Chiral metal catalysts

The detailed pathways of asymmetric induction during catalysis are not well understood even for the more widely studied hydrogenations (8, 22), and matching substrates with the most suitable transition-metal chiral catalyst remains very much an empirical art. We are not aware of kinetic studies except our own on catalyzed asymmetric hydrogenations, although they usually are assumed to follow well-studied nonchiral analogs for example, RhClP3 systems, where... 

This chemical bond between the metal and the hydroxyl group of ahyl alcohol has an important effect on stereoselectivity. Asymmetric epoxidation is weU-known. The most stereoselective catalyst is Ti(OR) which is one of the early transition metal compounds and has no 0x0 group (28). Epoxidation of isopropylvinylcarbinol [4798-45-2] (1-isopropylaHyl alcohol) using a combined chiral catalyst of Ti(OR)4 and L-(+)-diethyl tartrate and (CH2)3COOH as the oxidant, stops at 50% conversion, and the erythro threo ratio of the product is 97 3. The reason for the reaction stopping at 50% conversion is that only one enantiomer can react and the unreacted enantiomer is recovered in optically pure form (28). 

Chira.lHydrogena.tion, Biological reactions are stereoselective, and numerous dmgs must be pure optical isomers. Metal complex catalysts have been found that give very high yields of chiral products, and some have industrial appHcation (17,18). The hydrogenation of the methyl ester of acetamidocinnamic acid has been carried out to give a precusor of L-dopa, ie, 3,4-dihydroxyphenylalanine, a dmg used in the treatment of Parkinson s disease. 

In 1990, Jacobsen and subsequently Katsuki independently communicated that chiral Mn(III)salen complexes are effective catalysts for the enantioselective epoxidation of unfunctionalized olefins. For the first time, high enantioselectivities were attainable for the epoxidation of unfunctionalized olefins using a readily available and inexpensive chiral catalyst. In addition, the reaction was one of the first transition metal-catalyzed... 

Although the first metal-catalyzed asymmetric 1,3-dipolar cycloaddition reaction involved azomethine ylides, there has not been any significant activity in this area since then. The reactions that were described implied one of more equivalents of the chiral catalyst, and further development into a catalytic version has not been reported. 

Nitronates show a similar reacdvity to that of nitrones, and nitrones are one of 1,3-thpoles that have been successfully developed to catalyzed asymmetric versions, as discussed in the secdon on nitrones fSecdon 8 3 1 However, asymmetric nitronate cycloadthdons catalyzed chiral metal catalysts have not been reported Kanemasa and coworkers have demonstrated that nitronate cycloadthdon is catalyzed by Lev/is acids fEq 8 93 This may open a new way to asymmetric nitronate cycloadthdon catalyzed by chiral catalysts... 

The use of dirhodium(II) catalysts to generate ylides that, in turn, undergo a vast array of chemical transformations is one of the major achievements in metal carbene chemistry [1,103]. Several recent reviews have presented a wealth of information on these transformations [1, 103-106], and recent efforts have been primarily directed to establishing asymmetric induction, which arises when the chiral catalyst remains bound to the intermediate ylide during bond formation (Scheme 11). 

Transition metal coupling polymerization has also been used to synthesize optically active polymers with stable main-chain chirality such as polymers 33, 34, 35, and 36 by using optically active monomers.29-31 These polymers are useful for chiral separation and asymmetric catalysis. For example, polymers 33 and 34 have been used as polymeric chiral catalysts for asymmetric catalysis. Due... 

Can a chiral catalyst containing the same ligand/metal components promote the formation of both enantiomers enantioselectively The bis(oxazoline)magnesium perchlorate-catalyzed asymmetric Diels-Alder reaction [103]... 

In 2005, Carretero et al. reported a second example of chiral catalysts based on S/P-coordination employed in the catalysis of the enantioselective Diels-Alder reaction, namely palladium complexes of chiral planar l-phosphino-2-sulfenylferrocenes (Fesulphos). This new family of chiral ligands afforded, in the presence of PdCl2, high enantioselectivities of up to 95% ee, in the asymmetric Diels-Alder reaction of cyclopentadiene with A-acryloyl-l,3-oxazolidin-2-one (Scheme 5.17). The S/P-bidentate character of the Fesulphos ligands has been proved by X-ray diffraction analysis of several metal complexes. When the reaction was performed in the presence of the corresponding copper-chelates, a lower and opposite enantioselectivity was obtained. This difference of results was explained by the geometry of the palladium (square-planar) and copper (tetrahedral) complexes. 

These examples serve to illustrate several general points about use of chiral catalysts for D-A reactions. A cationic metal center is present in nearly all of the catalysts developed to date and has several functions. It is the anchor for the chiral ligands and also serves as a Lewis acid with respect to the dienophile. The chiral ligands establish the facial selectivity of the complexed dienophile. There are several indications of the importance of the anions to catalytic activity. Anions, in general,... 

As with D-A reactions, it is possible to achieve enantioselective cycloaddition in the presence of chiral catalysts.156 Many of the catalysts are similar to those used in enantioselective D-A reactions. The catalysis usually results from a lowering of the LUMO energy of the dipolarophile, which is analogous to the Lewis acid catalysis of D-A reactions. The more organized TS, incorporating a metal ion and associated... 

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