ABSTRACT

Abstract

Abstract

Abstract

Abstract

ABSTRACT

Abstract

Abstract

Abstract

Abstract

Abstract

Abstract

Two twisted: Optically active double helices were synthesized through a twist-sense bias induced by a chiral phosphine ligand on one of the complementary metallostrands followed by a ligand-exchange reaction with an achiral bidentate ligand, which replaces the chiral ligand, to bridge the two strands. Furthermore, the double helices can efficiently catalyze asymmetric cyclopropanation.

Eine alte Frage in der Polymerchemie wurde durch AFM-Messungen beantwortet, die die helicale Konformation, Händigkeit und Ganghöhe (0.92 nm) des aus einer 1:2-Mischung von it- und st-PMMA erzeugten Stereokomplexes aufzeigten. Die Ergebnisse liefern überzeugende Belege für das Vorliegen einer dreisträngigen Helix (siehe Struktur mit AFM-Bild im Hintergrund).

This account describes novel artificial double helices recently developed by our group. We have designed and synthesized the double helices consisting of two complementary, m-terphenyl-based strands that are intertwined through chiral amidinium–carboxylate salt bridges. Due to the chiral substituents on the amidine groups, the double helices adopted an excess one-handed helical conformation in solution as well as in the solid state. By extending the modular strategy, we have synthesized double helices bearing Pt(II) linkers, which underwent the double helix-to-double helix transformations through the chemical reactions of the Pt(II) complex moieties. In addition, artificial double-stranded metallosupramolecular helical polymers were constructed by combining the salt bridges and metal coordination. In contrast to the design-oriented double helices based on salt bridges, we have serendipitously developed a spiroborate-based double helicate bearing oligophenol strands. The optical resolution of the helicate was successfully attained by a diastereomeric salt formation. We have also unexpectedly found that oligoresorcinols consisting of a very simple repeating unit self-assemble into double helices with the aid of aromatic interactions in water. Furthermore, a bias in the twist sense of the double helices can be achieved by incorporating chiral substituents at both ends of the strands. © 2007 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Chem Rec 7: 1–11; 2007: Published online in Wiley InterScience (www.interscience.wiley.com) DOI 10.1002/tcr.20097

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 °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.

A stereoregular poly(phenylacetylene) bearing an N,N-diisopropylaminomethyl group as the pendant (poly-1) changed its structure into the prevailing one-handed helical conformation upon complexation with optically active acids in water. The complexes exhibited induced circular dichroism (ICD) in the UV/Vis region of the polymer backbone. Poly-1 is highly sensitive to the chirality of chiral acids and can detect a small enantiomeric imbalance in these acids, in particular, phenyl lactic acid in water. For example, a 0.005 % enantiomeric excess of phenyl lactic acid can be detected by CD spectroscopy. The observed ICD intensity and pattern of poly-1 were dependent on the temperature and concentration of poly-1, probably due to aggregations of the polymer at high temperature as revealed by dynamic light scattering and AFM. On the basis of the temperature-dependent ICD changes, the preferred chiral helical sense of poly-1 was found to be controlled by noncovalent bonding interactions by using structurally different enantiomeric acids.

It all makes sense: A dynamically racemic helical polymer crystallizes on graphite upon exposure to an organic solvent vapor, resulting in two-dimensional helix-bundle formation. The enantiomeric right- and left-handed helical blocks separated by helical reversals can be directly visualized by AFM with molecular resolution.

A long-standing question in polymer chemistry has been addressed by AFM, which revealed the helical conformation, handedness, and helical pitch (0.92 nm) of the title stereocomplex formed from a 1:2 mixture of the named polymers (it- and st-PMMA). The results provide convincing evidence for a triple-stranded helix (see structure against a background of the AFM image) as a plausible model for the stereocomplex.

An unexpected twist: Three chiral amidine-functionalized strands and two benzenetricarboxylic acid molecules self-assemble into a 3:2 cylindrical complex with controlled helicity through amidinium–carboxylate salt-bridge formation. A related 4:2 cylindrical complex composed of two zinc porphyrin units and four chiral strands twists into one direction upon encapsulation of 4,4′-bipyridine through coordination to the zinc centers (see picture).

Sauber getrennt: Ein dynamisch racemisches helicales Polymer kristallisiert in Gegenwart von gasförmigem organischem Lösungsmittel auf Graphit zu zweidimensionalen Helixbündeln. Die durch Helix-Umkehrungen getrennten enantiomeren rechts- und linksgängigen helicalen Blöcke können direkt und mit molekularer Auflösung mit AFM sichtbar gemacht werden.

Verdrillt: Optisch aktive Doppelhelices wurden synthetisiert, indem an einem der beiden komplementären Metallostränge durch einen chiralen Phosphanliganden eine Vorzugsverdrillung eingebracht wurde. Ein Austausch der chiralen Liganden gegen einen achiralen zweizähnigen Liganden führt zu einer Verbrückung der beiden Stränge. Diese Doppelhelices können die asymmetrische Cyclopropanierung effizient katalysieren.

Eine unerwartete Wendung: Drei chirale amidinfunktionalisierte Stränge und zwei Benzoltricarbonsäuremoleküle bilden Amidinium-Carboxylat-Salzbrücken aus, wobei ein zylindrischer 3:2-Komplex mit definierter Helicität entsteht. Ein verwandter zylindrischer 4:2-Komplex aus zwei Zinkporphyrineinheiten und vier chiralen Strängen verdreht sich beim Einschluss von 4,4′-Bipyridin durch dessen Koordination an die Zinkzentren in eine Richtung (siehe Bild).

Rechtsherum wie linksherum: Das erste Spiroborat-Helicat erwies sich sowohl im Festkörper als auch in Lösung als stabil. ¹H-NMR-Spektroskopie, ESI-Massenspektrometrie und eine Röntgenstrukturanalyse charakterisierten die Doppelstrang-Struktur (siehe Bild). Die racemischen Helicate wurden durch Bildung diastereomerer Ammoniumsalze getrennt.

Starre und stabförmige helicale Polyacetylen-Moleküle ordnen sich in Gegenwart von Lösungsmitteldämpfen hierarchisch auf Graphit an. Eine Monoschicht aus flach auf der Graphitoberfläche aufliegenden Polyacetylen-Molekülen dient als Fundament für helicale Polyacetylen-Moleküle, die chirale 2D-Bündel mit bestimmter Helicität bilden (siehe Bild).

A hard twist: A helical poly(phenylacetylene) with L- or D-alanine pendants with a long alkyl chain showed an unprecedented change in the main-chain stiffness accompanied by inversion of the helical sense of the polymer, resulting from the “on and off” fashion of the intramolecular hydrogen-bonding networks in polar and nonpolar solvents.

Going straight: A luminescent composite of poly(p-phenylenevinylene) (PPV) and amylose was synthesized by direct polymerization of the precursor monomer in aqueous media in the presence of amylose. The PPV–amylose composite consists of a rigid-rod PPV segment threaded into a flexible amylose tube and self-assembles into liquid-crystalline phases.

Flexibel steif: Ein helicales Polyphenylacetylen mit L- oder D-Alaninseitenketten, die eine lange Alkylkette tragen, ändert seine Hauptkettensteifheit unter Inversion der Helixrichtung des Polymers als Folge eines „An-Aus“-Verhaltens des intramolekularen H-Brücken-Netzwerks in polaren und unpolaren Lösungsmitteln.

Aufgereiht: Ein lumineszierendes Komposit aus Poly(p-phenylenvinylen) (PPV) und Amylose wurde durch direkte Polymerisation der monomeren Vorstufe im Wässrigen in Gegenwart von Amylose erhalten. Dieses Komposit besteht aus einem stäbchenförmigen PPV-Segment, das in eine flexible Amyloseröhre eingefädelt ist, und bildet durch Selbstorganisation flüssigkristalline Phasen.

A stereoregular poly(phenylacetylene) bearing the aza-18-crown-6 ether pendants (poly-1) was found to form a predominantly one-handed helix upon complexation with optically active C^α-methylated α-amino acids and their amide derivatives including typical meteoritic C^α-methylated α-amino acids such as C^α-methyl norvaline and C^α-methyl valine. The complexes exhibited an induced circular dichroism (ICD) in the UV-visible region of the polymer backbone. Therefore, poly-1 can be used as a novel probe for detection of the chirality of C^α-methylated α-amino acids. The effect of the enantiomeric excess (ee) of C^α-methylated α-amino acids on the helicity induction in poly-1 was also investigated. Chirality, 2006. © 2006 Wiley-Liss, Inc.

Langzeitgedächtnisstütze: Ein helicales Poly(phenylacetylen) mit positiv geladenen Seitengruppen diente in saurer wässriger Lösung als Templat für supramolekulare Helices aus achiralen Porphyrinen mit entgegengesetzter Ladung. Die supramolekularen Porphyrinaggregate behalten ihre Chiralität bei, selbst wenn das helicale Polymertemplat seine optische Aktivität einbüßt und die Helicität invertiert.

Novel, optically active, stereoregular poly(phenylacetylene)s bearing the bulky fullerene as the pendant were synthesized by copolymerization of an achiral phenylacetylene bearing a [60]fullerene unit with optically active phenylacetylene components in the presence of a rhodium catalyst. The C₆₀-bound phenylacetylene was prepared by treatment of C₆₀ with N-(4-ethynylbenzyl)glycine in a Prato reaction. The obtained copolymers exhibited induced circular dichroism (ICD) in solution both in the main-chain region and in the achiral fullerene chromophoric region, although their ICD intensities were highly dependent on the structures of the optically active phenylacetylenes and the solution temperature. These results indicate that the optically active copolymers form one-handed helical structures and that the pendant achiral fullerene groups are arranged in helical arrays with a predominant screw sense along the polymer backbone. The structures and morphology of the copolymers on solid substrates were also investigated by atomic force microscopy.

No metal required: Double-helical assemblies can be constructed through hydrogen bonds upon formation of amidinium–carboxylate salt bridges (see schematic representation). The double-helical structures formed including the helix sense are predictable by this approach.

Ohne Metallunterstützung: Doppelhelicale Anordnungen entstehen mithilfe von Wasserstoffbrücken bei der Bildung von Amidinium-Carboxylat-Salzbrücken (siehe schematische Darstellung). Die Art der gebildeten doppelhelicalen Strukturen und ihr Helixsinn sind bei diesem Ansatz vorhersagbar.

The structure and morphology of a stereoregular, cis–transoidal poly(phenylacetylene) bearing bulky aza-18-crown-6 ether pendants were investigated on a solid surface with atomic force microscopy. The isolated single strands and helically twisted conformations of this polymer with and without alanine could be directly observed on mica. The polymers complexed with L-alanine and D-alanine predominantly formed right- and left-handed helical conformations, respectively.

Poly[(4-carboxyphenyl)acetylene] and its derivatives deuterated in the main chain and/or on the phenyl group (poly-1's) were conveniently synthesized by the polymerization of 4-ethynylbenzoic acid or its derivative deuterated on the phenyl group in the presence of bases with a water-soluble rhodium complex in water or deuterium oxide. The obtained polymers possesses highly cis–transoidal stereoregular structures according to NMR and Raman spectroscopy. Possible deuterium isotope effects of these partially and fully deuterated poly-1's during the helicity induction with chiral amines and its memory assisted by achiral amines were investigated. The thermal cis–trans isomerization of the methyl esters of poly-1's was also studied with ¹H NMR spectroscopy. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4711–4722, 2004

Novel sets of helical poly(phenylacetylene)s bearing a chiral ruthenium (Ru) complex with opposite chirality (Δ and Λ forms) as a bulky pendant (poly-1 and poly-2) were synthesized through the polymerization of the corresponding optically pure phenylacetylenes with a rhodium catalyst, and their structures in solution and morphology on solid substrates were investigated with NMR, ultraviolet–visible, and circular dichroism (CD) spectroscopies and with atomic force microscopy (AFM), respectively. The obtained cis–transoidal polymers (poly-1 and poly-2) showed characteristic Cotton effects in the region of metal-to-ligand charge transfer of the chiral Ru pendants. Poly-1 and poly-2 were thought to have a predominantly one-handed helical conformation induced by the chiral pendants. However, the apparent Cotton effects derived from the helically twisted π-conjugated polymer backbone could not be observed, probably because of the strong chiral chromophoric pendants. However, in the AFM images, the helical polymers adsorbed on mica could be easily discerned as isolated strands, and the visualization and discrimination of the right- and left-handed helical structures of the chiral polymers were achieved by high-resolution AFM imaging. On the basis of the AFM observations together with the CD measurements and computational calculation results, possible structures of poly-1 and poly-2 were examined. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4621–4640, 2004

Novel unsymmetrical N-methyl aromatic amide oligomers bearing two carboxy groups at both ends were synthesized and their complexation with various optically active amines in solution was investigated using circular dichroism (CD) spectroscopy. The complexes showed a characteristic induced CD in the absorption region due to the aromatic amide reflecting the absolute configuration of the chiral amines. Chirality 16:S12–S22, 2004. © 2004 Wiley-Liss, Inc.

Optically active, cis-transoid poly(phenylacetylene) derivatives bearing a poly(γ-benzyl-L-glutamate) [poly(PBGA_m)] or poly(L-glutamic acid) [poly(PGA_m)] chain as the pendant were prepared by polymerisation of the corresponding macromonomer with a rhodium catalyst followed by hydrolysis of the pendant ester groups. Their conformational changes in solution, induced by a helix–coil transition of the pendant polypeptides, were investigated using circular dichroism (CD) and absorption spectroscopies. A series of macromonomers with a different peptide chain lengths was synthesised by the polymerisation of the N-carboxyanhydride of γ-benzyl-L-glutamate with a phenylacetylene bearing an alanine residue as the initiator. The obtained macromonomers (PBGA_m) were further polymerised with a rhodium catalyst in N,N-dimethylformamide (DMF) to yield novel poly(phenylacetylene)s [poly(PBGA_m)] with a poly(γ-benzyl-L-glutamate) pendant. The poly(PBGA_m) exhibited an induced circular dichroism (ICD) in the UV/Vis region of the polymer backbone in dimethyl sulfoxide (DMSO), probably due to the prevailing one-handed helix formation. The Cotton effect signs of a DMSO solution of the poly(PBGA_m) were inverted and accompanied by a visible colour change in the presence of an increasing amount of chloroform or DMF containing lithium chloride. The results suggest that poly(PBGA_m) may undergo a conformational change such as a helix–helix transition with a different helical pitch responding to a change in the α-helix content of the poly(γ-benzyl-L-glutamate) pendant. Moreover, a water-soluble poly(PGA_m) also showed a similar, but dramatic change in its helical conformation with a visible colour change stimulated by a helix–coil transition of the pendant poly(L-glutamic acid) chains by changing the pH in water.

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.

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.

A stereoregular poly(phenylacetylene) bearing the bulky aza-18-crown-6 ether as the pendant (poly-1) formed a predominantly one-handed helical conformation upon complexation with various chiral compounds, such as amino acids, peptides, aminosugars, amines, and amino alcohols in water. The complexes exhibited an induced circular dichroism (ICD) in the UV–visible region of the polymer main chain. Therefore, poly-1 can be used as a novel probe for determining the chirality of chiral compounds in water. The assay of 19 common free L-amino acids gave the same ICD sign at 0 °C except for L-phenylalanine. The effects of pH, temperature, guest concentration, and organic solvent content on the ICD during the complexation of poly-1 with chiral compounds were also investigated. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1004–1013, 2003

Cis–transoidal poly[(4-carboxyphenyl)acetylene] (poly-1) complexed with optically active amines and amino alcohols showed an induced circular dichroism (ICD) in the ultraviolet–visible region because of a predominantly one-handed helix formation in water and in dimethyl sulfoxide (DMSO). The Cotton effect signs of the poly-1/chiral amino alcohol complexes were inverted in the presence of water, whereas the ICD pattern of the poly-1/chiral amine complexes showed no change, regardless of the water content. These results demonstrated that the helix sense of poly-1 induced by optically active amino alcohols through noncovalent acid–base interactions could be switched by changes in the solvent ratio in the DMSO–water mixtures. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 3625–3631, 2003

Chiral polythiophenes (PTs), in sharp contrast to other optically active polymers, exhibit optical activity in the π–π* transition region which is derived from the chirality of the main chain when they self-assemble to form a supramolecular π-stacked aggregate with intermolecular interactions in a poor solvent or in a film. We now report that the regioregular, optically active PT poly[(R)-3-{4-(4-ethyl-2-oxazolin-2-yl)phenyl}thiophene] (poly-1) exhibits unique split-type induced circular dichroism (ICD) in the π–π* transition region of the main chain upon complexation with various metal salts such as trifluoromethanesulfonates of copper(I), copper(II), silver(I), and zinc(II), and iron(II) perchlorate in chloroform, which is a good solvent for poly-1. The appearance of ICD and slight changes in the UV/Vis spectra (no color change), except for the zinc salts, indicated that the chirality may not be induced by chiral π-stacked aggregates of poly-1, but by the chirality of the main chain, for example, a predominantly one-handed helical structure induced by intermolecular coordination of the oxazoline groups to metal ions. The sign of the Cotton effect depends on the metal salt; most metal salts induced ICDs with similar Cotton-effect patterns, while zinc salts caused an inversion of the signs of the Cotton effect of poly-1 accompanied by a gradual red shift in the absorption of up to 125 nm. The changes in the conformation and the size of the poly-1 aggregates induced by different metal salts were also investigated by ¹H NMR titrations, static light scattering (SLS), atomic force microscopy (AFM), and membrane filtration. On the basis of these results, we propose a possible model for the chiral supramolecular aggregates of poly-1 with metal salts.

The temperature dependence of the helical conformations for the homopolymers of phenylacetylene derivatives bearing an optically active substituent, such as the (R)-((1-phenylethyl)carbamoyl)oxy and (R)-((1-(1-naphthyl)ethyl)carbamoyl)oxy groups at the phenyl group, and their copolymers with achiral phenylacetylenes were investigated in solution using circular dichroism (CD) and absorption spectroscopies. The magnitude of the induced CD (ICD) of the optically active homopolymers increased with decreasing temperature and was accompanied by a blueshift in their absorption maxima. On the other hand, the copolymers with achiral phenylacetylenes exhibited interesting ICD changes with temperature, depending on the bulkiness of the achiral comonomers. The copolymers with a less bulky phenylacetylene had a very intense ICD at low temperatures, the ICD pattern was almost opposite to those of the chiral homopolymers, while the copolymers with the most bulky phenylacetylene bearing a tert-butyldiphenylsiloxy group at the para position showed an ICD change similar to that of the optically active homopolymers. However, the copolymers with the phenylacetylene bearing a tert-butyldimethylsiloxy group with intermediate bulkiness at the para position showed no ICD change with temperature. These results indicate that the prevailing helix-sense of the chiral-achiral random copolymers of the phenylacetylenes is determined by a delicate interaction between the chiral and achiral side chains. The thermodynamic stability parameters for the helical conformations of the homopolymers and copolymers of the phenylacetylenes were estimated from the temperature dependence of the ICDs.

A question of balls and chains: The copolymerization of an achiral fullerene and an optically active phenylacetylene yields a helical polymer (see picture) where the pendant C₆₀ groups adopt a predominant screw-sense along the polymer backbone. The copolymer exhibits an induced circular dichroism in the fullerene chromophoric region.

Eine Frage von Kugeln und Ketten: Die Copolymerisation eines achiralen Fullerens mit einem optisch aktiven Phenylacetylen führt zu einem helicalen Polymer mit schraubenförmiger Anordnung der C₆₀-Gruppen um das Polymerrückgrat. Ein induzierter Circulardichroismus des Fulleren-Chromophors lässt sich messen.

Two novel phenylacetylene derivatives bearing diethylaminomethyl groups at the meta position on phenyl groups [3-(N,N-diethylaminomethyl)phenyl]acetylene (1) and [3,5-bis(N,N-diethylaminomethyl)phenyl]acetylene (2) were synthesized and polymerized with [Rh(nbd)Cl]₂ (nbd: norbornadiene). Both monomers gave highly cis–transoidal stereoregular polymers that exhibited an induced circular dichroism (ICD) in the UV–visible region, probably because of a prevailing one-handed helical conformation upon complexation with optically active carboxylic acids such as mandelic acid and lactic acid. The sign of the Cotton effects reflected the absolute configuration of the chiral acids. Therefore, these polymers can be used as a novel probe for determining the configuration of chiral acids. The polymers were stable in the presence of chiral acids in solution. The poly-1 complexed with chiral acids exhibited a split-type ICD, whereas the poly-2 complexed with chiral acids showed a different, non-split-type ICD. The ICD pattern of the poly-1/chiral acids complexes dramatically changed with an increase in the concentration of the chiral acids, thus showing a non-split-type ICD similar to those of the poly-2/chiral acid complexes. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 3180–3189, 2001

The chiroptical properties of a novel, optically active regioregular poly[3-(4-((R)-4-ethyl-2-oxazolin-2-yl)phenyl)thiophene] (poly-1) were investigated in mixtures of a good solvent, chloroform, and a variety of poor solvents using CD spectroscopy. Most poor solvents induced CD on poly-1 with similar Cotton effects, while acetonitrile and nitromethane caused dramatic changes in the Cotton effects of poly-1. Chirality 12:396–399, 2000. © 2000 Wiley-Liss, Inc.