Ligands enantiopure

The reactions proceed with an e.e. of about 80% when the enantiopure ligand is used. Similar conditions using poly[oxy(methylsilylene)] (PMHS) as the hydride donor lead to reduction of aryl ketones with up to 98% e.e.188... 

These new generation catalysts have been applied to the asymmetric activation of an inactive racemic metal compound by a non-racemic enantiopure ligand, called a vitamer (Scheme 20a). In contrast, a racemic catalyst can interact with an enantiopure chiral poison (asymmetric... 

The synthesis of the enantiopure ligand Kbpa (8) will be described later in this review 48). The molecular structure, derived... 

Abstract The term Lewis acid catalysts generally refers to metal salts like aluminium chloride, titanium chloride and zinc chloride. Their application in asymmetric catalysis can be achieved by the addition of enantiopure ligands to these salts. However, not only metal centers can function as Lewis acids. Compounds containing carbenium, silyl or phosphonium cations display Lewis acid catalytic activity. In addition, hypervalent compounds based on phosphorus and silicon, inherit Lewis acidity. Furthermore, ionic liquids, organic salts with a melting point below 100 °C, have revealed the ability to catalyze a range of reactions either in substoichiometric amount or, if used as the reaction medium, in stoichiometric or even larger quantities. The ionic liquids can often be efficiently recovered. The catalytic activity of the ionic liquid is explained by the Lewis acidic nature of then-cations. This review covers the survey of known classes of metal-free Lewis acids and their application in catalysis. 

The enantiopure ligands (35a-c) can be synthesized by the same method in six steps starting from commercially available 2,6-dimethox-ypyridine (36) (Scheme 7). Enantiopure 35d can be conveniently prepared from enantiopure 41 via hydrogenation as shown in Scheme 7. 

The question may not be asked, but why develop a three-step synthesis when a one-pot protocol already exists The answer, of course, is simple. The three-step synthesis provides a chiral ligand from an enantiopure aminoalcohol whereas the one-pot reaction provides an achiral ligand at best and otherwise a racemic ligand that proves hard to convert to an enantiopure ligand [28]. 

Initial experiments showed that the asymmetric Heck cyclization of 13 can be realized with several chiral enantiopure ligands. For this transformation, the Pfaltz ligand 14 proved optimal providing the dienyl carbamate 15 in an enantioselectivity of 99 % ee. This Heck cyclization is slow under traditional heating (requiring more than 70 h at 100 °C) but can be accomplished in 30 min with no decrease of enantioselectivity at 170 °C in a microwave reactor. Addition of an excess of trifluoroacetic acid to the crude product furnishes (dihydro-iminoethano)carbazole 15 in 75 % yield over two steps. 

Enantioselective aziridination has also been achieved by use of an enantiopure ligand [70]. Reaction of a variety of N-enoyl oxazohdinones with N-aminophthalimide and lead tetraacetate in the presence of camphor-derived chiral ligands provided the N-phthahmidoaziridines in good to high enantiomeric excess (Scheme 13.49). The oxazohdinone moiety of the substrate played an indispensable role in this reaction. The use of aryl acrylates led either to low stereoselectivity or low chemical yield. Coordination of the hgand-mediated Lewis acid to the bidentate acyl oxazohdinone might account for these results. 

Dimethylphenyl isocyanide (0.132 g, 1 mmol) in solution in 3 mL of toluene was added via a double tipped needle to the solution prepared above and stirred overnight at room temperature. Solvents were evaporated to dryness and the solid residue was extracted with pentane (3 x 50 mL) and filtered on Celite 5 g. Enantiopure ligands 1(RP) or 1(SP) were obtained as yellow solids in 72% yield (0.276 g) after solvent removal. 

The case of kinetic resolution in asymmetric catalysis with enantiopure ligands was discussed in detail in section 4.5, which was devoted to kinetic coupling. In the case where the first steps are considered to be in quasi-equilibria, the enantioselectivity of two enantiomers is expressed by eq. (4.144), implying that selectivity is independent on substrate conversion. 

Infinite chains of (Co(L)Cl2.3CH3CN) (6.46) in which the metal centres are coordinated through the enantiopure ligand L (6.45). 

Novel ruthenium complexes of enantiopure ligands (e.g. 2,6-bis[4 (R)-phenyloxazolin-2 -il-pyridine) have been studied for their ability to interact with plasmidic DNA, as well as for their cytotoxic activity. For the first time, the different behaviour of ruthenium enantiomers on the cell cycle in HeLa tumor cells was pointed out. ... 

Starting from achiral phospholes, phosphine sulfides 123 were obtained as a racemate. Hydrolysis and coupling to (5)-valinol furnished 124 and 124, which were easily separated by column chromatography in 93% combined yield. From these precursors, the desired enantiopure ligands 125 could be prepared by conventional methods. 

DMPP itself is not a reactive diene in Diels-Alder reactions,but it is activated by coordination to transition metal ions. Complex 198 contains a labile perchlorato ligand that is easily displaced by the dienophile, which possesses a coordinating atom (O, S, As or P) in the group E. The cycloaddition reaction occurs intramolecularly in a highly organised environment, which leads to the coordinated exo cycloadduct 199 exclusively. A standard decoordination step affords the desired enantiopure ligands 200. Only the exo-syn isomers are formed, which bear the lone pair at the phosphorus atom and the dienophile functionalities at the same side of the molecule. ... 

The same study was performed with chiral enantiopure ligands to investigate the enantioselective version of the reaction, giving vinylic P-stereogenic phosphine boranes (Scheme 6.18). ... 

Asymmetric catalysis uses enantiopure ligands to generate products of high enantiomeric excess. 

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