Rhodium complexes ligands design

Nagel U. Asymmetric hydrogenation of a-(acetylamino)cin-namic acid with a novel rhodium complex the design of an optimal ligand. Angew. Chem. Int. Ed. Engl. 1984 23(6) 435 36. 

The enhanced synthetic potential of rhodium-complex-catalyzed enantioselective hydrogenation provided by these advances in ligand design has led to renewed interest in the reaction mechanism, and here we highlight four recent topics (i) the extended base of reactive intermediates (ii) an improved quadrant model for ligand-substrate interactions (iii) computational approaches to mechanism and (iv) (bis)-monophosphine rhodium complexes in enantioselective hydrogenation. These are discussed in turn. 

A range of metal catalysts have also been studied in aqueous solution for the transformation of carbon dioxide, including rhodium, ruthenium and iridium bipyridine or phenanthroline complexes.One of the most effective systems is the iridium complex shown in Figure 3.14. The ligand design concept used in this study is very clever. The catalytic activity of the complex and its solubility in aqueous solution can be tuned by the pH of the solution.Under acidic... 

Researchers have worked to alleviate the problems of separation and corrosion in processes such as the oxo process by designing catalysts that are confined in a separate phase from the reactants (see Section 14.2.4). A commercially successful approach for propene hydroformylation resulted from preparation of water-soluble rhodium complex catalysts by sulfonation of the phenyl rings of the triphenyl phosphine ligands. The catalyst is used in a reactor with two liquid phases the propene is concentrated in the organic phase and the catalyst in the aqueous phase near the interface. The CO -I- H2 is bubbled into a mixed reactor, and the two-phase liquid product flows to a settler the organic product flows to downstream separation devices, and the aqueous phase with the catalyst is recycled to the reactor. 

The introduction of fluorine substituents instead of hydrogen may be a useful strategy for the synthesis of metalation-resistant NHCs this has obvious parallels with the use of fluorine in medicinal chemistry to prevent metabolism of drugs by oxidation reactions at that C-H position. Af-Pentafluorophenylmethylene-substituted NHC complexes of iridium and rhodium showed no evidence of the activation of C-H, C-C or C-F bonds.Grubbs and co-workers also prepared a NHC ligand designed to be resistant to ort/ o-metalation by virtue of the presence of ort/ o-fluorine substituents, and the metathesis catalysts prepared from these ligands displayed increased activity when compared to traditional SIMes-based catalysts. 

---