Phosphine-catalyzed enantioselective [3+2] annulations of 2,3-butadienoates with imines

The first systematic screening of chiral phosphines in the cycloaddition reaction between 2,3-butadienoates and arylimines has led to the identification of fairly efficient catalysts. 2-Aryl-3-pyrrolines have been obtained with enantiomeric excesses up to 64%. In one instance, the enantiomeric excess could be increased to 91% ee by combining the enantioselective cyclization reaction with a crystallization step.     

Modified Hantzsch reaction in the presence of chiral organic catalysts

Modified Hantzsch reaction in three- and four-component system in the presence of new chiral organic catalysts was investigated and the advantages of this reaction in three-component system compared to four-component one were demonstrated, the influence of catalysts on the dynamics and stereochemistry of the reaction was elucidated. The enantiomeric excess of the main reaction product was evaluated with HPLC, its structure was proved by X-ray diffraction analysis.     

The Macromolecular Route to Chiral Amplification

Cooperative phenomena, described by one-dimensional statistical physical methods, are observed between the enantiomeric characteristics of monomeric materials and the polymers they produce. The effect of minute energies associated with this amplified chirality, although currently not interpretable, can be easily measured. Nonlinear relationships between enantiomeric excess or enantiomeric content and polymer properties may offer the possibility of developing chiral catalysts and chiral chromatographic materials in which the burden of large enantiomeric excess or content may be considerably alleviated. New approaches to information and sensor technology may become possible.     

Chiral Ti(IV) complexes of hexadentate Schiff bases as precatalysts for the asymmetric addition of TMSCN to aldehydes and the ring opening of cyclohexene oxide

Chiral dinuclear titanium(IV) complexes (generated in situ from hexadentate Schiff bases and titanium tetra-isopropoxide) have been found to be more effective catalysts for the asymmetric addition of trimethylsilyl cyanide to aldehydes and the ring opening of cyclohexene oxide than their mononuclear analogues. The best results were obtained for benzaldehyde (86% enantiomeric excess) and cyclohexene oxide (89% enantiomeric excess).     

手性催化剂的结构及其反应性能

以天然产物为前体对手性催化剂进行了分类，对每种手性催化剂列举了所催化的不对称反应及其对映选择性；并对催化剂的结构与反应性能的关系进行了初步的总结和分析。参考文献７０篇。     

High-throughput enzymatic method for enantiomeric excess determination of O-acetylated cyanohydrins

The reaction yield and enantiomeric excess of O-acetylated cyanohydrin reaction products from a library of chiral catalysts can be analyzed by a three-step screening method. Alcohol dehydrogenase and NADH are used to analyze unreacted substrate. A lipase with absolute specificity converts one enantiomer to a quantifiable product before the remaining enantiomer is hydrolyzed with an unspecific esterase and quantified.     

Asymmetric Activation

While nonracemic catalysts can generate nonracemic products with or without the nonlinear relationship in enantiomeric excesses between catalysts and products, racemic catalysts inherently give only a racemic mixture of chiral products. Asymmetric catalysts, either in nonracemic or racemic form, can be further evolved into highly activated catalysts with association of chiral activators. This asymmetric activation process is particularly useful in racemic catalysis through selective activation of one enantiomer of the racemic catalyst. Recently, a strategy whereby a racemic catalyst is selectively deactivated by a chiral additive has been reported to yield nonracemic products. However, reported herein is an alternative and conceptually opposite strategy in which a chiral activator selectively activates, rather than deactivates, one enantiomer of a racemic chiral catalyst. The advantage of this activation strategy over the deactivation counterpart is that the activated catalyst can produce a greater enantiomeric excess in the products-even with the use of a catalytic amount of activator relative to chiral catalyst-than that attained by the enantiomerically pure catalyst on its own. Therefore, asymmetric activation could provide a general and powerful strategy for not only the use of atropisomeric, racemic ligands but also chirally flexible and proatropisomeric ligands without enantiomeric resolution!     

Synthesis of new chiral aliphatic amino diselenides and their application as catalysts for the enantioselective addition of diethylzinc to aldehydes

[reaction: see text] A set of chiral aliphatic amino diselenides have been synthesized from readily available starting materials in a straightforward synthetic route via the ring-opening reaction of the parent aziridines. These ligands have been tested as catalysts for the enantioselective addition of diethylzinc to aldehydes. The influence of the alkyl group substituents on the stereoselectivity has been studied, and in the best case, an enantiomeric excess up to 99% could be obtained by using only 0.5 mol % of the chiral diselenide 3a.     

Enantiomer-Differentiation Induced by an Enantiomeric Excess during Chromatography with Achiral Phases

It is shown, in the case of the diketone 2 , that chromatography with achiral phases of a non-racemic mixture of enantiomers can furnish fractions which differ in enantiomeric excess. Such chromatography may, therefore, be used to further enrich a sample in one enantiomer. By the same token, chromatography is not a generally safe method for the purification of the product of an enantio-differentiating process, if the enantiomeric excess of a purified portion of that product is taken to be a measure of the efficiency of the process. The described effect represents an enantiomer differentiation induced solely by an alredy existing enantiomeric excess during chromatography. It thus belongs to a class of effects where the relative amounts of two enantiomers induce an observable difference between them. Such effects are called EE effects. The common principle underlying EE effects is explained by a simple symmetry argument. Since EE effects can also occur during reactions with achiral reagents, further transformations of an enantiomer-enriched product may furnish false information on its enantiomeric excess.     

The dynamic kinetic resolution of azlactones with thiol nucleophiles catalyzed by arylated, deoxygenated cinchona alkaloids

A significant improvement of the available organocatalytic methods (in terms of product substrate scope and product enantiomeric excess) for the generation of enantioenriched α-amino acid thioesters via the dynamic kinetic resolution of azlactones is reported. C-9 arylated cinchona alkaloid catalysts have been found to be considerably superior to other bifunctional alkaloid catalysts as the promoters of this asymmetric process.     

New substituted mono-and bis(imidazolyl)pyridines and their application in nitroaldolisation reaction

New chiral nitrogen ligands based on the substituted mono-and bis(imidazolyl)pyridines have been prepared and characterised. Their complexes with cupric acetate were used as catalysts in the nitroaldolisation reaction. In the case of optically pure complexes of mono(imidazolyl)pyridine, the isolated products were 2-nitro-1-(2-nitrophenyl)ethanols or 2-nitro-1-(4-nitrophenyl)ethanols in overall yields of 49–93 % and with the maximum enantiomeric excess of 15.6 %. The complexes of bis(imidazolyl)pyridine also catalyse the nitroaldol reaction, the yields being 64–90 %, but with zero enantioselective excess. Presented at the 57th Congress of Chemical Societies, Tatranské Matliare, 4–8 September 2005.     

On the way to chiral epoxidations with methyltrioxorhenium(VII) derived catalysts

Methyltrioxorhenium(VII) (MTO) is successfully applied as chiral epoxidation catalysts in the presence of H₂O₂ as oxidizing agent and excess chiral Lewis base ligands derived from pyrazole. Moderate enantiomeric excesses up to ca. 30% can be reached at low reaction temperatures (−30 °C), the conversions however, being quite low (<25%). The reason for this may be the fluctionality of the N-base ligand. Glycolate complexes of MTO, applied under the same conditions reach somewhat higher enantiomeric excesses (up to ca. 40%), however, again associated with low conversions (<30%). In this case the sensitivity of the catalyst to water induced ligand removal as well as to ligand exchange with other diols is the most likely reason. Nevertheless, the enantiomeric excesses reported here are among the best observed for MTO derived catalytic systems reported to date and more active and selective systems seem feasible.     

Kinetic resolution of chiral aminoalkenes via asymmetric hydroamination/cyclisation using binaphtholate yttrium complexes

Chiral binaphtholate yttrium aryl complexes are highly active and enantioselective catalysts for the asymmetric hydroamination of aminoalkenes, as well as the kinetic resolution of alpha-substituted 1-aminopent-4-enes to give trans-2,5-disubstituted pyrrolidines with good enantiomeric excess and high k(rel).     

Catalytic asymmetric epoxidation of chalcones under poly(ethylene glycol)-supported Cinchona ammonium salt catalyzed conditions

Dimeric cinchonine, cinchonidine, and quinine have been anchored (via nitrogen) to long linear PEG chains to afford soluble polymer-supported chiral ammonium salts, which were employed as phase-transfer catalysts in the asymmetric epoxidation of chalcones. The highest enantiomeric excess obtained was 86%.     

Enantiomeric impurities in chiral catalysts,auxiliaries, and synthons used in enantioselective syntheses. Part 4

The enantiomeric purity of chiral reagents used in asymmetric syntheses directly affects the apparent reaction selectivity and the product’s enantiomeric excess. Herein, 46 recently available chiral compounds were evaluated in order to determine their actual enantiomeric compositions. They have not been assayed previously and/or have been introduced after 2006, when the last comprehensive evaluation of commercially available chiral compounds was reported. These compounds are widely used in asymmetric syntheses as chiral synthons, catalysts, and auxiliaries. The enantioselective analysis methods include HPLC approaches using Chirobiotic, Cyclobond and LARIHC series chiral stationary phases, and GC approaches using Chiraldex chiral stationary phases. Accurate, efficient assays for selected compounds are given. All enantiomeric test results were categorized within five impurity levels (i.e., <0.01%, 0.01–0.1%, 0.1–1%, 1–10% and >10%). Different batches of the same reagent from the same company can have different levels of enantiomeric impurities. Many of the reagents tested were found to have less than 0.1% enantiomeric impurities. Only one of the chiral compounds was found to have an enantiomeric impurity exceeding 10%.     

Readily available hydrogen bond catalysts for the asymmetric transfer hydrogenation of nitroolefins

This paper focuses on readily accessible thiourea hydrogen bond catalysts derived from amino acids, whose steric and electronic features are modulated by their degree of substitution at the carbinol carbon center. These catalysts were applied in the asymmetric transfer hydrogenation of nitroolefins furnishing the chiral products in up to 99% yield and 86% enantiomeric excess. The proposed catalyst's mode of action is supported by mechanistic investigations.     

Magnetically recoverable chiral catalysts immobilized on magnetite nanoparticles for asymmetric hydrogenation of aromatic ketones

Novel heterogenized asymmetric catalysts were synthesized by immobilizing preformed Ru catalysts on magnetite nanoparticles via the phosphonate functionality and were characterized by a variety of techniques, including TEM, magnetization, and XRD. These nanoparticle-supported chiral catalysts were used for enantioselective heterogeneous asymmetric hydrogenation of aromatic ketones with very high enantiomeric excess values of up to 98.0%. The immobilized catalysts were easily recycled by magnetic decantation and reused for up to 14 times without loss of activity and enantioselectivity. Orthogonal nature of the present catalyst immobilization approach should allow the design of other superparamagnetic nanoparticle-supported asymmetric catalysts for a wide range of organic transformations.     

Prediction of enantiomeric excess in a combinatorial library of catalytic enantioselective reactions

A quantitative structure-enantioselectivity relationship was established for a combinatorial library of enantioselective reactions performed by addition of diethyl zinc to benzaldehyde. Chiral catalysts and additives were encoded by their chirality codes and presented as input to neural networks. The networks were trained to predict the enantiomeric excess. With independent test sets, predictions of enantiomeric excess could be made with an average error as low as 6% ee. Multilinear regression, perceptrons, and support vector machines were also evaluated as modeling tools. The method is of interest for the computer-aided design of combinatorial libraries involving chiral compounds or enantioselective reactions. This is the first example of a quantitative structure-property relationship based on chirality codes.     

二苯基脯氨醇及其衍生物对前手性酮的不对称催化还原研究进展

对(S)-α,α-二苯基脯氨醇及其衍生物对前手性酮的不对称催化还原研究进行了综述, 表明它们具有催化还原产率高、对映选择性好的作用, 是一类性能优秀的不对称还原反应的有机小分子催化剂.