New approaches to chiral discrimination in coupling between molecules

Results in the coupling of chiral molecules are reviewed from elementary points of view and some new results are given. We show that interactions between chiral molecules can be treated by using molecular quantum electrodynamics in electric and magnetic dipole approximation in ways different from standard diagrammatic perturbation theory. The interactions are the dispersive coupling of ground-state chiral molecules and excitation transfer, with emphasis on chiral discrimination. For ground-state molecules the coupling is dealt with first by calculating the coupling, at all separation distances, of electric and magnetic dipoles induced in the two molecules by fluctuations in the vacuum radiation field. The second method is the response by one chiral molecule to the field generated by the other. Excitation transfer is treated as the response by the accepting ground-state molecule to the dipole field of the donor. A novel variant in finding the rate of excitation transfer is by using Poynting's theorem. Received: 17 June 1998 / Accepted: 6 October 1998 / Published online: 16 March 1999     

Chirality induction of polyaniline derivatives through chiral complexation

Chirality induction of π-conjugated polyaniline derivatives was achieved by chiral complexation with chiral palladium(II) complexes. The crystal structure of the chiral conjugated complex with a model compound of the polyaniline, N,N-bis(4^′-dimethylaminophenyl)-1,4-benzoquinonediimine, revealed a chiral propeller twist conformation of the π-conjugated moiety.     

手性胶束的不对称诱导作用不对称苯偶姻缩合反应

胶束体系是模拟酶的简单模型之一。手性胶束对反应有手性诱导作用。在表面活性剂(1R，2S)-(-)-N-十二烷基-N-甲基麻黄素溴化物和(1R，2S)-(-)-N-十六烷基-N-甲基麻黄素溴化物形成的胶束体系中进行的苯偶姻缩合反应，生成了光学活性的α-羟基酮。     

One-pot synthesis of chiral azetidines from chloroaldehyde and chiral amines

A variety of chiral azetidinepiperidines have been synthesized utilizing an expedient one-pot union of piperidine chloroaldehyde with chiral amines. This two step one-pot procedure provides access to an interesting set of compounds that retain the chiral purity of the starting chiral amine.     

手性在锆原子上的二茂锆配合物的合成

由新孟基环戊二烯和三甲基硅基环戊二烯制备了两个前手性的二茂锆配合物。由它们与亲核试剂的反应,合成了一些新的,手性在锆原子上的锆配合物。^1HNMR和^1^3CNMR谱图表明,配体上的手性基团对锆原子上的取代反应有明显的手性诱导效果。     

Investigation of brush-type chiral stationary phases based on O,O'-diaroyl tartardiamide and O,O'-bis-(arylcarbamoyl) tartardiamide

Four chiral organosilanes based on O,O'-dibenzoyl tartardiamide, O,O'-bis-(3,5-dimethylbenzoyl) tartardiamide, O,O'-bis-(phenylcarbamoyl) tartardiamide and O,O'-bis-[(3,5-dimethylphenyl)carbamoyl] tartardiamide were synthesized and immobilized on silica to afford corresponding brush-type chiral stationary phases (CSPs) with well-defined structures. Using 54 compounds containing a wide variety of structures as analytes, the enantioselectivities of the four CSPs were evaluated under normal-phase modes. 3,5-Dimethyl substituent in the aryl group was found to significantly affect the enantioselectivity of CSPs containing aryl ester moieties. Aryl carbamate moieties in CSPs were observed more beneficial for enantioseparation than aryl ester moieties. The additional hydrogen-bond donors (NH) present in the carbamate groups contributed greatly to the enantioselectivity of CSPs, which is contrary to the results that have been found in network-polymeric CSPs.     

Scalemic and racemic imprinting with a chiral crosslinker

The development of molecularly imprinted chiral stationary phases has traditionally been limited by the need for a chiral pure template. Paradoxically, availability of a chiral pure template largely defeats the purpose of developing a chiral stationary phase. To solve this paradox, imprinting of scalemic and racemic template mixtures was investigated using both chiral (N-α-bismethacryloyl-l-alanine) and achiral (N,O-bisacrylamide ethanolamine) crosslinkers. Imprinting of scalemic mixtures provided polymers capable of partial separation of Boc-tyrosine enantiomers with virtually the same results when using either the chiral or achiral crosslinker. However, the chiral crosslinker was required for chiral differentiation by the racemic imprinted polymers which were evaluated in both batch rebinding and chromatographic modes. Batch rebinding analysis revealed intersecting binding isotherms for the L- and D-Boc-tyrosine, indicating bias for the D or L enantiomer is concentration dependent. Partial chromatographic separation was achieved by the racemic imprinted polymers providing variable D or L bias in equal probability over multiple replicates of polymer synthesis. Correlation of enantiomer bias with the batch rebinding results and optimization of HPLC parameters are discussed.     

A Chiral Phosphorous Derivatizing Agent for the Determination of the Enantiomeric Excess of Chiral Alcohols, Amines by ~(31)P NMR

The suitable chiral derivatizing agent (CDA) is the essential factor for the determination of the optical purity of the enantiomeric substances by NMR method. Among different kinds of CDAs, the chiral phosphorous derivatizing agents1 have become very popular. In this paper, we report a simple and highly efficient 31P NMR method for the ee determination of chiral alcohols and amines, based on the phosphorous CDA 12. R1R2XH+X=O, NH+123A3BOOPClXR1R2HOOPOOPXR1R2H The po…     

Application of acyclic chiral auxiliaries on alkylation reactions

The application in alkylation reactions of an acyclic chiral auxiliary is described. The synthesis is straightforward from a chiral primary amine and a double acylation. A characteristic of this auxiliary is its modular design formed by an achiral part (acyl) and a chiral component (primary amine) so it can be tuned for different reactions without difficulty. The alkylation proceeds with excellent diastereoselectivity because the conformational flexibility of the enolate is restricted by the formation of a chelate and the allylic 1,3-strain.     

Spontaneous segregation on a hybrid chiral surface

Segregation of enantiomers in two-dimensional adsorbed layers is a process that is usually controlled by anisotropic directional interactions between adsorbed molecules. In this contribution, we propose a simple theoretical model in which the chiral segregation occurs even though the lateral interactions are neglected. In particular, we consider a solid surface composed of two domains with different patterns of active sites being mirror images of each other. The domains of opposite handedness represent crystal facets of a composite chiral material which are adjoined to form a heterochiral adsorbing surface. To explore equilibrium properties of the system, we use Canonical Ensemble Monte Carlo method for a square lattice. The influence of factors such as energetic properties of the surface and density of the adsorbed layer on the extent of separation is examined. The obtained results indicate that effective two-dimensional separation on the hybrid chiral surface assumed in our model can be achieved only at sufficiently low adsorbate densities. The results also suggest that the segregation on the hybrid surface would be a promising method of enantiodiscrimination for those chiral molecules which do not exhibit strong lateral interactions.     

Addition of lactate-derived chiral allyltrichlorostannanes to chiral aldehydes

Chiral lactate-derived allyltrichlorostannanes reacted with chiral α-methyl β-alkoxy and syn and anti α-methyl-β-alkoxy aldehydes to give the corresponding homoallylic alcohols with moderate to high 1,4-syn-diastereoselectivities.     

Exploring the Synthesis of a New Group of Chiral Ammonium Salts with Specific Configurations at the Stereogenic Nitrogen Centers

A group of new chiral dications with a fixed, specific configuration at the stereogenic nitrogen center was created. Stereoselective synthesis and recrystallization give the diastereomerically and enantiomerically pure dications, including a chiral amphiphile with surface‐active properties.     

Direct chiral discrimination in NMR spectroscopy

Conventional nuclear magnetic resonance spectroscopy is unable to distinguish between the two mirror-image forms (enantiomers) of a chiral molecule. This is because the NMR spectrum is determined by the chemical shifts and spin–spin coupling constants which – in the absence of a chiral solvent – are identical for the two enantiomers. We discuss how chirality may nevertheless be directly detected in liquid-state NMR spectroscopy: In a chiral molecule, the rotating nuclear magnetic moment induces an electric dipole moment in the direction perpendicular to itself and to the permanent magnetic field of the spectrometer. We present computations of the precessing electric polarization following a π/2 pulse. Our estimates indicate that the electric polarization should be detectable in favourable cases. We also predict that application of an electrostatic field induces a chirally sensitive magnetization oscillating in the direction of the permanent magnetic field. We show that the electric-field-perturbed chemical shift tensor, the nuclear magnetic shielding polarizability, underlies these chiral NMR effects.     

Chirality induction of π-conjugated chains through chiral complexation

Chirality induction of π-conjugated polyanilines through chiral complexation with the chiral palladium(II) complexes was demonstrated to afford the chiral conjugated polymer complexes. Complexation of the emeraldine base of poly(o-toluidine) (POT) with the chiral palladium(II) complex bearing one labile coordination site led to the formation of the chiral conjugated polymer complex, which exhibited an induced circular dichroism (ICD) based on the chirality induction into a π-conjugated backbone. The mirror image of the CD signal was observed with the chiral conjugated polymer complex, which was obtained from the chiral palladium(II) complex possessing the opposite configuration. The chirality of the podand ligand moieties of the palladium complex is considered to induce a propeller twist of the π-conjugated molecular backbone. The crystal structure of the chiral conjugated complex of N-bis(4′-dimethylaminophenyl)-1,4-benzoquinonediimine (L³) as a model compound of the polyaniline revealed a chiral propeller twist conformation of the π-conjugated backbone. Furthermore, chiral complexation with the cationic palladium(II) complexes provided the ionic chiral conjugated complexes.     

Chiral discrimination of aliphatic amines and amino alcohols using NMR spectroscopy

Two methods are compared for analyzing the enantiomeric purity of aliphatic amines and amino alcohols using NMR spectroscopy. The first employs (+)‐(18‐crown‐6)‐2,3,11,12‐tetracarboxylic acid as a chiral NMR solvating agent in methanol‐d₄. The second involves a derivatization scheme in which the amine is reacted with naphtho[2,3‐c]furan‐1,3‐dione to form the corresponding amide. The naphthyl amide is then mixed with a chiral calix[4]resorcinarene in deuterium oxide. The crown ether only produces sufficient enantiomeric discrimination to determine enantiomeric purity for three of the nine substrates studied. The system with the naphthyl amide and a calix[4]resorcinarene produces enantiomeric discrimination of sufficient magnitude to determine enantiomeric purity for all nine substrates. The H1 and H4 resonances of the naphthyl ring are especially suitable to monitor for enantiomeric discrimination. The order of the (R)‐ and (S)‐enantiomers of the H1 and H4 resonances exhibit specific trends for aliphatic amines and amino alcohols that correlate with the absolute configuration. Copyright © 2012 John Wiley & Sons, Ltd.     

Theoretical study of chiral discrimination in the hydrogen bonding complexes of the hydrazine dimer

In this work, the discrimination of different chiral forms of the hydrazine dimer were investigated using Density Functional Theory (DFT) and second‐order Moller–Plesset Perturbation (MP2) theory at basis set levels from 6‐31g to 6‐31++g(d,p). Four chiral structures were studied. The optimized geometric parameters, interaction energies, and chirodiastatic energy for various isomers at different levels were estimated. Finally, the solvent effects on the geometries of the hydrazine dimers were also investigated using self‐consistent reaction‐field (SCRF) calculations at the B3LYP/6‐31++g(d,p) level. The results indicate that the polarity of the solvent has played an important role in the structures and relative stabilities of different isomers. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

On the influence of chiral auxiliaries in the stereoselective cross-coupling reactions of titanium enolates and acetals

Titanium enolates from chiral N-propanoyl-1,3-thiazolidine-2-thiones containing bulky substituents at C4 turned out to be excellent platforms to get highly stereocontrolled cross-coupling reactions with acetals. Related oxazolidinethiones also afforded good results, but the corresponding oxazolidinones resulted completely unselective for such reactions, which proves that an exocyclic CS bond is essential to attain a synthetically useful stereocontrol.     

Synthesis of new chiral fluorescent sensors and their applications in enantioselective discrimination

Five novel sensors (R,R)-3–6 and (S, S)-6 were synthesized and developed for enantioselective recognition of chiral compounds. Sensor 6 with two thiourea groups and steric π-conjugation frameworks could discriminate different chiral substrates, including acidic compounds, basic compounds, and neutral compounds. These results disclosed that the outstanding performance of enantioselective discrimination could be attributed to the thiourea group which acted as a hydrogen-bonding donor and the bulky steric moiety of the hosts which provided appropriate chiral environment. This result will be of great practical value in the designation of chiral sensors and high-throughput assay of chiral products.