Chirality asymmetric metal catalysis

Apart from the asymmetric metal catalysis, enantioselective Baeyer-Villiger oxidations mediated by enzymes have been known for some time [32,33,34]. Both whole-cell cultures and isolated enzymes, usually flavin-dependent monooxygenases, can be used to oxidize ketones enantioselectively. For future improvements in the asymmetric Baeyer-VilHger oxidation the use of chiral Lewis acids in combination with an appropriate oxidant seems worthy of intensive investigation. 

Asymmetric Metal Catalysis The first asymmetric ct-sulfenylation using prochiral substrates under a chiral catalyst was reported by Mukaiyama and co-workers in 1986 (Scheme 46.45). In this pioneering study, the reaction between tin(II) enolates of ketones or 3-acyl-oxazolidin-2-ones and thiosulfonates 394 proceeded with modest-to-high enantioselectivities in the presence of chiral diamine 396. 

Nishiyama H., Motoyama Y. Other Transition Metal Reagents Chiral Transition-Metal Lewis Acid Catalysis for Asymmetric Organic Synthesis in Lewis Acid Reagents 1999 225, Ed Yamamoto H., Pb. Oxford Univ. Press, Oxford Keywords asymmetric Diels-Alder reactions, chiral transition metal Lewis-acid catalysis, asymmetric synthesis... 

In contrast to the maturity of asymmetric synthesis utilizing chiral transition metal catalysts, asymmetric phase transfer catalysis is still behind it and covers organic reactions to lesser extent. Thus, it is further necessary in wide range to explore efficient asymmetric phase transfer catalysis keeping its superiority of easy operation, mild reaction conditions, and environmental binignancy. 

Methods in asymmetric synthesis, particularly those which employ transition metal catalysis, have and continue to contribute enormously to the synthetic chemists armoury and, providing both enantiomers of the catalyst ligand are available, one can synthesise either enantiomer of a chiral intermediate at will. 

As shown in Figure 1.26, a chiral Sm(III) complex catalyzes asymmetric reduction of aromatic ketones in 2-propanol with high enantioselectivity. Unlike other late-transition-metal catalysis, the hydrogen at C2 of 2-propanol directly migrates onto the carbonyl carbon of substrate via a six-membered transition state 26A, as seen in the Meerwein-Ponndorf-Verley reduction. ... 

The epoxidation of enones using chiral phase transfer catalysis (PTC) is an emerging technology that does not use transition metal catalysts. Lygo and To described the use of anthracenylmethyl derivatives of a cinchona alkaloid that are capable of catalyzing the epoxidation of enones with remarkable levels of asymmetric control and a one pot method for oxidation of the aUyl alcohol directly into... 

The mechanism of the asymmetric induction in asymmetric catalysis by transition metal complexes is only speculation. In principle, the asymmetric induction can result from direct interaction between ligand and substrate as proposed, for instance, by McQuillin et al. (32) for the asymmetric hydrogenation. But it might take place simply by interaction between substrate and chiral transition metal atoms without... 

There are several possibilities for asymmetric synthesis in catalysed cyclopropanation and very substantial progress has already been made especially with catalyst development. The option of covalently attaching chiral auxiliaries to diazo compounds or to substrates, e.g. alkenes for cyclopropanation, has been discussed above in the subsection on diastere-oselectivity. The fact that many of the processes require metal catalysis makes the alternative option of using chiral catalysts particularly attractive and potentially more rewarding for commercial exploitation. The double option of combining the use of a chiral catalyst with a diazo compound carrying a chiral auxiliary is also available. For convenience, the double option is also included in this subsection. 

As many endeavors in transition-metal catalysis, the design, synthesis and screening of chiral ligands have played a pivotal role in the development of the asymmetric allylic alkylation reaction. A series of C2-symmetric diphosphines such as DiPAMP, chiraphos, DIOP. and BINAP,... 

The catalytic system described above has been further developed to an asymmetric catalytic complexation of prochiral 1,3-dienes (99% yield, up to 86% ee) using an optically active camphor-derived 1-azabutadiene ligand [56]. This method provides for the first time planar-chiral transition metal 7t-complexes by asymmetric catalysis. 

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