Assessing chiral self-recognition using chiral stationary phases

Chiral stationary phases were synthesized and their ability to separate racemic precursors from which they were derived was assessed. Taken in conjunction with homochiral recognition previously observed in the solid state, the results of this study reveal that a geometrically controlling π-π interaction has a profound influence on molecular recognition.     

Synthesis of a chiral hexa-host molecule

The synthesis of the first chiral hexa-host molecule (III) is described: an X-ray analysis of the acetic acid adduct of (III) allows the first direct observation of dimeric acetic acid.     

Anapole moment of a chiral molecule

Due to the absence of the centre of symmetry a chiral molecule acquires a specific electromagnetic moment, anapole one, which is of the usual atomic order of magnitude. This moment is calculated by us explicitly for a model of a chiral radical. Molecular anapole moments lead to the polarization of chiral radicals in a conducting solution by an applied voltage. The analogous effect can in principle constitute a physical background for the experimental searches forT-violation in atoms and molecules. In this connection we consider also anapole moments of atoms and diatomic molecules caused by parity nonconservation.     

Helicates of chiragen-type ligands and their aptitude for chiral self-recognition

Two (-)-5,6-pinene-bipyridine moieties connected by a para-xylylene bridge (so-called chiragen-type ligands), (-)-L1, undergo self-assembly upon reaction with equimolar amounts of CuI to form enantiopure circular hexanuclear P-helicates. If both enantiomers of L1 are used, mixtures of P and M hexanuclear helicates are exclusively obtained through a complete chiral recognition; that is, no mixing of the (+) and (-) ligands, respectively, occurs upon complexation. This was proven by a) NMR spectroscopy where identical spectra to those for complexes with the enantiomerically pure ligands were obtained and b) circular dichroism (CD) spectroscopy. The reaction is completely changed by the use of the corresponding meso-L1. Instead of well-defined species, oligomeric mixtures are observed, a result demonstrating the crucial role played by ligand chirality in self-assembly processes. Structural variations on the chiral ligand L1, such as a meta-xylylene bridge instead of a para-xylylene one (in L4) or four pinene groups instead of two (in L5 and L6), favor nondiscrete coordination assembly.     

Application of a chiral metal-organic framework in enantioselective separation

A modular approach for the synthesis of highly ordered porous and chiral auxiliary (Evans auxiliary) decorated metal-organic frameworks is developed. Our synthesis strategy, which uses known porous structures as model materials for incorporation of chirality via linker modification, can provide access to a wide range of porous materials suitable for enantioselective separation and catalysis. Chiral analogues of UMCM-1 have been synthesized and investigated for the enantioseparation of chiral compounds in the liquid phase and first promising results are reported.     

Decoherence effects on superpositions of chiral states in a chiral molecule

The superposition of chiral states of chiral molecules, as delocalized quantum states of a many-particle system, can be used for the experimental investigations of decoherence theory. In this regard, a great challenge is the precise quantification of the robustness of these superpositions against environmental effects. The methods so far proposed need the detailed specification of the internal states of the molecule, usually requiring heavy numerical calculations. Here, by using the linearized quantum Boltzmann equation and by borrowing ideas employed for analyzing other quantum systems, we present a general and simple approach, of wide applicability, which can be used to compute the dominant contribution to the decoherence rate for the superpositions of chiral states of chiral molecules, due to environmental scattering.     

Use of a chiral surfactant for enantioselective reduction of a ketone

The influence of a chiral surfactant and a polymer-supported chiral additive on reduction of ketones using sodium borohydride will be described. Initial preparations involved methylation of (S)-leucinol to give (2S)-N , N-dimethyl-2-amino-4-methyl-1-pentanol (1) (67%). The chiral surfactant (2) was synthesized by reacting (1) with bromohexadecane (71%). The functionalized styrene for the polymer-supported chiral additive (5) was synthesized by reacting (1) with 4-vinylbenzyl chloride. Polymerization was carried out with 10% of the functionalized monomer (4), 5% cross-linking agent divinylbenzene, and 85% styrene with AIBN as the initiator. The activity of the chiral surfactant and polymeric additive were examined by using them as additives in a standard reduction of 2-pentanone with sodium borohydride to yield (R)- and (S)-2-pentanol (3) (20%). The resulting alcohol was analyzed by polarimetry (ee 9.5%) and also esterified with (2S)-methylbutyric acid prior to characterization by NMR. 13C NMR indicated an enantiomeric excess of 5.2% when the chiral surfactant was used, and 7% when the polymeric additive was used.     

Application of a chiral scaffolding ligand in catalytic enantioselective hydroformylation

The synthesis of β-amino-aldehydes has been achieved through enantioselective hydroformylation of PMP-protected allylic amines. The reaction is accomplished by using a scalemic scaffolding ligand that covalently and reversibly binds to the substrate. These ligands behave like chiral auxiliaries because they are covalently attached to the substrate during hydroformylation; however, similar to traditional asymmetric ligands, they can be used in catalytic quantities. The directed hydroformylation of disubstituted olefins occurs under mild conditions (35 °C and 50 psi CO/H(2)), and Z-olefins afford excellent enantioselectivities (up to 93% ee).     

Catalytic enantioselective synthesis of axially chiral allenes

The conventional procedures for preparing optically active axially chiral allenes generally require stoichiometric chiral sources as either substrates or reagents. On the other hand, examples of catalytic asymmetric synthesis of axially chiral allenes are rare and it is a relatively underdeveloped area in synthetic organic chemistry. In this review article, various methods for preparing enantiomerically enriched axially chiral allenes using substoichiometric chiral sources are surveyed. Some reactions with stoichiometric but recoverable chiral sources are also mentioned. Most of the asymmetric reactions in these categories are transition-metal-catalyzed reactions, and there are a few examples of organocatalytic reactions. In addition, some enzymatic/microbial systems are also known.     

Permanent microporosity and enantioselective sorption in a chiral open framework

A homochiral microporous material is presented. The phase has 47% permanently porous void volume and is shown to have >1 nm diameter pores with three-dimensional channels using probe molecule sorption. Enantioselective guest sorption is strongly dependent on guest size. The homochiral microporous phase was identified by reactive selection from a first-generation chiral but nonporous framework. Chiral permanent porosity is established by directional noncovalent interactions between framework-forming and nonframework forming components of the stable second-generation material, which become stronger upon loss of the guests from the pore system.     

A novel enantioselective cyclization of a chiral epoxide to a benzofuran system

A convenient method has been established for the stereoselective construction of a dihydrobenzofuran ring system found in several bioactive natural products. The final step of the synthesis involves the concerted and efficient intramolecular displacement of mesylate 12 to give 2′R,3′R-dihydrobenzofuran a bicyclic analog of 1. This route was based on a proposed biosynthetic pathway patterned on related rotenoids.     

Highly enantioselective synthesis of optically active hydroxyaldehydes using a chiral catalyst

Optically active hydroxyaldehydes arc synthesized in 88–94% e.e. by the catalytic enantioselective addition of dialkylzinc using N,N-dibutylnorephedrine.     

Highly enantioselective cyanosilylation of aldehydes catalyzed by a chiral oxazaborolidinium ion

The chiral oxazaborolidinium salts 1 and 2 are excellent catalysts for the enantioselective cyanosilylation of a wide variety of aldehydes (see Table 1) using trimethylsilyl (TMS) cyanide and triphenylphosphine oxide as the source of a new reactive cyanide donor. This donor appears to be the isocyanophosphorane Ph3P(OTMS)(N=C:) (4). The novel process described herein has several advantages: predictability of absolute configuration of cyanohydrin products from a mechanistic model (3), high yields and very good enantiomeric purity of products (>/=90%), and, finally, easy and efficient recovery of the catalytic ligand.     

Self-supported chiral catalysts for heterogeneous enantioselective reactions

The development of heterogeneous chiral catalysts for enantioselective reactions is highly desirable in order to overcome some drawbacks of homogeneous catalysts. Different from the conventional approaches by using various types of supports or biphasic systems for the recovery and reuse of homogeneous catalysts, a conceptually new strategy for heterogenization of homogeneous chiral catalysts, that is, a "self-supporting" approach, has been developed to use homochiral metal-organic coordination polymers generated by the self-assembly of chiral multitopic ligands with metal ions, and thus obviates the use of any support. In this concept article, the success of this "self-supporting" strategy will be exemplified in heterogeneous catalysis of asymmetric carbonyl-ene, sulfoxidation, epoxidation, and asymmetric hydrogenation reactions.     

Catalytic enantioselective allylation with chiral Lewis bases

The advent of chiral Lewis base-promoted allylation of aldehydes has opened a new direction in the catalytic enantioselective construction of homoallylic alcohols. This short review outlines the conceptual framework for the creation of this new process and the interplay of mechanistic investigations and synthetic studies that have conspired to produce a useful new reaction. The current state-of-the-art in catalyst design and application of the reaction in synthesis are briefly illustrated.     

An enantioselective approach to chiral pseudoguaianolide intermediates

The viability of an enantioselective route to pseudoguaianolides has been established with the construction of hydroazulenone 3, whose absolute configuration parallels that of radiatin.     

In-adamantane, a small inside-out molecule

The hydrocarbon in-adamantane (1), a high-energy adamantane isomer in which one methine hydrogen atom is inside the cage, is predicted by ab initio calculations to be isolable at dry ice temperature. It has 440 kJ/mol of hydrogenic strain but appears to be stable against dimerization, moisture, and air. The inverted CH bond is compressed, and the IR and NMR spectra are unusual. The symmetrical pentadecafluoro derivative (2) has an estimated half-life of 100 years at room temperature.     

Gold-catalyzed highly enantioselective synthesis of axially chiral allenes

Axially chiral allenes are synthesized from chiral propargylamines catalyzed by KAuCl4 in high yields (up to 93% yield) and excellent enantioselectivities (up to 97% ee) in CH3CN at 40 degrees C. The reaction has been applied to the synthesis of novel allene-modified artemisinin derivatives with the delicate endoperoxide moieties remaining intact. A tentative mechanism regarding gold(I)-catalyzed intramolecular hydride transfer was proposed on the basis of deuterium-labeling experiments and ESI-MS analysis of the reaction mixture.