Immobilized Rh complexes

It was recently found that the modification of neutral phosphine ligands with cationic phenylguanidinium groups represents a very powerful tool with which to immobilize Rh-complexes in ionic liquids such as [BMIM][PFg] [76]. The guani-dinium-modified triphenylphosphine ligand was prepared from the corresponding iodide salt by anion-exchange with [NH4][PFg] in aqueous solution, as shown in Scheme 5.2-15. The iodide can be prepared as previously described by Stelzer et al. [73]. 

Table 2.1.6.2 Hydrogenation over free and immobilized Rh complexes on AI-SBA-15 (3 bar H2, 24h reaction time).

Carbon monoxide is compressed and sparged into the reactor riser. The reactor has no mechanical moving parts, and is free from leakage/ maintenance problems. The ACETICA Catalyst is an immobilized Rh-complex catalyst on solid support, which offers higher activity and operates under less water conditions in the system due to heterogeneous system, and therefore, the system has much less corrosivity. 

What was not known at the time when these first PHIP experiments with heterogeneous catalysts were performed was the fact that metal clusters and particles can produce PHIP effects as well. In fact, it was widely believed that the mechanism of catalytic hydrogenation on metal surfaces was incompatible with the requirement of the pairwise H2 addition to a substrate. Therefore, the possibility cannot be excluded that the immobilized Rh complexes used in the early studies were precatalysts rather than the actual catalysts, especially in some of the gas - solid hydrogenations. Extended catalyst activation periods may have resulted in the reduction of supported metal complexes and the production of nanoparticulate Rh catalysts. However, the main conclusion that PHIP effects can be produced in heterogeneous processes is still valid. 

With tethering technology, immobilized Rh(DiPFc) catalyst, a veiy selective hydrogenation catalyst was developed by Engelhard (26) and Chirotech (27). Rh(DiPFc)(COD)BF4 is one of most interesting homogeneous catalysts due to its chemo-selectivity and sulfur tolerant behavior. The anchored complex y-... 

The insoluble polymer-supported Rh complexes were the first immobilized chiral catalysts.174,175 In most cases, however, the immobilization of chiral complexes caused severe reduction of the catalytic activity. Only a few investigations of possible causes have been made. The pore size of the insoluble support and the solvent may play important roles. Polymer-bound chiral Mn(III)Salen complexes were also used for asymmetric epoxidation of unfunctionalized olefins.176,177... 

Lee et al. [103] synthesized a chiral Rh-complex with a bisphosphine-contain-ing cation as ligand (Fig. 41.8, 2) to improve the immobilization of the transition-metal complex within the ionic liquid. 

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. 

Pardey and coworkers159,161 also reported results for water-gas shift and nitrobenzene reduction for the related Rh complex, [Rh(COD)(amine)2](PF6), where COD = 1,5-cyclooctadiene, immobilized on P(4-VP), and results are reported in... 

BF4 affords the immobilized [Rh(COD)(PPM)]BF4 complex 36. The supported Rh-PPM catalysts were then tested for their ability to hydrogenate methyl a-acetamidocinnamate (substrate Rh = 200 1, 25 °C, methanol, 0.1 MPa H2). 

The use of surface bound triflate ions has been exploited by Raja et al. to immobilize the complexes [Rh(COD) fSj-(-i-)-PMP ], [Pd(allyl) fSj-(-i-)-PMP ], [Rh(COD) fSj-(-)-AEP rand[Rh(COD) flR,2Rj-(-t)-DED ]"in the pores of silicas possessing various pore sizes with narrow distributions [128]. These constrained chiral catalysts were then tested for the asymmetric hydrogenation of methyl ben-zoylformate to its corresponding methyl mandelate (40°C, methanol, 2 MPa H2). In the homogenous form, only the catalysts [Rh(COD) fSj-(-i-)-PMP ], [Pd(allyl) (Sj-(-i-)-PMP ] exhibit any signiflcant e.e.s under the reaction conditions (53%... 

Immobilized Rh Diphosphino Complexes as Catalysts for Asymmetric Hydrogenation... 

Rhodium(I) complexes immobilized on silica using 3-(3-silylpropyl)-2,4-pentanedio-nato ligands (38) show good activity in the hydrosilylation of 1-octene with HSi(OEt)3 at 100°C60. The immobilized Rh catalysts are prepared by (i) reaction of (EtO)3Si(CH2)3C(COMe)2Rh(CO)2 with untreated silica (Catalyst A), (ii) reaction of Rh(acac)(CO)2 (acac = acetylacetonato = 2,4-pentanedionato) with silica modified by [(EtO)3Si(CH2)3C(COMe)2] prior to the complexation (Catalyst B), (iii) reaction of [Rh(CO)2Cl]2 with a polycondensate of [(EtO)3Si(CH2)3C(COMe)2] , Si(OEt)4 and water (Catalyst C) and (iv) sol-gel processing of (EtO)3Si(CH2)3C(COMe)2Rh(CO)2 and Si(OEt)4 (Catalyst D). The Catalysts A and B show ca three times better activity than their homogeneous counterparts, while the Catalyst D exhibits only low activity and the Catalyst C is inactive60. 

In a manner similar to the four methods mentioned above, rhodium complex catalysts immobilized on silica modified by 2-(MeO)3Si(CH2)2C5H4N and (Me0)3Si(CH2)30C0CMe=CH2 using [Rh(CO)2Cl]2 as the precursor are prepared61. These immobilized pyridine-Rh complexes are shown to be active catalysts in the... 

In the application of membrane technology (cf. Section 3.2.3) for the separation of the Rh complexes and the re-immobilized ligands after the reaction, a further remarkable enlargement of the ligands was desirable. Unfortunately, the combination of diamines with TPPTS yields highly crosslinked polymeric materials, which cannot be handled. A reduction of the degree of crosslinking is possible by use of the disulfonated TPPDS (cf. Section 3.1.1.1). So, in a combination with TCD-diamine (tricyclodecane diamine) a salt was formed that was partly soluble in toluene and soluble in THF. The same was the case with the use of MA -dimethyl-TCD-diamine. These salts may be useful in water-free two-phase catalyst systems. 

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