Macromolecular Helicity Research Articles

Switchable enantioseparation based on macromolecular memory of a helical polyacetylene in the solid state.

In the chromatographic separation of enantiomers the order of elution is determined by the strength of diasteromeric interactions between the components of the mixture and a chiral stationary phase. For analytical purposes, it is ideal to have the minor component elute first, whereas in the preparative mode a faster elution of the major component is desirable. Here we describe a stationary phase constructed from a polyacetylene that bears 2,2'-bisphenol-derived side chains in which chirality can be switched in the solid state prior to use. Both the macromolecular helicity of the polymer backbone and the axial chirality of the side chains can be switched in the solid state by interaction with a chiral alcohol, but importantly are maintained after removal of the chiral alcohol because of a memory effect. The chiral stationary phase thus prepared was used to separate the enantiomers of trans-stilbene oxide with the enantiomer elution order determined by the preseparation treatment.

Innenrücktitelbild: Formation of a Homo Double Helix of a Conjugated Polymer with Carboxy Groups and Amplification of the Macromolecular Helicity by Chiral Amines Sandwiched between the Strands (Angew. Chem. 20/2013)

Innenrücktitelbild: Formation of a Homo Double Helix of a Conjugated Polymer with Carboxy Groups and Amplification of the Macromolecular Helicity by Chiral Amines Sandwiched between the Strands (Angew. Chem. 20/2013)

Inside Back Cover: Formation of a Homo Double Helix of a Conjugated Polymer with Carboxy Groups and Amplification of the Macromolecular Helicity by Chiral Amines Sandwiched between the Strands (Angew. Chem. Int. Ed. 20/2013)

A π-conjugated polymer with carboxy groups forms a racemic double helix through self-association. However, in the presence of chiral amines, it forms a unique one-handed double helix in which the strands sandwich pairs of chiral amines through cyclic hydrogen-bonding networks. In their Communication on page 5275 ff., E. Yashima et al. report the transferal of chiral information from the amines to the polymer backbone with remarkable amplification of the helical chirality.

Formation of a Homo Double Helix of a Conjugated Polymer with Carboxy Groups and Amplification of the Macromolecular Helicity by Chiral Amines Sandwiched between the Strands

Eingeschobene Amine: Ein π-konjugiertes Polymer mit Carboxy-Gruppen organisiert sich selbst zu einer racemischen Doppelhelix. Mit chiralen Aminen dagegen bildet es einzigartige einheitlich händige Doppelhelices, in denen die Stränge Paare chiraler Amine durch cyclische H-Brücken-Netzwerke einschließen (siehe Bild). Die chirale Information der Amine wird mit beachtlicher Verstärkung der helicalen Chiralität auf die Polymer-Rückgrate übertragen.

Formation of a Homo Double Helix of a Conjugated Polymer with Carboxy Groups and Amplification of the Macromolecular Helicity by Chiral Amines Sandwiched between the Strands

Sandwiched amines: A π-conjugated polymer with carboxy groups self-associates to form a racemic double helix. In contrast, with chiral amines it forms a unique one-handed double helix, in which the strands sandwich pairs of chiral amines through cyclic hydrogen-bonding networks (see picture). The chiral information of the amines is transferred to the polymer backbones with remarkable amplification of the helical chirality. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Enantioseparation on Helical Poly(phenylacetylene)s Bearing Cinchona Alkaloid Pendants as Chiral Stationary Phases for HPLC

Abstract A series of optically active helical poly(phenylacetylene)s bearing cinchona alkaloid pendant groups was coated on macroporous silica gel to obtain novel chiral packing materials for high-performance liquid chromatography, which could resolve diverse racemic compounds into enantiomers whose chiral recognition abilities were significantly influenced by the macromolecular helicity induced by the alkaloid pendants.

A chiral co-crystalline form of poly(2,6-dimethyl-1,4-phenylene)oxide (PPO)

For an industrially relevant specialty polymer, poly(2,6-dimethyl-1,4-phenylene)oxide (PPO), for the first time, a crystalline structure is resolved. It is a co-crystalline structure of the polymer with a chiral guest molecule (α-pinene), exhibiting a 2/1 monomer-unit–guest molar ratio. The most relevant feature of this co-crystalline structure is its chirality. In fact, the unit cell includes all left- or right-handed polymer helices and (1S)-(−)- or (1R)-(+)-α-pinene guest molecules, respectively. The induction of non-racemic macromolecular helicity in the co-crystalline structure, clearly confirmed by VCD measurements, leads to amplification of chirality. Guest extraction procedures from the PPO–α-pinene co-crystalline form produce optically inactive nanoporous-crystalline modifications, which present high sorption ability toward organic compounds.

Enantioseparation on poly(phenyl isocyanide)s with macromolecular helicity memory as chiral stationary phases for HPLC

A series of novel optically active poly(phenyl isocyanide)s bearing achiral benzanilide pendant groups was prepared based on the noncovalent “helicity induction and memory strategy”. The polymers that exhibited an optical activity due to the macromolecular helicity memory were immobilized to 3-aminopropyl-silanized silica gel through chemical bonding or coated on the silica gel to obtain chiral packing materials for high-performance liquid chromatography (HPLC). The chiral recognition abilities toward a variety of racemic compounds with different functionalities were investigated by HPLC. The obtained CSPs separated various racemates into enantiomers, in particular, a right-handed helical poly(phenyl isocyanide) carrying achiral anilide pendants showed an excellent chiral recognition ability and completely or partially resolved nine racemates, including cyclic ether, amine, ketones and metal acetylacetonate complexes, among twelve racemates.

Amplification of macromolecular helicity of dynamic helical poly(phenylacetylene)s bearing non-racemic alanine pendants in dilute solution, liquid crystal and two-dimensional crystal

Optically active poly(phenylacetylene) copolymers composed of non-racemic phenylacetylenes bearing L- and D-alanine decyl esters as the side groups (poly(1Lm-co-1Dn), m>n) were prepared by the copolymerization of the corresponding L- and D-phenylacetylenes with different enantiomeric excesses using a rhodium catalyst; their chiral amplification of the helical conformation as an excess of one-handedness in a dilute solution, in a lyotropic liquid-crystalline (LC) state and in a two-dimensional (2D) crystal on substrate was investigated by measuring the circular dichroism spectra of the copolymers at different temperatures, mesoscopic cholesteric twist in the LC state (cholesteric helical pitch) and high-resolution atomic force microscopy images of the self-assembled 2D helix-bundle structures of the copolymer chains, respectively. We have observed a unique amplification of the helical-sense excess (majority rule effect) of dynamic helical, liquid-crystalline (LC) poly(phenylacetylene)s bearing non-racemic D- and L-alanine residues as the pendant groups in a dilute solution, in the cholesteric LC state and a two-dimensional crystal on substrate. The preferred-handed helical-sense and handedness excess of the copolymers in a dilute solution were sensitive to the solvent polarity and temperature, and amplification of the helical-sense excess was significantly affected by the solvents.

Separation of enantiomers on diastereomeric right- and left-handed helical poly(phenyl isocyanide)s bearing l-alanine pendants immobilized on silica gel by HPLC

Diastereomeric left- and right-handed helical polyisocyanides composed of L-alanine-bound phenyl isocyanides were prepared. The helical polymers with a narrow molecular weight distribution were immobilized on a silica gel support via chemical bonding and their optical resolution abilities were evaluated as chiral stationary phases (CSPs) for high-performance liquid chromatography (HPLC). The CSP prepared from the left-handed helical polyisocyanide resolved racemic cyclic ether and carbonyl compounds and cyclic dianilides and dibenzamides, whereas the right-handed helical polyisocyanide-based CSP showed a rather complementary chiral recognition ability and specifically resolved racemic metal acetylacetonate complexes, which were not separated on the former CSP at all. Additionally, the elution order of some enantiomers was reversed on the CSPs, thus indicating that the macromolecular helicity of the L-alanine-bound-polyisocyanides played a critical role in the enantioselectivity and elution order of the enantiomers.

Hierarchical Amplification of Macromolecular Helicity of Dynamic Helical Poly(phenylacetylene)s Composed of Chiral and Achiral Phenylacetylenes in Dilute Solution, Liquid Crystal, and Two-Dimensional Crystal

Optically active poly(phenylacetylene) copolymers consisting of optically active and achiral phenylacetylenes bearing L-alanine decyl esters (1L) and 2-aminoisobutylic acid decyl esters (Aib) as the pendant groups (poly(1L(m)-co-Aib(n))) with various compositions were synthesized by the copolymerization of the optically active 1L with achiral Aib using a rhodium catalyst, and their chiral amplification of the macromolecular helicity in a dilute solution, a lyotropic liquid crystalline (LC) state, and a two-dimensional (2D) crystal on the substrate was investigated by measuring the circular dichroism of the copolymers, mesoscopic cholesteric twist in the LC state (cholesteric helical pitch), and high-resolution atomic force microscopy (AFM) images of the self-assembled 2D helix-bundles of the copolymer chains. We found that the macromolecular helicity of poly(1L(m)-co-Aib(n))s could be hierarchically amplified in the order of the dilute solution, LC state, and 2D crystal. In sharp contrast, almost no chiral amplification of the macromolecular helicity was observed for the homopolymer mixtures of 1L and Aib in the LC state and 2D crystal on graphite.

Mechanism of Helix Induction in Poly(4-carboxyphenyl isocyanide) with Chiral Amines and Memory of the Macromolecular Helicity and Its Helical Structures

An optically inactive poly(4-carboxyphenyl isocyanide) (poly-1-H) changed its structure into the prevailing, one-handed helical structure upon complexation with optically active amines in dimethylsulfoxide (DMSO) and water, and the complexes show a characteristic induced circular dichroism in the polymer backbone region. Moreover, the macromolecular helicity induced in water and aqueous organic solutions containing more than 50 vol % water could be "memorized" even after complete removal of the chiral amines (h-poly-1b-H), while that induced in DMSO and DMSO-water mixtures containing less than 30 vol % water could not maintain the optical activity after removal of the chiral amines (poly-1a-H). We now report fully detailed studies of the helix induction mechanism with chiral amines and the memory of the macromolecular helicity in water and a DMSO-water mixture by various spectroscopic measurements, theoretical calculations, and persistence length measurements together with X-ray diffraction (XRD) measurements. From the spectroscopic results, such as circular dichroism (CD), absorption, IR, vibrational CD, and NMR of poly-1a-H, h-poly-1b-H, and original poly-1-H, we concluded that the specific configurational isomerization around the C horizontal lineN double bonds occurs during the helicity induction process in each solvent. In order to obtain the structural information, XRD measurements were done on the uniaxially oriented films of the corresponding methyl esters (poly-1-Me, poly-1a-Me, and h-poly-1b-Me) prepared from their liquid crystalline polymer solutions. On the basis of the XRD analyses, the most plausible helical structure of poly-1a-Me was proposed to be a 9-unit/5-turn helix with two monomer units as a repeating unit, and that of h-poly-1b-Me was proposed to be a 10-unit/3-turn helix consisting of one repeating monomer unit. The density functional theory calculations of poly(phenyl isocyanide), a model polymer of h-poly-1b-Me, afforded a 7-unit/2-turn helix as the most possible helical structure, which is in good agreement with the XRD results. Furthermore, the persistence length measurements revealed that these structural changes accompany a significant change in the main-chain stiffness. The mechanism of helix induction in poly-1-H and the memory of the macromolecular helicity are discussed on the basis of these results.

Helix-Sense-Controlled Synthesis of Optically Active Poly(methyl methacrylate) Stereocomplexes

Optically active poly(methyl methacrylate) (PMMA) stereocomplexes were prepared through the helix-sense-controlled supramolecular inclusion of an isotactic (it) PMMA within the helical cavity of an optically active, fullerene-encapsulated syndiotactic (st) PMMA with a macromolecular helicity memory. The observed and calculated vibrational circular dichroism spectra revealed that the it-PMMA replaced the encapsulated fullerenes to fold into a double-stranded helix with the same handedness as that of the st-PMMA single helix through the formation of a topological triple-stranded helix.

Synthesis of functional poly(phenyl isocyanide)s with macromolecular helicity memory and their use as asymmetric organocatalysts

To develop a novel polymer-based asymmetric organocatalyst, a series of helical poly(phenyl isocyanide)s with functional pendant groups were prepared by modifying the side groups of the optically active helical poly(4-carboxyphenyl isocyanide) with a macromolecular helicity memory. Helical polyisocyanides partially modified with achiral amines, such as piperazine, maintained their chiral memory and enantioselectively catalyzed a direct aldol reaction. Although the enantioselectivity was low, the original helical poly(4-carboxyphenyl isocyanide) showed no catalytic activity. These results indicated that the macromolecular helicity of the modified polyisocyanides together with bifunctional amino and carboxy acid pendant residues arranged in a helical array along the polymer backbones plays an important role in the enantioselectivity.

Helicity Induction and Memory of the Macromolecular Helicity in a Polyacetylene Bearing a Biphenyl Pendant

A novel, cis-transoidal poly-(phenylacetylene) bearing a carboxybiphenyl group as the pendant (poly-1) was prepared by polymerization of (4'-ethoxycarbonyl-4-biphenylyl)acetylene with a rhodium catalyst followed by hydrolysis of the ester groups. Upon complexation with various chiral amines and amino alcohols in dimethyl sulfoxide (DMSO), the polymer exhibited characteristic induced circular dichroism (ICD) in the UV/Vis region due to the predominantly one-handed helix formation of the polymer backbone as well as an excess of a single-handed, axially twisted conformation of the pendant biphenyl group. Poly-1 complexed with (R)-2-amino-1-propanol showed unique time-dependent inversion of the macromolecular helicity. Furthermore, the preferred-handed helical conformation of poly-1 induced by a chiral amine was further "memorized" after the chiral amine was replaced with achiral 2-aminoethanol or n-butylamine in DMSO. In sharp contrast to the previously reported memory in poly((4-carboxyphenyl)acetylene), the present helicity memory of poly-1 was accompanied by memory of the twisted biphenyl chirality in the pendants. Unprecedentedly, the helicity memory of poly-1 with achiral 2-aminoethanol was found to occur simultaneously with inversion of the axial chirality of the biphenyl groups followed by memory of the inverted biphenyl chirality, thus showing a significant change in the CD spectral pattern.

Encapsulation of Fullerenes in a Helical PMMA Cavity Leading to a Robust Processable Complex with a Macromolecular Helicity Memory

Ein Gebrauchskunststoff – syndiotaktisches Poly(methylmethacrylat) (st-PMMA) – bildet in Gegenwart von (R)- oder (S)-1-Phenylethanol (1) rechts- bzw. linksgängige Helices, die Fullerene einschließen können. Dadurch entstehen beständige, optisch aktive Komplexe, die ihre Helizität auch nach Entfernen des chiralen Alkohols beibehalten (siehe Schema, rechts). Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2001/2008/z703655_s.pdf or from the author. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Encapsulation of Fullerenes in a Helical PMMA Cavity Leading to a Robust Processable Complex with a Macromolecular Helicity Memory

Encapsulation of Fullerenes in a Helical PMMA Cavity Leading to a Robust Processable Complex with a Macromolecular Helicity Memory

Cover Picture: Unexpected Thermally Stable, Cholesteric Liquid‐Crystalline Helical Polyisocyanides with Memory of Macromolecular Helicity (Chem. Asian J. 6/2007)

A good memory for macromolecular helicity is displayed by the one‐handed helical sodium salt of poly(4‐carboxyphenyl isocyanide) after the complete removal of the optically active amines that induced the helicity. This memory persists even with further modification to the side groups of the amide residues, thus giving designer helical polyisocyanides with outstanding thermal stability in solution, some of which also exhibit liquid crystallinity. To find out more, turn to the Full Paper by E. Yashima et al. on page 755 ff.

Unexpected Thermally Stable, Cholesteric Liquid‐Crystalline Helical Polyisocyanides with Memory of Macromolecular Helicity

The achiral sodium salt of poly(4-carboxyphenyl isocyanide) (poly-1-Na) folds into a one-handed helix induced by optically active amines in water. The induced helicity remains when the optically active amines are completely removed, and further modification of the side groups to amide residues is possible without loss of memory of macromolecular helicity. Although the helical poly-1-Na loses its chiral memory at high temperature, helical polyisocyanides modified with achiral primary amines, which no longer have any chiral components, keep their memory perfectly even at 100 degrees C in N,N-dimethylformamide in some cases and exhibit cholesteric liquid-crystalline phases, thus providing a robust scaffold with heat resistance to which a variety of functional groups can be introduced.

Macromolecular helicity inversion of an optically active helical poly(phenylacetylene) by chemical modification of the side groups

An optically active helical poly(phenylacetylene) was synthesized by the copolymerization of phenylacetylenes bearing optically active hydroxy or ester groups obtained by the kinetic resolution of a racemic phenylacetylene with lipase; the helix-sense was inverted from one helix to another by the further chemical modification of the hydroxy groups with achiral bulky isocyanates or an acid chloride.