Thermomorphic ligands

Keywords Immobilization Thermoregulated catalysis Thermomorphic ligands Hydroformylation Polyethylene glycol modified phosphites... 

Our approach is to use the inexpensive ligands that are already used industrially as well as conventional solvents. The goal of this project is to develop a thermomorphic approach to the rhodium-catalyzed hydroformylation of higher olefins (>Ce) that enhances conversion rates and ease of product recovery while minimizing catalyst degradation and loss. 

On the other hand thermomorphic solvent systems can be used for industrial applications within existing equipment. In principal, it is also possible to use unmodified ligands and catalyst complexes for homogeneously catalyzed reactions. Therefore, the economic hurdles to use the system in industrial practice are not very high. 

From our cooperation partners. Profs. Gladysz and Dinjus, we received ligands with perfluorinated chains ( ponytails ), which show a thermomorphic solubility in organic solvents (P(et-Rf8)2(m-me-bz)) or maybe extracted with fluorous solvents (P(et-Rfs)3). P(et-Rf6)(z-pr)2 with only one perfluorinated... 

Suitable thermomorphic solvent systems were appUed to the telomeriza-tion of butadiene with ethylene glycol or with carbon dioxide, to the isomer-izing hydroformylation of trans-4-octene and to the hydroaminomethylation of 1-octene with morpholine. Further investigations for the carboxytelomer-ization and for the synthesis of 4-nitrodiphenylamine were also carried out. In addition to common Ugands, PEG-modified ligands and fiuorous Ugands were used for the telomerization and the carboxytelomerization. 

An extension of these dendritic phosphine ligands for Pd-catalyzed allylic ami-nation was recently described using a thermomorphic system for separation [72]. 

Mizugaki et al. 74) have recently utilized thermomorphic properties of Pd(0)-complexed phosphinated dendrimers for dendritic catalyst recycling. Using the method developed by Reetz 16), they prepared dendritic ligands containing, respectively, 2, 8, 16, and 32 chelating diphosphines. Palladium dichloride was com-plexed to the dendrimers, and a reduction in the presence of triphenylphosphine gave the Pd(0)-complexed dendrimers (80—83). The dendritic complexes were active... 

Figure 6 Representative examples of ligands used in thermomorphic catalysis.

Polymers are a general alternative to low molecular weight ligands in always biphasic liquid/liquid systems. Just as the nonpolar or polar polymers above imparted phase-selective solubility to catalysts in thermomorphic systems, the appropriate polymer can impart aqueous or fluorous phase-selective solubility to a catalyst. Several recent examples illustrate this for aqueous, fluorous, and other biphasic catalysts. 

The Plenio group recently described the advantages of using simple PEG supports with sterically demanding phosphines in biphasic and thermomorphic systems. In their work, the best ligand was the diadamantylbenzylphos-phine 138 (Eq. 79). They chose to use a 2,000-Da polymer because H NMR... 

Gladysz showed that a thermomorphic fluorous paUadacyde acts as a PdNP catalyst precursor for the Heck reaction at 80-140 C in DMF with very high turnover numbers [24a]. Molecular palladium complexes such as palladacycles and other palladium salts have also been used as PdNP precursors upon treatment with CO in DMF or toluene at room temperature, and these PdNPs catalyzed nudeophiUc substitution/carbonylation/amination affording iso-indolinones at room temperature [24bj. PdNPs capped with spedal ligands such as polyoxometal-... 

The idea of thermomorphic catalysts, which is largely similar to the concept of catalysts with modified poly(ethylene oxide)s, was implemented by the example of modified poly(A -isopropylacrylamides) (PNIPAMs). At low temperatures, PNIPAMs can dissolve in water and at high temperatures, in two-phase systems, PNIPAMs pass into the organic phase, and also retain partly in water (Fig. 11-4). The upper critical solution temperature depends on substituents at the amide group [83, 91-93]. Several macromolecular metal complexes with PNIPAM ligands have been designed. 

Also, 1-dodecene can be selectively reacted with syngas in the presence of a Rh(BIPHEPHOS) catalyst to give n-tridecanal (Table 6.1, entry 5). A related Rh catalyst based on nanoparticles (2.7-4.8 nm) could be three times recycled without loss of activity when the reaction was conducted in a thermomorphic solvent system [61]. Also, under the conditions illustrated in Scheme 6.6 ( = 4), tridecanal is formed, but in comparison to shorter chain olefins the conversion dropped and the aldehyde was obtained only in a moderate Ub ratio [54]. Other attempts have been based on more sophistic ligands such as amphiphilic sulfonated monophosphines [62] and diphosphines [55, 63], phosphine-functionalized phosphonium ionic liquids [64], and MeO-substituted phosphines [65]. Tridecanal has a slightly floral scent reminiscent of citrus and grapefruit peel. 

---