediamine complex on different supports [71]. These solids were used for the hydride-transfer reduction of prochiral ketones (Scheme 2) in a continuous flow reactor. The inorganic support plays a crucial role. The chiral complex was easily... 

The most spectacular results, in terms of comparison between CFPs- and carbon-supported metal catalysts, were likely provided by Toshima and co-workers [33,34]. As illustrated in Section 3.3.3, they were able to produce platinum and rhodium catalysts by the covalent immobilization of pre-formed, stabilized metal nanoclusters into an amine functionalized acrylamide gel (Scheme 5). To this purpose, the metal nanopartides were stabilized by a linear co-polymer of MMA and VPYR. The reaction between its ester functions and the amine groups of the gel produced the covalent link between the support and the... 

Rhodium also has been reported as a catalyst for [2+2+2] alkyne cycloaddition in water. Uozumi et al. explored the use of an amphiphilic resin-supported rhodium-phosphine complex as catalyst (Eq. 4.60). The immobilized rhodium catalyst was effective for the [2+2+2] cycloaddition of internal alkynes in water,113 although the yields of products were not satisfactory. 

When the rhodium-catalyzed reaction is performed under a high pressure of CO in the presence of phosphite ligands, aldehyde products (159) are formed by insertion of CO into the rhodium-alkyl bond followed by reductive elimination (Eq. 31) [90]. The bimetallic catalysts were immobilized as nanoparticles, giving the same products and functional group tolerance, with the advantage that the catalyst could be recovered and reused without loss of... 

They observed the complete deactivation of the rhodium catalyst whether immobilized or not in the presence of free amines. When no amine was present, styrene formation was not observed. After 17 h of a reaction in which both catalysts were immobilized, the yield of the product, ethylbenzene, was 52%, again demonstrating the principle of enabling two otherwise incompatible catalysts to work concomitantly in order to achieve process intensification. 

Ester formation catalyzed by lipase (Mucor miehei) in conjunction with hydrogenation catalyzed by a rhodium complex Sol-gel immobilization of both catalysts... 

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)). 

Only a slight decrease in the conversion was observed during the recycling. No leaching of the immobilized rhodium complex occurred as indicated by ICP-AES analysis of the mother liquor. 

Both the approaches discussed above are addressed to the desire for immobilization of the metal species, while the corrosive and volatile promoter must still be trapped and recycled. Recently, Webber et al. (28) have attempted to achieve immobilization of both the metal species and the promoter by attaching rhodium to a polymer functionalized with chlorinated thiophenol groups. This imaginative approach is presently plagued by both low reactivity and rhodium loss from the polymer surface. 

The first rhodium-catalyzed reductive cyclization of enynes was reported in I992.61,61a As demonstrated by the cyclization of 1,6-enyne 37a to vinylsilane 37b, the rhodium-catalyzed reaction is a hydrosilylative transformation and, hence, complements its palladium-catalyzed counterpart, which is a formal hydrogenative process mediated by silane. Following this seminal report, improved catalyst systems were developed enabling cyclization at progressively lower temperatures and shorter reaction times. For example, it was found that A-heterocyclic carbene complexes of rhodium catalyze the reaction at 40°C,62 and through the use of immobilized cobalt-rhodium bimetallic nanoparticle catalysts, the hydrosilylative cyclization proceeds at ambient temperature.6... 

The immobilization of catalysts on a solid support is a well-known approach to render a system recycleable, and this has been performed recently by the immobilization of rhodium-MonoPhos 29 a on aluminosilicate A1TUD-1. The resultant system showed high efficiency in water, and could be recycled [80]. 

Another type of chiral rhodium complex [Rh-MeDuPHOS] was also immobilized in [BMIM][PF6] and used in the enantiomeric hydrogenation of related enamides [95] (Fig. 41.5). Geresh et al. focused their research on the stabilization of the air-sensitive catalyst in the air-stable ionic liquid, so that the complex was protected from attack by atmospheric oxygen and recycling was possible. 

Fig. 41.13 Supported ionic liquid phase (SILP) catalyst. The ionic liquid phase containing a rhodium complex is immobilized on the surface of a silica gel support material.

A similar type of immobilization was obtained by reacting the phosphonylated 2,2 -bipyridine ligand depicted in Figure 42.10 with excess titanium alkoxide. Rhodium and iridium complexes of this immobilized ligand showed activity for... 

Using two types of specially synthesized rhodium-complexes (12a/12b), pyruvate is chemically hydrogenated to produce racemic lactate. Within the mixture, both a d- and L-specific lactate dehydrogenase (d-/l-LDH) are co-immobilized, which oxidize the lactate back to pyruvate while reducing NAD+ to NADH (Scheme 43.4). The reduced cofactor is then used by the producing enzyme (ADH from horse liver, HL-ADH), to reduce a ketone to an alcohol. Two examples have been examined. The first example is the reduction of cyclohexanone to cyclohexanol, which proceeded to 100% conversion after 8 days, resulting in total TONs (TTNs) of 1500 for the Rh-complexes 12 and 50 for NAD. The second example concerns the reduction of ( )-2-norbornanone to 72% endo-norbor-nanol (38% ee) and 28% exo-norbornanol (>99% ee), which was also completed in 8 days, and resulted in the same TTNs as for the first case. 

L. V. Dinh, J. Gladysz, Transition Metal Catalysis in Fluorous Media Extension of a New Immobilization Principle to Biphasic and Monophasic Rhodium-Catalyzed Hydrosilylations of Ketones and Enones , Tetrahedron Lett. 1999, 40,8995. 

Ohtaki, M., Komiyama, M., Hirai, H., and Toshima, N., Effects of polymer support on the substrate selectivity of covalently immobilized ultrafme rhodium particles as a catalyst for olefrn hydrogenation, Macromolecules, 24, 5567, 1991. 

Another way of getting around the problem of the separation of the catalyst from the substrate is via use of a flow reactor [38], Supercritical carbon dioxide has been used successfully as a medium for the hydroformylation of 1-octene using an immobilized rhodium catalyst. The catalyst is covalently fixed to silica through the modifying ligand A-(3-trimethoxysilyl-n-propyl)-4,5-bis(diphenylphosphino)phenoxazine (Figure 8.13). Selectivity was found to be... 

The synthesis of aldehydes via hydroformylation of alkenes is an important industrial process used to produce in the region of 6 million tonnes a year of aldehydes. These compounds are used as intermediates in the manufacture of plasticizers, soaps, detergents and pharmaceutical products [7], While the majority of aldehydes prepared from alkene hydroformylation are done so in organic solvents, some research in 1975 showed that rhodium complexes with sulfonated phosphine ligands immobilized in water were able to hydroformylate propene with virtually complete retention of rhodium in the aqueous phase [8], Since catalyst loss is a major problem in the production of bulk chemicals of this nature, the process was scaled up, culminating in the Ruhrchemie-Rhone-Poulenc process for hydroformylation of propene, initially on a 120000 tonne per year scale [9], The development of this biphasic process represents one of the major transitions since the discovery of the hydroformylation reaction. The key transitions in this field include [10] ... 

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