Ligand polyether-functionalized

Another interesting technique to solve the problem of mass transport in bipha-sic reactions was developed by Fell and Jin, the catalysis with so-called thermo-regulated phase transfer ligands [177-185] (details see Section 4.6.3). In the examples given so far, a smart solvent system controlled the phase behaviour of the reaction mixture. Lemaire et al. have found that also polyether-functionalized chiral mono- or diphosphites are active in thermoregulated conversion [186]. The hy-droformylation of styrene yields conversions of 99%, n/i-ratios of 85/15 and an enantioselectivity of up to 25%. A recycling of the catalyst proved to be possible, however, with a certain leaching of the rhodium catalyst. 

Another group of macrocyclic ligands that have been extensively studied are the cycHc polyethers, such as dibenzo-[18]-crown-6 (5), in which the donor atoms are ether oxygen functions separated by two or three carbon atoms. The name crown ethers has been proposed (2) for this class of compounds because of the resemblance of their molecular models to a crown. Sandwich stmctures are also known in which the metal atom is coordinated with the oxygen atoms of two crown molecules. 

Poly (macrocyclic) compounds. The analytical application of compounds such as crown polyethers and cryptands is based on their ability to function as ligands and form stable stoichiometric complexes with certain cations. Special importance is due to their preference for alkali metal ions which do not form complexes with many other ligands. A number of these compounds are commercially available and their properties and analytical applications have been described by Cheng et a/.11... 

The second ligand type consists of a large group of cyclic compounds incorporating numbers of ether functions as donors. Structure (22) illustrates a typical example. Such crown polyethers usually show strong complexing ability towards alkali and alkaline earth ions but their tendency to coordinate to transition metal ions is less than for the above... 

Frechet et al. were able to coordinate polyether dendrons having carboxylate functional groups at the focal point with lanthanide ions up to the fourth generation (Fig. 2.11) [41]. Preparation by straightforward ligand exchange starting from lanthanide triacetates with dendron carboxylates was made possible by the... 

The importance of the chelate effect combined with the construction of multidentate ligands is well known in lanthanide chemistry. This is expressed in the rich coordination chemistry of / -diketonates [88] or complexes with Schiff bases [89] and macrocyclic polyethers [90] where lanthanide cations achieve steric saturation by high coordination numbers. Entrapment of the cation in a macrocyclic cavity results in greater complex stability. However, simply functionalized ligands such as ethanolamines can also supply a suitable ligand sphere [91-93],... 

Following up this work, the same group recently published the synthesis of hybrid dendritic ligands containing a combination of dendritic chiral DPEN and Frechet polyether dendrons (Scheme 7) [50]. The solubility of these hybrid dendrimers was found to be controlled by the polyether den-dron. Compared with the simple core-functionalized systems displayed in Fig. 21, the hybrid dendrimers showed similar catalytic activity but reduced recyclability. 

The organometallic ferrocene moiety is an attractive redox center to integrate into macrocyclic polyether ligands because, apart from its established functional group organic chemistry, ferrocene itself is elec-trochemically well behaved in most common solvents undergoing a reversible one-electron oxidation (43). 

On their own polyethers have had limited use in supramolecular chemistry. Low molecular mass compounds have been shown to bind a number of metal ions, notably those in the lanthanide series that can accommodate ligands with large numbers of oxygen donor atoms. One of the main problems is the lack of diversity in the compounds functional groups which limits the range of their ligating opportunities. Where polyethers have been highly successful is as substituents to other molecules, such as calixarenes, and in their cyclic forms as the crown ethers, which will be described later. 

In order to facilitate recycling of the multiple TsDPEN-functionalized dendrimer catalysts, the same group recently reported the synthesis of a novel form of hybrid dendrimer ligands by coupling polyether dendrons with peripherally TsDPEN-functionahzed Newkome-type poly(ether-amide) dendrimer (Figure 4.28) [90]. The solubility of these hybrid dendrimers was found to be affected by the generation of the polyether dendron. The ruthenium complexes produced were applied in the asymmetric transfer hydrogenation of ketones, enones, imines and activated... 

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