Catalysis, chiral

This example illustrates a subtle control of a chemical reaction by a delicate manipulation of tire stereochemical environment around a metal centre dictated by tire selection of tire ligands. This example hints at tire subtlety of nature s catalysts, tire enzymes, which are also typically stereochemically selective. Chiral catalysis is important in biology and in tire manufacture of chemicals to regulate biological functions, i.e., phannaceuticals. 

Design of chiral catalysis and asymmetric autocatalysis for diphenyl-(l-methyl-pyrrolidin-2-yl) methanol-catalyzed enantioselective additions of organozinc reagents 97YGK994. 

Formation of C-C Bonds Using Dialkylzinc Reagents Under Chiral Catalysis... 

Table 26. Addition of Dialkylzinc Reagents to Aldehydes under Chiral Catalysis...

Enzyme-mediated oxidation reactions offer highly diverse options for the modification of existing functional groups as well as for the introduction of novel function in chiral catalysis. Biooxidations often enable us to obtain complementary solutions to metal-assisted transformations and organocatalysis and are considered one of the important strategies of green chemistry . 

Synthesis of optically pure compounds via transition metal mediated chiral catalysis is very useful from an industrial point of view. We can produce large amounts of chiral compounds with the use of very small quantities of a chiral source. The advantage of transition metal catalysed asymmetric transformation is that there is a possibility of improving the catalyst by modification of the ligands. Recently, olefinic compounds have been transformed into the corresponding optically active alcohols (ee 94-97%) by a catalytic hydroxylation-oxidation procedure. 

The production of enantiomerically pure products is of great importance in chemical industry. The most desirable way to obtain these products is by chiral catalysis. Homogeneous complexes can often be used as chiral catalysts however, because of their difficult regenerability, the development of heterogeneous chiral catalysts by immobilization of these complexes is difficult but highly desired. 

Zeolite-anchored complexes may exhibit a greater activity and selectivity in chiral catalysis. This has been demonstrated by Corma and colleagues.181-184 They prepared a chiral ligand from L-proline (1 in Fig. 7.3). 

Bein, T., ACS Chiral Catalysis Symposium, Orlando, 1996. [mentioned in footnte of reference... 

There are, then, three critical requirements of any catalyst if it is to be exploited on a commercial scale these are activity, selectivity and stability. It has been widely demonstrated and generally accepted that homogeneous catalysts are superior to their heterogeneous counterparts in terms of both activity (certainly under mild reaction conditions) and selectivity (the classical example is chiral catalysis). 

X. Zhang, Chim. Oggi/Chem. Today 2004, 22(5), Supplement on Chiral Catalysis, 10. 

Attempts have also been made to explore chiral catalysis in the Pudovik reaction (the addition of dialkylphosphites to aldehydes). Rath and Spilling158... 

Control of enantioselectivity in addition of hydroxylamines to prochiral activated double bonds has also been achieved through chiral catalysis. Substoichiometric amounts of magnesium bromide-chiral oxazoUdine complex 54 (equation 35) catalyze asymmetric addition of 0-benzylhydroxylamine to pyrazolides of a,/3-unsaturated acids 53 with moderate to good enantioselectivity ". 

Metal-mediated processes represent an important advance in azomethine ylide chemistry, allowing reactions to proceed at ambient temperatures. They also allow for the possibility of chiral catalysis, which is a little explored area as yet, but represents an exciting development allowing for the induction of enantiocontrol without the need for stoichiometric stereocontrolling units. 

Of greater synthetic interest is asymmetric induction by the use of chiral catalysis. Grigg was the first to report chiral catalysis of 1,3-dipolar cycloadditions in 1991 (101). A study of metal salts and chiral ligands revealed that 358 underwent cycloaddition with methyl acrylate to furnish adduct 359 in the presence of C0CI2 and (IR, 25)-A-methylephedrine as the chiral ligand. The pyrrolidine product was isolated in 55% yield with an ee of 84%. The use of methyl acrylate as solvent led to an improved yield of 84% with an excellent ee of 96% (Scheme 3.121). 

Chiral catalysis was introduced in industrial synthesis in the mid-1970 s. The standard example is the catalytic hydrogenation of dehydroamino acid derivatives such as (Z)-2-acetyl-amino-3-phenylpropenoic acid with chiral rhodium complexes to give /V-acetylphenylalanine in high optical purity1. 

Recently, a striking synthesis of (—)-menthol (18) has been introduced8, based on chiral catalysis in the isomerization of 13 to the enamine 14 ... 

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