Relationship between the helical structure and optical activity of some chiral cyclic esters

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Transition polarizability model of induced resonance Raman optical activity

Induced resonance Raman optical activity (IRROA) proved to be a very sensitive method to detect molecular chirality. It is exhibited, for example, by complexes of lanthanides with chiral alcohols or ketones. So far, the phenomenon has not been understood at a quantitative level. To elucidate its mechanisms and to correctly relate the spectra to the structure, a transition polarizability model (TPM) is developed and applied to a camphor‐europium complex. The model well reproduces the high ROA/Raman intensity ratio of the IRROA observed experimentally. The results additionally indicate a fundamental role of the nonchiral fod ligand in the Eu(fod)₃ compound for the chirality enhancement. The TPM model thus serves as a guidance for both experimental and theoretical studies to come. © 2013 Wiley Periodicals, Inc.     

Theoretical study of helical structure caused by chirality of cysteine dimer

In this work we report a theoretical study of the helix structure and chiral discrimination on the interactions between the chiral cysteine–cysteine. Two reasonable geometries on the potential energy hypersurface of the cysteine–cysteine system are considered with the global minimum. Accurate geometric structures, relative stabilities, harmonic vibrational frequencies, and infrared (IR) intensities were investigated. To take into account the water solvation effect, the Onsager model within the self‐consistent reaction field (SCRF) method and the polarized continuum (PCM) method were used to evaluate the interaction energy, ΔG_solv at the same level employed in the gas phase. The results indicate that the polarity of the solvent plays an important role in the structures and relative stabilities of different isomers. Computational results indicate that the global minimum should be conformer I regardless of whether in the gas phase or in aqueous solution, which differs from previous theoretical reports. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Investigation of molecular and chiroptical characteristics of optically active functional vinyl copolymers possessing main-chain chirality

Optically active styrene-methyl methacrylate and styrene-methacrylonitrile copolymers were obtained by template assisted asymmetric copolymerization of 3,4-cyclohexyhdene-D-mannitol-1,2;5,6-bis-O[(4-vinylphenyl)boronate] ( 1 ) with the corresponding comonomers followed by removal of the template and deboronation. By polymer analog modification methods, several optically active functional copolymers were obtained starting with these copolymers. The resulting polymers were subjected to different types of spectroscopic analysis. Thus, the results obtained from IR, ¹³C-NMR and GPC were of great value for detailed structural analysis of these polymers providing both qualitative and quantitative informations in terms of mode and neatness of substitution and degree of functionalization. The chiroptical measurement results obtained from circular dichroic (CD) studies have been utilized to analyze precisely the substituent dependent local solution stereochemistry of these chiral polymers. © 1992 John Wiley & Sons, Inc.     

Detection and amplification of chirality by helical polymers

A unique feature of synthetic helical polymers for the detection and amplification of chirality is briefly described in this article. In sharp contrast to host-guest and supramolecular systems that use small synthetic receptor molecules, chirality can be significantly amplified in a helical polymer, such as poly(phenylacetylene)s with functional pendants, which enable the detection of a tiny imbalance in biologically important chiral molecules through a noncovalent bonding interaction with high cooperativity. The rational design of polymeric receptors can be possible by using chromophoric helical polymers combined with functional groups as the pendants, which target particular chiral guest molecules for developing a highly efficient chirality-sensing system. The chirality sensing of other small molecular and supramolecular systems is also briefly described for comparison.     

Vibrational Raman optical activity of pi-conjugated helical systems: hexahelicene and heterohelicenes

Helicenes constitute a special class of molecules combining helical conformation with pi-electron delocalization. These confer to helicenes specific chirooptical properties. In this article, we investigate the vibrational signatures thanks to the simulation of vibrational Raman optical activity (VROA) spectra. For that, four representative helicenes: hexahelicene, tetrathia-[7]-helicene, and its pyrrole and furan analogs have been simulated and interpreted using a recently implemented analytical scheme. Helicenes show intense VROA peaks attributed to their pi-conjugated structure and associated with collective vibrational modes. In hexahelicene, the dominant VROA features are due to vibrational modes involving motions of the carbon skeleton and H-wagging, but the intensity finds its source almost exclusively in the former. In the case of the three heterohelicenes, the previous statement is also verified, and on changing the heteroatoms, similar modes presenting comparable atomic contribution patterns have been highlighted, though the vibrational and electronic properties are modified. Some fingerprints could therefore be associated with the helicity of the system. In particular, in forward spectra, most of the VROA bands are positive for left-handed helicenes. Nevertheless, the spectral patterns are quite complex, and no easy rule-of-thumb could distinguish between the different heterohelicenes. Then, considering the fact that most of the contributions originate from the C atoms (group coupling matrices decomposition), it can be concluded that the major role of the heteroatom is restricted to modifying the geometry and the normal modes. At last, the small impact of the gauge-origin on the calculated spectra using a relatively modest basis set (rDPS:3-21G) is demonstrated here in the case of the tetrathia-[7]-helicene molecule presenting a C(2) symmetry. This further demonstrates the adequacy of this basis set for VROA calculations.     

Supramolecular helical columns from the self-assembly of chiral rods

Chiral-bridged rod molecules (CBRs) that consisted of bis(penta-p-phenylene) conjugated to an opened or closed chiral bridging group as a rigid segment and oligoether dendrons as flexible segments were synthesized and characterized. In the bulk state, both molecules self-assemble into a hexagonal columnar structure, as confirmed by X-ray scatterings and transmission electron microscopy (TEM) observations. Interestingly, these structures display opposite Cotton effects in the chromophore of the aromatic unit in spite of the same chirality (R,R) of the chiral bridging groups. The molecules were observed to self-assemble into cylindrical micellar aggregates in aqueous solution, as confirmed by light scattering and TEM investigations, and exhibit intense signals in the circular dichroism (CD) spectra, which are indicative of one-handed helical conformations. The CD spectra of each molecule showed opposite signals to each other, which were similar to those in the bulk. Notably, when the opened CBR was added to a solution of closed CBRs up to a certain concentration, the CD signal of the closed CBR was amplified. This implies that both molecules co-assemble into a one-handed helical structure because the opened chiral bridge is conformationally flexible, which is inverted to co-assemble with the closed CBR. These results demonstrate that small structural modifications of the chiral moiety can transfer the chiral information to a supramolecular assembly in the opposite way.     

Prediction of Raman Optical Activity Spectra of Chiral 3‐Acetylcamphorato‐Cobalt Complexes

We examine calculated vibrational Raman optical activity (ROA) spectra of octahedral cobalt complexes containing different combinations of acetylacetonato and 3‐acetylcamphorato ligands. Starting from the Δ‐tris(acetylacetonato)cobalt(III) complex, the ROA spectra of isomers generated by successive replacement of acetylacetonato ligands by chiral (+)‐ or (?)‐3‐acetylcamphorato ligands are investigated. In this way, it is possible to assess the influence of the degree of ligand substitution, ligand chirality, and geometrical isomerism on the ROA spectra. In addition, the effect of the Λ‐configuration is studied. It is found that the ROA spectra contain features that make it possible to identify each of the isomers, demonstrating the great sensitivity of ROA spectroscopy to the chiral nature of the various complexes.     

Chiral amplification in macromolecular helicity assisted by noncovalent interaction with achiral amines and memory of the helical chirality

Poly[(4-carboxyphenyl)acetylene] (poly-1) exhibits an intense induced circular dichroism (ICD) in the UV-visible region upon complexation with excess (R)-1-(1-naphthyl)ethylamine ((R)-2), owing to the formation of a predominantly single-handed helical conformation of the polymer backbone. In the presence of a small amount of (R)-2, poly-1 showed a very weak ICD due to the lack of a single-handed helical conformation. However, we have found that the co-addition of the excess bulky, achiral 1-naphthylmethylamine (5) with a small amount of (R)-2 caused a dramatic increase in the ICD magnitude, comparable to the full ICD induced by excess (R)-2. This indicates that an almost single-handed helix can be induced on poly-1 upon complexation with a small amount of (R)-2 assisted by achiral 5. Furthermore, the induced single-handed helical poly-1 could be successfully memorized by the replacement of (R)-2 and 5 with achiral 2-aminoethanol or n-butylamine.     

Theoretical determination of the vibrational Raman optical activity signatures of helical polypropylene chains.

Raman and vibrational Raman optical activity (VROA) spectra of helical conformers of polypropylene chains are simulated using ab initio methods to unravel the relationships between the vibrational signatures and the primary and secondary structures of the chains. For a polypropylene chain containing three units, conformational effects are shown to lead to more acute signatures for VROA than for Raman spectra. In addition to regular polypropylene chains, which can display right and left helicities with the same probability, chirality and therefore helicity are enforced by substituting one chain end with a phenyl group. The simulations predict that the threefold helical structures, which correspond to (TG)(N) conformations of the backbone, have a specific VROA backward signature in the form of an intense couplet around 1100 cm(-1). This couplet is associated with collective wagging and twisting motions, while most of its intensity comes from the anisotropic invariants combining normal coordinate derivatives of the electric dipole-electric dipole polarizability and of the electric dipole-magnetic dipole polarizability. A similar signature has already been found in model helical polyethylene chains, whereas it is very weak in forward VROA.     

Lanthanide class of a trinuclear enantiopure helical architecture containing chiral ligands: synthesis, structure, and properties

The pinene-bipyridine carboxylic derivatives (+)- and (-)-HL, designed to form configurationally stable lanthanide complexes, proved their effectiveness as chiral building blocks for the synthesis of lanthanide-containing superstructures. Indeed a self-assembly process takes place with complete diastereoselectivity between the enantiomerically pure ligand L(-) and Ln(III) ions (La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er), thus leading to the quantitative formation of a trinuclear supramolecular architecture with the general formula [Ln(3)(L)(6)(mu(3)-OH)(H(2)O)(3)](ClO(4))(2) (abbreviated as tris(Ln[L](2))). This class of C(3)-symmetrical compounds was structurally characterized in the solid state and solution. Electrospray (ES) mass spectrometric and (1)H NMR spectroscopic analyses indicated that the trinuclear species are maintained in solution (CH(2)Cl(2)) and are stable in the investigated concentration range (10(-2)-10(-6) m). The photophysical properties of the ligand HL and its tris(Ln[L](2)) complexes were studied at room temperature and 77 K, thus demonstrating that the metal-centered luminescence is well sensitized both for the visible and near-IR emitters. The chiroptical properties of tris(Ln[L](2)) complexes were investigated by means of circular dichroism (CD) and circularly polarized luminescence (CPL). A high CD activity is displayed in the region of pi-pi* transitions of bipyridine. CPL spectra of tris(Eu[(+)-L](2)) and tris(Tb[(+)-L](2)) present large dissymmetry factors g(em) for the sensitive transitions of Eu(III) ((5)D(0)-->(7)F(1), g(em)=-0.088) and Tb(III) ((5)D(4)-->(7)F(5), g(em)=-0.0806). The self-recognition capabilities of the system were tested in the presence of artificial enantiomeric mixtures of the ligand. (1)H NMR spectra identical to those of the enantiomerically pure complexes and investigations by CD spectroscopic analysis reveal an almost complete chiral self-recognition in the self-assembly process, thus leading to mixtures of homochiral trinuclear structures.     

Optically active helical polymer from radical polymerization of menthyl vinyl ketone

Menthyl vinyl ketone (MVK) was radically polymerized to obtain a polymer with excess of one‐handed helical sense. Like as the anionically polymerized poly‐MVK, the radically polymerized poly‐MVK also kept stable one‐handed helical conformation in solvent. The optical rotation and the circular dichroism signal intensity of the radically polymerized poly‐MVK were larger than that of the anionically polymerized poly‐MVK. The molecular weight of the radically polymerized poly‐MVK was much larger than that of the THF‐soluble part of the anionically polymerized poly‐MVK. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010     

Chirality sensing of chiral pyrrolidines and piperazines with a liquid crystalline dynamic helical poly(phenylacetylene) bearing ethyl phosphonate pendant groups

Stereoregular cis‐transoidal poly(phenylacetylene) bearing a phosphonic acid monoethyl ester as the pendant group (poly‐ 1 ‐H) was found to form a preferred‐handed helix upon complexation with various optically active pyrrolidines and piperazines in dilute dimethyl sulfoxide and water, and the complexes exhibited characteristic induced circular dichroisms (ICDs) in the UV‐vis region of the polymer backbone. The Cotton effect signs in water reflect the absolute configuration of the pyrrolidines. The sodium salt of poly‐ 1 ‐H (poly‐ 1 ‐Na) and poly‐ 1 ‐H in the presence of optically active amines formed lyotropic nematic and cholesteric liquid crystalline phases in concentrated water solutions, respectively, indicating the rigid‐rod characteristic of the polymer main chain regardless of the lack of a single‐handed helix, as evidenced by the long persistence length of about 18 nm before and after the preferred‐handed helicity induction in the polymer. X‐ray diffraction of the oriented films of the nematic and cholesteric liquid crystalline polymers exhibited almost the same diffraction pattern, suggesting that both polymers have the same helical structure; dynamically racemic and one‐handed helices, respectively. On the basis of the X‐ray analysis, a possible helical structure of poly‐ 1 is proposed. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 1383–1390, 2010