Olefin hydrogenation of prochiral

In general, of the mixed phosphorus-thioether ligands that have been used in the asymmetric hydrogenation of prochiral olefins, the thioether-phosphinite ligands have provided some of the best results. As an example, a new class of thioether-phosphinite ligands developed by Evans et al. has recently proved to be very efficient for the rhodium-catalysed asymmetric hydrogenation of a... 

Several S/N ligands have also been investigated for the asymmetric hydrogenation of prochiral olefins. Thus, asymmetric enamide hydrogenations have been performed in the presence of S/N ligands and rhodium or ruthenium catalysts by Lemaire et al., giving enantioselectivities of up to 70% ee. Two... 

In addition, several S/S ligands were also investigated for the asymmetric hydrogenation of olefins. In 1977, James and McMillan reported the synthesis of various disulfoxide ligands, which were applied to the asymmetric ruthenium-catalysed hydrogenation of prochiral olefinic acid derivatives, such as itaconic acid. These ligands, depicted in Scheme 8.16, were active to provide... 

In recent years, the asymmetric hydrogenation of prochiral olefins have been developed in the presence of various chiral sulfur-containing ligands combined with rhodium, iridium or more rarely ruthenium catalysts. The best results have been obtained by using S/P ligands, with enantioselectivities of up to 99% ee in... 

The tetradentate ligands (340) and (341) form 1 1 metakligand complexes with [IrCl(cod)]2.548 The complexes were tested in the asymmetric hydrogenation of prochiral olefins, providing enantioselectivities up to 36%. The multitopic ligands L, (342) and (343), bind to Ir1 to form [IrL] species which have been characterized by elemental analysis, mass spectrometry, IR and NMR spectroscopy.549 The complexes show enantioselectivities of up to 30% for the hydrogenation of prochiral olefins under mild reaction conditions. 

For details, see Koenig, K. E. Asymmetric Hydrogenation of Prochiral Olefins in Kosak, J. R. ed. Catalysis of Organic Reactions, Marcel Dekker, New York, 1984, Chap. 3. 

Effect of amphiphiles on the enantioselective hydrogenation of prochiral olefins in water... 

The maximum enantioselectivity of 18 % achieved so far in aqueous hydroformylations may not seem very promising. However, the history of asymmetric hydrogenation of prochiral olefins and ketones demonstrates that such a situation may change fast if there is a strong drive behind the case. 

These heterogeneous catalysts have shown excellent reactivity in the enantioselective hydrogenation of prochiral olefins with enantiomeric excess (ee s) > 92% and regioselectivities (>99%) with quantitative conversion. 

Enantioselective Hydrogenation of Prochiral Olefins - The presence of SDS increased both the rate and the enantioselectivity of hydrogenation of prochiral dehydroaminoacid derivatives using rhodium catalysts modified with the diphosphine 75a (Table 4) in aqueous media.500 For example, addition of 0.13 mmol of SDS to the Rh/75a catalyst shortened the reaction half time (tj/2) from... 

In the late sixties, several research groups in the U.S. (6), Europe (7), and Japan (8) initiated studies of homogeneous catalytic asymmetric syntheses. Of these efforts, the catalytic asymmetric hydrogenation of prochiral olefins reported by Knowles, et al. (6,9) attracted most attention. The importance of this technology was shown by its application in the Monsanto L-DOPA process which had since become an industrial flagship in catalytic asymmetric syntheses (Figure 1). 

The use of soluble rhodium catalysts containing chiral ligands to obtain high stereoselectivity in the asymmetric hydrogenation of prochiral olefins represents one of the most important achievements in catalytic selectivity, rivaling the stereoselectivity of enzyme catalysts [J. Halpem, Science, 217 (1982) 401]. Many chiral ligands... 

Using these sugar-based ligands, the Zheng/Chen group has explored a variety of asymmetric hydrogenation of prochiral olefins. Three types of hydrogenation examined are... 

In the pioneering studies of Homer et al. [57] and Knowles and Sabacky [58], chirally modified Wilkinson catalysts were introduced in the homogeneous enantioselective hydrogenation of prochiral olefins. To this end, in Wilkinson-type catalysts the triphenylphosphine ligand was replaced by the optically active phosphine ligands (-i-)-PMePr"Ph and H-PMePfPh, chiral at the phosphoms atom. 

Enantioselective hydrogenation of prochiral carbonyl compounds with Wilkinson-type catalysts is less successful than the hydrogenation of prochiral olefins. Both rates and enantioselectivities are greatly diminished in the hydrogenation of ketones, compared with olefins. Enantioselectivities only occasionally reach 80% ee, e. g., in the hydrogenation of acetophenone with the in-situ catalyst [Rh(nbd)Cl]2/DIOP, where nbd = norbomadiene [71]. The Ru-based BINAP catalysts improved this situation, by allowing the hydrogenation of a variety of functionalized ketones in enantioselectivities close to 100% ee [72]. 

Most recently, these BlNAP-cored dendrimers were further employed in the Ir-catalyzed asymmetric hydrogenation of quinolines by Fan et al. (Scheme 4.3) [33]. Unlike the asymmetric hydrogenation of prochiral olefins, ketones and imines, the hydrogenation of heteroaroniatic compounds proved to be rather difficult [34—37]. All four generations of dendrimer catalysts generated in situ from BlNAP-cored dendrimers and [lr(COD)Cl]2 were found to be effective, even at an extremely high substrate catalyst ratio in the asymmetric hydrogenation of quinaldine with... 

The successful industrial application of the homogeneous catalytic asymmetric hydrogenation of prochiral olefins depends on the ability of the catalyst systems to offer high activity both in terms of reaction rate and efficient utilization of the catalyst (as the molar substrate to catalyst ratio S/C). It is also essential for the reaction to deliver the product with high enantiomeric excess and in good yield under conditions appropriate for industrial manufacture. In addition, the catalysts should be accessible in appropriate quantities for commercial manufacture. 

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