Chiral Spacer Research Articles

Synthesis and Host-abilities of some New Corands Bearing Uncommon Chiral Spacer Units

Benzo- and naphtho-ethylene glycols are prepared from dihydroxy-benzenes or -naphthalenes and the corresponding chloroethoxyethanols using known procedures. Cyclocondensation reactions of these open-chain diols with the chiral and concave tetra-t-butyl-2,6,9-trioxabicyclo[3.3.1]nonadiene (“bridged bisdioxine”) bisacid chloride afford several new corands. In one case, besides the expected macrocycle, an open-chain podand is obtained and further cyclized to a large 2:2 macrocycle. The bridged bisdioxine spacer may also be converted into the tetraoxaadamantane scaffold as shown with an example. Extraction experiments employing selected corands towards various metal ions show a significant host-property towards Hg(II) salts for those macrocycles having o-disubstituted aryl groups incorporated, thus forming 1:1 complexes. NMR titration studies with sodium thiocyanate as well as with some organic guests do not exhibit significant host–guest interactions, in the case of Hg(II) thiocyanate a weak complexation was determined.

Short wavelength light reflecting films from side‐chain liquid crystal homopolymers with chiral spacers

AbstractA series of acrylate monomers with alkoxy tails of varying lengths are synthesised and polymerised. The butoxy analogue had a stable enantiotropic cholesteric liquid crystalline phase which formed a grandjean texture when prepared as a thin film between glass slides. The polymer was mixed with a low molar mass nematic liquid crystal in various proportions and the pitch of the chiral nematic phases were determined using a cano‐wedge cell technique. The polymer prepared from (S)‐2‐(4‐butoxyphenyl‐4′‐benzoyloxy)‐1‐methyl ethyl acrylate had a pitch length of 113 nm which indicates that the polymer film could be employed in optical devices requiring selective reflection of light with short wavelengths in the region of 170 nm. Copyright © 2001 John Wiley & Sons, Ltd.

Dinuclear pincer-palladium(II) complexes and their use as homogeneous or heterogeneous catalyst for the aldol reaction of methyl isocyanoacetate

Two new bimetallic Pd complexes have been synthesized and characterized and their catalytic activity checked for the aldol reaction of aldehydes with methyl isocyanoacetate. Each palladium atom is coordinated to an SCS-type ligand and the two pincer units are linked by a chiral spacer. The catalytic aldol reaction of methyl isocyanoacetate with aldehydes proceeds quickly but no significant diastereoselectivity and enantioselectivity is found. The comparison with a homologous mononuclear Pd complex shows no differences with the bimetallic compounds, concluding that there is no cooperativity between the metal centers. Two silica-supported catalysts prepared with a bimetallic compound show catalytic activity with very minor enantioselectivity.

Synthesis and characterization of liquid-crystalline polyesters with chiral and achiral twin spacers

A series of novel thermotropic main-chain chiral liquid–crystalline random copolyesters consisting of spacers of two different types—chiral and achiral—was synthesized. Polyesters (BmTa) with tartaric acid as the chiral spacer (Ta), aliphatic diols (with ‘m’ = 2–10 methylene groups) as the achiral spacers, and 4,4′-dihydroxy biphenyl (B) as the mesogen were synthesized via condensation polymerization in solution after duly protecting the 2,3-dihydroxy groups of tartaric acid by acetylation. The copolymers were characterized by Fourier transform infrared spectroscopy, 1H and 13C NMR spectra, gel permeation chromatography, and thermogravimetric analysis. Transition temperatures for phase changes recorded by DSC were corroborated with the textures observed by a hot-stage optical polarizing microscope. The wide-angle X-ray diffraction (WAXD) profiles indicated a SmE phase at room temperature. The lower angle region at 2θ = 0.5–2.45 covered by WAXD indicated a layer of thickness of 161 Å, less than the molecular length for B0Ta. The [αD] values were recorded on a digital polarimeter. The birefringence was lost at higher mesophase temperatures in lower members with m < 5, a behavior found in certain chiral systems, and the higher members with m > 5 showed a lesser number of phase transitions. On cooling, the polyesters produced a texture with the formation of transition bars. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 1743–1752, 2001

Synchrotron X-ray Study of the Phase Transitions in Liquid Crystal Polyesters Derived from p,p‘-Bibenzoic Acid and racemic- and (R)-3-Methyl-1,6-hexanediol

The phase transitions in two liquid crystalline polyesters have been studied by diffraction techniques employing synchrotron radiation. The two polyesters, derived from p,p‘-bibenzoic acid and 3-methyl-1,6-hexanediol, only differ on the optical activity of the chiral spacer: both the active and racemic polymers have been investigated. Time-resolved experiments at variable temperature were performed, analyzing the cooling from the isotropic melt and the subsequent melting. The analysis of the different diffraction profiles (covering WAXS, MAXS, and SAXS regions) shows important differences in the phase behavior of the two polyesters, more important than those anticipated from the analysis of the DSC curves. Nevertheless, a common feature is the obtention of SC mesophases in both polymers, characterized by a continuous increase of the tilting angle as the temperature is decreased (before the appearance of high-order mesophases).

Nonapeptide analogues containing (R)-3-hydroxybutanoate and beta-homoalanine oligomers: synthesis and binding affinity to a class I major histocompatibility complex protein.

Crystal structures of antigenic peptides bound to class I MHC proteins suggest that chemical modifications of the central part of the bound peptide should not alter binding affinity to the MHC restriction protein but could perturb the T-cell response to the parent epitope. In our effort in designing nonpeptidic high-affinity ligands for class I MHC proteins, oligomers of (R)-3-hydroxybutanoate and(or) beta-homoalanine have been substituted for the central part of a HLA-B27-restricted T-cell epitope of viral origin. The affinity of six modified peptides to the B2705 allele was determined by an in vitro stabilization assay. Four out of the six designed analogues presented an affinity similar to that of the parent peptide. Two compounds, sharing the same stereochemistry (R,R,S,S) at the four stereogenic centers of the nonpeptidic spacer, bound to B2705 with a 5-6-fold decreased affinity. Although the chiral spacers do not strongly interact with the protein active site, there are configurations which are not accepted by the MHC binding groove, probably because of improper orientation of some lateral substituents in the bound state and different conformational behavior in the free state. However we demonstrate that beta-amino acids can be incorporated in the sequence of viral T-cell epitopes without impairing MHC binding. The presented structure-activity relationships open the door to the rational design of peptide-based vaccines and of nonnatural T-cell receptor antagonists aimed at blocking peptide-specific T-cell responses in MHC-associated autoimmune diseases.

A new chiral polyisocyanate: an optical switch triggered by a small amount of photochromic side groups

A new isocyanate monomer was synthesized which contains an azobenzene dye connected to the isocyanate group via a chiral spacer. Copolymers with the achiral hexyl isocyanate were prepared by anionic polymerization with NaCN. CD measurements in solution show that the chiral center in the spacer induces a shift in the equilibrium between P- and M-helical main chain conformations toward the left-handed conformation. The magnitude of this chiral induction can be varied reversibly by photochemical isomerization of the azo chromophore (trans → cis). The preference for one twist sense of the polymer chain is much larger than in previously studied systems, if copolymers with the same amount of chiral monomer units are compared. It is thus possible to build chirooptical switches with only small amounts of chiral monomers.

Amplification of the twisting power by modification of the chiral spacer in a smectic liquid-crystalline phase

We investigated twisting power of chiral twin molecules, (S)-2-methylnonanedioic acid bis[4-(5-octyl-2-pyrimidinyl)phenyl]ester (MNB-8-PYP) and (S)-2-methyldecanedioic acid bis[4-(5-octyl-2-pyrimidinyl)phenyl] ester (MDB-8-PYP), in N* and SmC* phases. The inverse of the SmC* pitch induced by MDB-8-PYP was found to vary non-linearly as a function of changing concentration, indicating cooperative twisting mechanism for MDB-8-PYP which induces stronger helical structure than MNB-8-PYP.

Base-specific interaction of polymers containing adenine: effect of chiral spacer on the interaction with polynucleotide

Polyethyleneimine derivatives of adenine with l- and d-serine as spacers were prepared. The polymer having d-serine spacer 2 gave a stable polymer complex with poly(uridylic acid) (poly U), but the polymer having l-serine spacer 1 gave an unstable polymer complex. The reason was concluded to be caused by the steric repulsion of l-serine units along the polymer chain when the polymer formed polymer complex with poly U.

ChemInform Abstract: Hydroxyacids as Efficient Chiral Spacers for Asymmetric Intramolecular (2 + 2) Photocycloadditions.

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Axially Chiral Catenanes and π‐Electron‐Deficient Receptors

AbstractThe design of a new class of chiral [2]catenanes is reported. The self‐assembly of [2]catenanes comprising one or two 3,3′‐bitolyl spacers in the π‐electron‐deficient component, and bis‐p‐phenylene‐34‐crown‐10 (BPP 34 C 10) as the π‐electron‐rich component, is described. The X‐ray crystal structures, together with solution‐state dynamic 1H NMR spectroscopic studies, show that the degree of order characterizing the molecular structures is substantially different from that of the “parent” [2]‐catenane, comprising cyclobis(paraquat‐p‐phenylene) and BPP34C 10. When appropriately substituted in their ortho positions, bitolyl compounds can support axial chirality: the self‐assembly of axially chiral [2]catenanes, comprising one or two 3,3′‐disubstituted‐2,2′‐dihydroxy‐1,1′‐binaphthyl spacers, has been achieved in good yields, showing that the introduction of the bulky, axially chiral spacer and the consequent distortion of the cavity of the π‐electron‐deficient component still permits good molecular recognition between the components leading to efficient catenane production. X‐ray crystallography suggests that this recognition is driven by hydrogen bonding and π‐π stacking interactions between the complementary subunits. The hydroxyl groups on the chiral spacer were further functionalized as benzoyl esters in a [2]catenane as well as in the tetracationic cyclophanes; that is, chemistry can be done on these catenanes. The chiral tetracationic cyclophanes exhibit good enantiomeric differentiation toward the D‐ and L‐enantiomers of aromatic amino acids in water and their N‐acetylated derivatives in organic solvents.

Hydroxyacids as efficient chiral spacers for asymmetric intramolecular [2+2] photocycloadditions

An indirect and efficient enantioselective [2+2] photocycloaddition between 3-carboxycyclohex-2-enone and allyl alcohol, involving a three step sequence, is described. When the reagents are linked by covalent bonds to a chiral spacer such as L-lactic or (R)-3-hydroxybutyric acids, the corresponding photocycloadducts are isolated in high yields. with very high regio and diastereostereoselectivies. The reaction affords polyfunctional cyclobutanes as pure enantiomers.

Cholesteric and photoreactive polyesters

Several classes of cholesteric and photoreactive homo- and copolyesters were synthesized and characterized. Most of these polyesters were prepared in such a way that a chiral spacer (e.g. isosorbide) was polycondensed with a photoreactive dicarboxylic acid, such as 4-carboxycinnamic acid, benzene- 1,4-bisacrylic acid, 4-(4′-carboxyphthalimido)cinnamic acid. In several cases other dicarboxylic acids, such as naphthalene-2,6-dicarboxylic acid or 4-aminobenzoic acid trimellitimide were cocondensed to favor the formation of a Grandjean texture. When ‘sugar diols’ such as isosorbide or isomannide were used as chiral building blocks, suitable diphenols were required as comonomers to stabilize the LC phase. Most polyesters were capable of forming a selectively reflecting Grandjean texture, which can be fixed by crosslinking using UV light. An alternative synthetic strategy based on chiral, substituted terephthalic acids is discussed.

Liquid crystalline copolymers containing electron donor and acceptor mesogens with a chiral group in the spacer unit

AbstractSide‐chain liquid crystalline polymers containing both mesogenic (carbazolylmethylene)aniline and (4′‐nitrobenzylidene)aniline units with various spacer groups were prepared to examine effects of the structure of spacer groups on the liquid crystalline properties. The copolymer containing (R)‐(+)‐2‐methylpropylene as a chiral group in the spacer unit induced a smectic phase; the copolymer with a trimethylene spacer of similar length to the chiral spacer exhibited a nematic phase. Smectic phases were observed for the copolymer containing the chiral spacer group when the proportion of the carbazolyl group was in the range of 0.55–0.88. For example, the copolymer with the proportion of the carbazolyl group of 0.68 expressed the smectic phase from 88° to 167°C. This isotropic temperature was 37° higher than the calculated value (130°C) based on an assumed copolymer composition without the electron donor–acceptor interaction. Thus, it is assumed that for the chiral copolymer containing both electron donor and acceptor groups, the thermal stability and the induction of the smectic phase were caused by both the electron donor–acceptor interaction and the existence of the chiral group in the spacer unit. © 1995 John Wiley &amp; Sons, Inc.

Liquid‐crystalline polyimides, 23. Photoreactive, cholesteric copoly(ester‐imide)s derived from 1,4‐phenylenediacrylic acid and 4‐aminobenzoic trimellitimide

AbstractThe dichlorides of commercial 1,4‐phenylenediacrylic acid3,3′‐(1,4‐Phenylene)di‐2‐propenoic acid. and N‐(4‐carboxyphenyl)‐trimellitimide were polycondensed with a chiral spacer prepared from (S)‐3‐bromo‐2‐methylpropanol and 4‐mercaptophenol. Optical microscopy revealed that all the resulting copoly(ester‐imide)s form an enantiotropic cholesteric melt and adopt a Grandjean texture upon slight shearing. The WAXD powder patterns indicate the formation of a layered structure in the solid state. Irradiation with UV light of wavelength ⩽ 360 nm allows crosslinking at any temperature above 25°C.

Design of molecular architectures for polymeric mesophase formation

AbstractBased on the tendency of low molar mass liquid crystals composed of extended mesogens symmetrically disubstituted with long n‐alkoxy substituents to exhibit smectic C mesophases, we have proposed that SCLCPs with laterally attached (vs. terminally attached) mesogens offer an ideal architecture for obtaining sC* mesophases. In particular, mesogens that typically form the desirable sC*‐n phase sequence can be laterally attached to the polymer backbone through a chiral spacer, which should result in high values of spontaneous polarization. Not only are we using mesogens which exhibit sC*‐n phase sequences, we are also attempting to induce smectic layering into systems which typically form nematic mesophases by using immiscible hydrocarbon/fluorocarbon components and electron‐donor‐acceptor interactions. Thus far, the thermotropic behavior of poly{5‐[[[2', 5'‐bis[(3″‐fluoro‐4″‐dimethoxyphenyl)ethynyl]benzyl]oxy]carbonyl[2.2.1]hept‐2‐ene]s and poly(5‐[[[2',5'‐bis[(3″‐fIuoro‐4″‐methoxybenzoyl)oxy]benzyl]oxy]carbonyl]‐bicyclo[2.2.1]hept‐2‐ene)s correspond to that of their low molar mass analogs. Preliminary results demonstrate that smectic layering is successfully induced in 2,5‐bis[(4'‐n‐alkoxybenzoyl)oxy]toluenes and polynorbornenes with laterally attached 2,5‐bis[(4'‐n‐alkoxybenzoyl)oxy]benzyl mesogens by terminating the n‐alkoxy substituents with perfluorinated segments.

Photoreactive Cholesteric Polyesters Derived from 4-Carboxycinnamic Acid and Novel Chiral Spacers

A homopolyester was prepared from 4-carboxycinnamic acid and a diol derived from 4-mercaptophenol and (S)-3-bromo-2-methyl-1-propanol. With this spacer and a 1 :1 combination of 4-carboxycinnamic acid and naphthalene-2,6-dicarboxylic acid, copolyesters with random and with alternating sequences of the dicarboxylic acids were synthesized. The homopolyester and the random copolyester are noncrystalline materials with a broad cholesteric phase, whereas the alternating copolyester is semicrystalline. In contrast to the homopolyester, the copolyesters form a blue Grandjean texture over a broad temperature range. Upon irradiation in solution the cinnamoyl moiety undergoes trans-cis photoisomerization and (2 + 2) photocycloaddition. The cinnamic, benzoic, and naphthalene-2,6-dicarboxylic phenyl esters undergo photo-Fries rearrangements. The dominating reaction in spin-coated films is the photocycloaddition causing photocrosslinking. The efficiency of cycloaddition and their competition with the Fries reaction are dependent on the composition of the polymers and the conversion. The rearrangement is completely suppressed in films of the homopolyester 4, and the photocycloaddition is more efficient than in the copolymers.

LC‐Polyimides. XIX. Chiral and thermotropic poly(esterimide)s based on N‐(4′‐caboxyphenyl)trimellitimide and novel chiral spacers

AbstractStarting from commercial S‐ or R‐3‐bromo‐2‐methylpropanol, several new spacer diols were prepared. These spacers were polycondensed with the acid chloride of N‐(4′‐carboxyphenyl)trimellitimide. The resulting poly(ester‐imide)s were characterized by elemental analyses, viscosity measurements, 1H‐NMR spectroscopy, DSC‐ and WAXD‐measurements and optical microscopy. The poly(ester‐imide)s derived from chiral, aliphatic spacers form layer structures in the solid state, but no liquid crystalline phase. With nonsymmetrical, nonchiral semialiphatic spacers, poly(ester‐imide)s were obtained, which form a smectic E or H phase in the solid state, a smectic‐A or ‐C phase in the melt, and a nematic phase, when the spacer possesses an odd number of CH2 groups. The polycondensation of a chiral semialiphatic spacer yielded thermotropic poly(ester‐imide)s with either S‐ or R‐configuration. WAXD patterns measured with synchrotron radiation at various temperatures proved that a layer structure exists in the solid state (smectic‐E* or H*) and a chiral smectic‐A* or ‐C* phase plus a cholesteric phase in the melt. A 1 : 1 blend of the S‐ and R‐polyesters was also studied, but did not show unusual features. © 1995 John Wiley &amp; Sons, Inc.

Coupling between chirality and odd-even effect of twin materials in smectic liquid-crystalline phases

Novel chiral twin molecules, (S)-2-methylnonanedioic acid bis[4-(5-octyl-2-pyrimidinyl)phenyl] ester (MNB-8-PYP) and (S)-2-methyldecanedioic acid bis[4-(5-octyl-2-pyrimidinyl)phenyl] ester (MDB-8-PYP), have been prepared and the odd-even effect of the chiral spacer on the helical twisting power investigated. Pitch measurements in a mixture with a host material revealed that MDB-8-PYP with an even number of atoms in the spacer induces a stronger helical structure in the cholesteric phase and particularly in the chiral smectic C phase than MNB-8-PYP with an odd-numbered spacer. The marked odd-even effect observed in the SC* pitch can be explained by our proposed ‘intercalated chirality’ model for twisting power in smectic phases induced by a chiral twin molecule.

New Architectures for Side Chain Liquid Crystalline Polymers with Chiral Smectic C Mesophases

All attempts at synthesizing side chain liquid crystalline polymers (SCLCPs) with chiral smectic C (sc*) mesophases simply functionalize one terminal group of the mesogen with a chiral substituent and attach the other terminus to the polymer backbone through a spacer. If a sc* mesophase is observed, it is usually in the less desirable sc*-sA phase sequence. We propose that SCLCPs with laterally attached (vs terminally attached) mesogens offer an ideal architecture for obtaining sc* meso- phases. This is because extended mesogens symmetrically disubstituted with long n-alkoxy groups can be attached to the polymer backbone through a chiral spacer. Thus, mesogens which typically form the desirable sc*-n phase sequence can be used, and the chiral group can be introduced at the center of the mesogen which should result in high values of spontaneous polarization. We are not only using mesogens which exhibit sc*-n phase sequences, but are also attempting to induce smectic layering into laterally attached systems...