Hydrogenation enantioselective catalysts

The titanocene dichloride complexes derived from the camphor- and pinene-annulated ligands 126 and 127 were tested as enantioselective hydrogenation catalyst and using 2-phenylbutene as substrate 2-phenylbutane was obtained with ee up to 34% [148, 149]. 

These examples are part of a broader design scheme to combine catalytic metal complexes with a protein as chiral scaffold to obtain a hybrid catalyst combining the catalytic potential of the metal complex with the enantioselectivity and evolvability of the protein host [11]. One of the first examples of such systems combined a biotinylated rhodium complex with avidin to obtain an enantioselective hydrogenation catalyst [28]. Most significantly, it has been shovm that mutation-based improvements of enantioselectivity are possible in these hybrid catalysts as for enzymes (Figure 3.7) [29]. 

Fig. 2.6 Chiral functionalised NHC complexes as enantioselective hydrogenation catalysts with dihydrogen...

The fundamental concepts of enantioselective hydrogenation were introduced in Section 2.5.1 of Part A, and examples of reactions of acrylic acids and the important case of a-acetamido acrylate esters were discussed. The chirality of enantioselective hydrogenation catalysts is usually derived from phosphine ligands. A number of chiral phosphines have been explored in the development of enantioselective hydrogenation catalysts,21 and it has been found that some of the most successful catalysts are derived from chiral 1, l -binaphthyldiphosphines, such as BINAP.22... 

Enantioselective hydrogenation catalysts with chiral nitrogen ligands have been rather neglected, however. Until now, it seems that ligands containing only... 

Early transition-metal complexes have been some of the first well-defined catalyst precursors used in the homogeneous hydrogenation of alkenes. Of the various systems developed, the biscyclopentadienyl Group IV metal complexes are probably the most effective, especially those based on Ti. The most recent development in this field has shown that enantiomerically pure ansa zirconene and titanocene derivatives are highly effective enantioselective hydrogenation catalysts for alkenes, imines, and enamines (up to 99% ee in all cases), whilst in some cases TON of up to 1000 have been achieved. 

A number of chiral ligands, especially phosphines, have been explored in order to develop enantioselective hydrogenation catalysts.9 Some of the most successful catalysts... 

Development of molecularly imprinted enantioselective hydrogenation catalysts based on immobilised rhodium complexes was reported by Gamez et al. [29]. The imprinted catalysts were prepared by polymerising Rh(I)-(A,A -dimethyl-l,2-diphe-nylethanediamine) with di- and tri-isocyanates, using a chiral alkoxide as the template (9). The imprinted polymer, after removal of the template, was tested for the reduction of ketones to alcohols. An enhanced enantioselectivity was observed in the presence of the imprinted polymeric catalyst, in comparison to the control polymer. 

A particularly important example is the 2,2 -diol, which is called BINOL. Another important type includes 1, I -binaphthyl diphosphines, such as BINAP. BINOL and BINAP are useful chiral ligands in organometallic compounds that serve as catalysts for hydrogenations and other reactions. In Section 2.5.1.1, we discuss how compounds such as BINOL and BINAP have been used to develop enantioselective hydrogenation catalysts. 

A number of chiral ligands have been explored in order to develop enantioselective hydrogenation catalysts.Some of the most successful catalysts are derived from chiral 1,1 -binaphthyldiphosphines such as BINAP. These ligands are chiral by virtue of the sterically restricted rotation of the two naphthyl rings (see Section 2.1.5). Scheme 2.10 gives the structures and common names of some other important chiral diphosphine ligands. 

As to the applications of proline, it can modify the surface of palladium on charcoal to give an enantioselective hydrogenation catalyst (Section D.2.3.1.). Forming amides with aromatic carboxylic acids, proline has served as an auxiliary in the enantioselective Birch reduction of the aromatic system (Section D. 1.1.1.3.1.). An interesting application is the catalysis of the aldol-type cyclization of triketones. Among the amino acids tested, it is often the best choice for high enantiomeric excess3 4. 

De Smet et al. introduced a continuous process combining SRNF with homogeneous catalysis. The concept was proven with two enantioselective hydrogenation catalysts, Rh-EtDUPHOS and Ru-BINAP, separated from a methanol reaction mixture by the commercial MPF-60 membrane with rejection values of 97-98%. Hydrogen played the role of reagent for the catalysis and its pressure provided the driving force for the separation [17]. 

Klabunovskii, E.l. (1991) On some general regularities in the action of enantioselective hydrogenation catalysts, J. Organomet. Chem, 417, 181-191, Chem. Abstr. 1991,115, 231385p. 

Fish, M.J., and Ollis, D.J. (1977) Characterization of enantioselective hydrogenation catalysts transient electrochemical oxidation of D-(+)-tartaric acid on Nickel, J. Catal. 50, 353-363. 

This book contains many publications which represent analyses of the steps of elaboration of effective heterogeneous enantioselective hydrogenation catalysts, of their significant role in the theory of catalysis, and of their role in the practice of asymmetric catalysis. In addition to reviewing the first works on catal Tic hydrogenation of C=C double bond in prochiral compounds on metal catalysts supported on chiral carriers, which admittedly have only historical interest, the Chapters 1-3 review data on asymmetric adsorption of enantiomers and separation of racemic mixtures on organic and inorganic adsorbents. 

The use of polymer-protected nanoparticles can contribute new features to catalysis. A platimun sol as well as Pt on AI2O3 are well-known enantioselective hydrogenation catalysts. The possible uses of hybrid nanocomposites in catalysis are far from all realized. ... 

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