Supramolecular ligand

Scheme 36.18 Use of a supramolecular ligand library in enantioselective hydrogenation.

The use of supramolecular ligand libraries in homogeneous catalysis was reviewed by Breit [54]. 

Assemblies based on 8 and pyridine phosphorus ligands 5-7 were used as supramolecular ligands in the rhodium-catalyzed hydroformylation and showed typical bidentate behavior. The chelating bidentate assembly exhibited lower activities (a factor of three) than the monodentate analogue. Only a slightly higher selectivity for the linear aldehyde was observed. The chiral ligand assemblies based... 

The chelating behavior was also evident from H P-NM R experiments. The addition of triphenylphosphine (a) to a catalyst solution of [HRh(CO)2(13 b)] did not affect the complex. Moreover, the addition of 1 equiv. of b to a solution of HRh(CO)2(13) PPhj resulted in the exclusive formation of [HRh(CO)2(13 b)] upon release of free triphenylphosphine. The chelating effect of the supramolecular ligand assembly effectively competes with triphenylphosphine, leading to exclusive formation of the rhodium complex of 13 b. In the complex [HRh(CO)2(13 b) the supramolecular ligand 13 b coordinates in an equatorial-equatorial fashion to the rhodium metal center, whereas the HRh(CO)2(13)PPh3 exists in a mixture of complexes (ee and ea). 

J 0.2 Secondary Phosphines or Phosphites as Supramolecular Ligands 259 Table 10.1 Hydroformylation of 1-heptene with Pt(cod)2 as precursor and SPOs"... 

It appeared finally that the support can no longer be considered as a pure inert stabilizing partner. Actually, the support acts as a supramolecular ligand and has been claimed to promote specific electronic properties and/or geometrical features of the nano-sized supported metal particles. Any metal-support interaction (MSI) does occur in any case when small particles are deposited on a carrier. However, the extent of their interaction depends on the nature of the metal, but much more on the size of the particles and the nature of the support. 

Higher (and supramolecular) ligands based on sugar, porphyrin, dendrimers, cyclodextrins, calix[4]arenes, etc., have also been tested for water-soluble conversions, the hydroformylation of water-insoluble olefins included [219]. In some cases the water-soluble, macromolecular cpds. act as inverse phase-transfer catalysts, e. g., when crown ethers are involved [269]. 

A. Corma et al. assume that the positive effect of the support on the activity can be attributed to the increase concentration of hydrogen due to the zeolitic support. A preferential interaction between the substrate and the zeolitic surface increases the enantioselectivity. Another explanation can be given assuming that the zeolite acts as a supramolecular ligand. The activity and selectivity of the catalyst are then a consequence of the solvatation by the zeolite framework (surface curvature) (24). 

Catalyzing 1,3-dipolar cycloadditions may be the most unusual feature of this supramolecular ligand, though by far not the only one. The size of cucurbituril... 

Supramolecular ligands can also be used to stabilize metastable nuclearities, bringing further the strategy mentioned above. 

In comparison to -butanal, isobutyraldehyde is formed in these conditions only to 15%. It can be reacted with formaldehyde to produce 2,2-dimethyl-l,3-propanediol (neopentyl glycol) [28]. Preferential iso-selective hydroformylation of propene was recently announced by Eastman in collaboration with Reek s group by using supramolecular ligands for rhodium, which were constructed by the complexation of pyridylphosphines in Mg-centered porphyrine complexes (Scheme 4.3) [29]. 

A multifunctional rhodium catalyst has been developed (Scheme 8) that enables one-pot hydroformylation of alkenes (157) followed by hydrogenation of the arising linear aldehyde to produce the linear alcohol (160). This cascade is controlled by the cooperative action of two different supramolecular ligands (158) and (159). ° ... 

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