Preferred-handed Helical Conformation Research Articles

Chiral Transfer of Linear Polysiloxane with Preferred-Handed Helical Conformation.

We report the preparation of chiral silica using a linear polysiloxane main chain with a preferred-handed helical structure as the template. Poly(methylvinyl siloxane) (PMVS) with a cysteine derivative side chain designated as PMVS-Cys was prepared using anionic polymerization and an ene-thiol reaction. PMVS-Cys forms a helical conformation in both solution and film via hydrogen bonding between amide groups at side chains. The helical structure remains during the calcination process, resulting in silica with helical structure. The silica with a helical structure shows optical activity.

Helicity induction and memory of a lipophilic Brønsted acid-type poly(phenylacetylene) in non-polar solvents.

A novel lipophilic Brønsted acid-type PPA (poly-1-H) bearing a sulfonylphosphoramidic acid moiety, covered with a dynamically axially chiral biphenyl pendant, was synthesized and found to form a preferred-handed helical conformation in non-polar solvents upon complexation with non-racemic amines. This induced helicity was maintained after replacement with achiral amines ("dynamic helicity memory"). In addition, poly-1-H exhibited "static helicity memory" in which the induced helical conformation was maintained in non-polar solvents, even after acidification with a stronger acid without replacement with achiral amines.

Synthesis of a poly(diphenylacetylene) bearing optically active anilide pendants and its application to a chiral stationary phase for high-performance liquid chromatography

Poly(diphenylacetylene) having optically active anilide pendants (poly-1) were synthesized by the condensation reaction of an optically active carboxylic acid with a key precursor polymer containing amino (-NH2) groups, which was prepared by the polymerization of a phthalimide-protected diphenylacetylene monomer using WCl6-Ph4Sn as a catalyst, followed by phthalimide deprotection in the resulting polymer using hydrazine monohydrate. Poly-1 formed a preferred-handed helical conformation (h-poly-1) upon thermal annealing in DMF because of chirality of the pendant group. Poly-1 and h-poly-1 showed different chiral recognition abilities from the analogous poly(diphenylacetylene)s, having the corresponding optically active amide pendants, as chiral stationary phases (CSPs) for high-performance liquid chromatography. The resolution results with the h-poly-1-based CSP were much better than those with the poly-1-based CSP owing to the preferred-handed macromolecular helicity. Among the tested racemates, the h-poly-1-based CSP exhibited superior chiral recognition ability, especially toward binaphthyl compounds and chiral metal complexes.

Chiral Self-Assembly of Nanoparticles Induced by Polymers Synthesized via Reversible Addition-Fragmentation Chain Transfer Polymerization.

Chiral inorganic nanomaterials are of great interest because of their excellent optical properties. Most of the attention has been focused on the utilization of biomolecules or their derivatives as linkers or templates to control the chiral structure of assembled inorganic nanoparticles. Chiral polymers are promising synthetic materials that can be used to replace their biological counterparts. Here, by using poly(methacrylate hydroxyethyl-3-indole propionate) (PIPEMA) and poly(2-hydroxyethyl methacrylate) (PHEMA) synthesized via syndioselective reversible addition-fragmentation chain transfer polymerization, we successfully realized chiral self-assembly of gold nanorods with strong circular dichroism response in the vis-NIR region. Moreover, the intensity of the chiral signal of the assemblies can be regulated by the molecular weight of the polymers. Notably, although the monomers are achiral and no chiral reagents are involved in their synthesis, the main chains of PIPEMA and PHEMA exhibit a preferred-handed helical conformation, which is the origin of chirality of the nanorod assemblies. The preferred-handed helical conformation of polymers is attributed to their syndiotacticity and stabilized by the steric hindrance of the side groups. The addition of chiral carbon atoms at the side groups doesnot change the preferred-handedness of the polymer main chain, resulting in the assembled nanorod structures with the same chirality. This strategy provides inspiration for the rational design and synthesis of optically active functional synthetic polymers for the preparation of promising chiral nanomaterials.

"Helix-in-Helix" Superstructure Formation through Encapsulation of Fullerene-Bound Helical Peptides within a Helical Poly(methyl methacrylate) Cavity.

A one‐handed 310‐helical hexapeptide is efficiently encapsulated within the helical cavity of st‐PMMA when a fullerene (C60) derivative is introduced at the C‐terminal end of the peptide. The encapsulation is accompanied by induction of a preferred‐handed helical conformation in the st‐PMMA backbone with the same‐handedness as that of the hexapeptide to form a crystalline st‐PMMA/peptide‐C60 inclusion complex with a unique optically active helix‐in‐helix structure. Although the st‐PMMA is unable to encapsulate the 310‐helical peptide without the terminal C60 unit, the helical hollow space of the st‐PMMA is almost filled by the C60‐bound peptides. This result suggests that the C60 moiety can serve as a versatile molecular carrier of specific molecules and polymers in the helical cavity of the st‐PMMA for the formation of an inclusion complex, thus producing unique supramolecular soft materials that cannot be prepared by other methods.

Synthesis of Optically Active Poly(diphenylacetylene)s Using Polymer Reactions and an Evaluation of Their Chiral Recognition Abilities as Chiral Stationary Phases for HPLC.

A series of optically active poly(diphenylacetylene) derivatives bearing a chiral substituent (poly-2S) or chiral and achiral substituents (poly-(2Sx-co-31−x)) on all of their pendant phenyl rings were synthesized by the reaction of poly(bis(4-carboxyphenyl)acetylene) with (S)-1-phenylethylamine ((S)-2) or benzylamine (3) in the presence of a condensing reagent. Their chiroptical properties and chiral recognition abilities as chiral stationary phases (CSPs) for high-performance liquid chromatography (HPLC) were investigated. Poly-2S and poly-(2Sx-co-31−x) (0.06 < x < 0.71) formed a preferred-handed helical conformation with opposite helical senses after thermal annealing despite possessing the same chiral pendant (h-poly-2S and h-poly-(2Sx-co-31−x)). Furthermore, h-poly-2S and h-poly-(2S0.36-co-30.64) emitted circularly polarized luminescence with opposite signs. h-Poly-2S showed higher chiral recognition abilities toward a larger number of racemates than poly-2S without a preferred-handed helicity and the previously reported preferred-handed poly(diphenylacetylene) derivative bearing the same chiral substituent on half of its pendant phenyl rings. h-Poly-(2S0.36-co-30.64) also exhibited good chiral recognition abilities toward several racemates, though the elution order of some enantiomers was reversed compared with h-poly-2S.

Helical Poly(phenylacetylene) Bearing Chiral and Achiral Imidazolidinone-Based Pendants that Catalyze Asymmetric Reactions due to Catalytically Active Achiral Pendants Assisted by Macromolecular Helicity.

A series of optically active helical copolymers of phenylacetylenes are prepared by the rhodium-catalyzed copolymerization of the imidazolidinone-linked, catalytically active achiral phenylacetylenes and catalytically inactive chiral phenylacetylenes. The obtained chiral/achiral copolymers exhibit an induced circular dichroism in the UV-vis regions of the copolymer backbones resulting from a preferred-handed helical conformation biased by the chiral imidazolidinone units incorporated in the copolymers. The copolymers are found to catalyze the asymmetric Diels-Alder reaction and produce the products with a moderate enantioselectivity in spite of the fact that the catalytically active units of the copolymers are achiral, indicating that the observed enantioselectivity totally originates from the helical chirality dynamically induced by the optically active, but catalytically inactive imidazolidinone units incorporated in the copolymers.

Helix-sense-selective radical polymerization of (S)-6-acryloyl-2,2′-bisalkoxy-1,1′-binaphthyl

A series of (S)-6-acryloyl-2,2′-bisalkoxy-1,1′-binaphthyl 1 were radically polymerized to obtain polymers. The optical rotation and the circular dichroism (CD) signal intensity of the radically polymerized poly-1 were close to that of the anionically polymerized poly-1 indicating that the radically polymerized poly-1 should adopt stable preferred-handed helical conformation in solution just as the anionically polymerized poly-1.

Dual Memory of Enantiomeric Helices in Poly(phenylacetylene)s Induced by a Single Enantiomer through Helix Inversion and Dual Storage of the Enantiomeric Helicity Memories

A series of stereoregular poly(phenylacetylene)s carrying prochiral phosphonic acid monoesters as the pendant groups (monophenyl ester: poly-1a; monoethyl ester: poly-1b; monoisopropyl ester: poly-1c) were synthesized. The polymers formed a preferred-handed helical conformation upon complexation with nonracemic amines, whose helix senses were inverted by temperature or solvent composition. The resulting diastereomeric right- and left-handed helices induced by chiral amines were retained or memorized after the chiral amines were completely replaced with achiral diamines, thus producing the corresponding enantiomeric helices. The helicity memory, however, is temporary and will be gradually lost due to its inherently dynamic nature. In order to permanently store or save the helicity memory, the phosphorus residues of the polymers were methyl esterified with diazomethane, which proceeded in an enantioselective way to generate a chiral center at the phosphorus residues with optical activity due to chirality tr...

Synthesis of polyisocyanides bearing oligothiophene pendants: higher-order structural control through pendant framework design

L-Alanine-based polyisocyanides bearing oligothiophene (OT) pendants (poly-3a and poly-3b) were synthesized and the influence of the OT pendants on the chiroptical properties and helix-forming abilities of the polymers was investigated. Poly-3a bearing a quinquethiophene unit with two n-hexyl chains successfully formed a preferred-handed helical conformation by simultaneous interactions of the OT-pendant π–π stacking and hydrogen bonding of the amide groups. In contrast, poly-3b, which possesses a branched alkyl chain on behalf of the terminal n-hexyl chain of poly-3a, did not form a higher-ordered helical conformation. The results suggest that the structure of the alkyl chains positioned far from the polymer backbones remotely and sensitively affected the ability of the polymer to form a helical conformation. We also found that the structure of the OT pendants significantly affect the molecular weight as observed by AFM measurements. The polyisocyanide poly-3c bearing a bithiophene unit was additionally synthesized to examine the influence of the number of thiophene rings within the OT pendants on the higher-order structure formation. L-Alanine-based polyisocyanides bearing oligothiophene (OT) pendants were synthesized and the influence of the OT pendants on the helix-forming abilities of the polymers was investigated. The polyisocyanide possessing quinquethiophene units with two n-hexyl chains readily formed a right-handed helical conformation with a helical array of the OT pendants biased by the chirality of the L-alanine residues. This study revealed that the simultaneous interactions of the π–π stacking and the hydrogen bonding between the pendants are required to arrange the OT units into a helically twisted manner along the backbone.

Macromolecular Helicity Induction and Memory in a Poly(biphenylylacetylene) Bearing an Ester Group and Its Application to a Chiral Stationary Phase for High-performance Liquid Chromatography

Abstract An optically inactive poly(biphenylylacetylene) bearing an ester group at the 4′-position of the pendants formed a preferred-handed helical conformation and biased axial chirality in the pendants through a noncovalent interaction with a chiral alcohol, both of which were retained (memorized) even after removal of the chiral alcohol. The chiral stationary phase for high-performance liquid chromatography, prepared by coating the polymer with macromolecular helicity memory on silica gel, showed good chiral recognition ability towards various racemates.

Synthesis and chiral recognition ability of helical polyacetylenes bearing helicene pendants

Novel polyacetylenes bearing an optically active or racemic [6]helicene unit as the pendant groups directly bonded to the main-chain (poly-1s) were prepared by the polymerisation of the corresponding acetylenes (1-rac, 1-P and 1-M) using a rhodium catalyst. The optically active polyacetylenes (poly-1-P and poly-1-M) formed a preferred-handed helical conformation biased by the optically active helicene pendants, resulting in the induced circular dichroism (ICD) in their π-conjugated polymer backbone regions. The optically active helical polymers, when employed as an enantioselective adsorbent, showed a high chiral recognition ability towards racemates, such as the monomeric [6]helicene and 1,1′-binaphthyl analogues, and enantioselectively adsorbed one of the enantiomers.

Remarkable Enhancement of the Enantioselectivity of an Organocatalyzed Asymmetric Henry Reaction Assisted by Helical Poly(phenylacetylene)s Bearing Cinchona Alkaloid Pendants via an Amide Linkage.

A series of novel helical poly(phenylacetylene)s bearing amino-functionalized cinchona alkaloid pendant groups connecting to the phenyl rings through an amide linkage were prepared by the polymerization of the corresponding phenylacetylenes using a rhodium catalyst. All of the polymers formed a preferred-handed helical conformation biased by the optically active pendants, resulting in the induced circular dichroism in their π-conjugated polymer backbone regions. The optically active helical polymers efficiently catalyzed the asymmetric Henry reaction of benzaldehydes with nitromethane, giving optically active products up to 94% enantiomeric excess (ee) when the poly(phenylacetylene) bearing an amino-functionalized quinine pendant group was used as the polymeric organocatalyst; the enantioselectivity was remarkably higher than those catalyzed by the corresponding nonhelical poly(phenylacetylene) (18% ee) and the monomer (28% ee).

Molecular chirality induction to an achiral π-conjugated polymer by circularly polarized light

A preferred-handed helical conformation was induced to poly(9,9-dioctylfluoren-2,7-diyl) (PDOF) in a thin film form upon irradiation with single-handed circularly polarized light (CPL) where the induction was reversible.

A chiral π-stacked vinyl polymer emitting white circularly polarized light

An optically active, π-stacked poly[2,7-bis(4-tert-butylphenyl)dibenzofulvene] having a preferred-handed helical conformation was synthesized by anionic polymerization. A thin film sample of the polymer exhibited broad-band white circularly polarized light (CPL) emission on photo excitation.

A photo-degradable helix: Synthesis, structure, and photolysis of optically active poly[2,7-bis(4-t -butylphenyl)-9-methylfluoren-9-yl acrylate

2,7-Bis(4-t-butylphenyl)-9-methylfluoren-9-yl acrylate (BBPMFA) was synthesized and polymerized using α,α′-azobisisobutyronitrile or n-Bu3B-air as a radical initiator and using the complex of 9-fluorenyllithium with (S)-(+)-1-(2-pyrrolidinylmethyl)pyrrolidine as an optically active anionic initiator. Although the radical polymerization led to rather low-molecular-weight products at low yields, the anionic polymerization afforded polymers with higher molecular weights in higher yields. The poly(BBPMFA) obtained by the anionic polymerization was slightly rich in isotacticity (meso diad 57%) and showed an intense circular dichroism (CD) spectrum and large dextrorotation. The intensity of the CD spectrum and magnitude of optical activity increased with an increase in Mn, suggesting that the polymer possesses a preferred-handed helical conformation. The CD spectrum disappeared within 1 s on irradiation to the polymer in a CHCl3 solution using a 500-W Hg-Xe lamp. This was ascribed to fast photolysis of the ester linkage leading to a loss of helical conformation of the entire chain. Photolysis products of poly(BBPMFA) were poly(acrylic acid) and 2,7-bis(4-t-butylphenyl)-9-methylenefluorene (2,7-bis(4-t-butylphenyl)dibenzofulvene). The photolysis reaction seemed to proceed through the “unzipping” mechanism. The rate constant of photolysis of poly(BBPMFA) under irradiation at monochromated 325 nm was around 0.01 s−1 independent of molecular weight. Photolysis at 325 nm was approximately 2400 times faster than that for chemical ester solvolysis under a neutral condition in the dark. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011

Redox-triggered switching of helical chirality of poly(phenylacetylene)s bearing riboflavin pendants

Two novel optically active poly(phenylacetylene)s bearing riboflavin (vitamin B2) residues as the pendant through different covalent linkages (poly-1 and poly-2) were prepared by the polymerization of the corresponding monomers (1 and 2) with a rhodium catalyst, and their chiroptical properties were investigated by UV-visible and circular dichroism (CD) spectroscopies. Riboflavin-linked poly-2 through the acetal linkage with the ribityl group to the phenylacetylene exhibited a relatively intense induced CD (ICD) in the polymer backbone region. The ICD was temperature-dependent, indicating that the main chain of poly-2 adopts a dynamic preferred-handed helical conformation induced by the optically active ribityl group. In contrast, poly-1 having the riboflavin pendant via the N3-methylene linker showed almost no apparent ICD due to the main-chain helical conformation, probably because the optically active ribityl group of poly-1 is located relatively far from the polymer backbone compared with poly-2. The cyclic voltammetric measurements of poly-1 and poly-2 revealed that these polymers possess reversible redox properties originating from the electron transfer process of the riboflavin pendants. The switching of the chiroptical properties in response to the redox stimuli was also investigated using a chemical reductant (Na2S2O4) and oxidant (O2).

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.

Syntheses and Chiroptical Properties of Optically Active Helical Poly(phenylacetylene)s Bearing [60]Fullerene Pendants

Two novel C60-bound chiral (3) and achiral (2) phenylacetylenes were synthesized by the treatment of C60 with (R)-N-[1-(4-ethynylphenyl)ethyl]glycine and N-(4-ethynylbenzyl)glycine by the Prato reaction in the presence of paraformaldehyde and dipentadecyl ketone, respectively. The homopolymerization of the achiral 2 bearing long alkyl chains and copolymerization with optically active phenylacetylenes including 3 using a rhodium catalyst in chloroform homogeneously occurred, giving the C60-containing helical poly(phenylacetylene)s soluble in chloroform, while the homopolymers of the analogous achiral C60-bound phenylacetylenes with no alkyl chain (1) and 3 were totally insoluble in common organic solvents. The obtained copolymers exhibited an induced circular dichroism (ICD) both in the main-chain region and in the fullerene chromophoric region, indicating that the optically active copolymers possess a preferred-handed helical conformation and that the pendant fullerene groups are arranged in helical arrays with a predominant screw-sense along the polymer backbones. The formation of a hierarchical supramolecular assembly of the homopolymers of 1 and 3 bearing fullerene pendants on each repeating unit was also studied by electron microscopy.

Chirality-Responsive Helical Polymers

Unique macromolecules that fold into a preferred-handed helical conformation induced by external chiral stimuli are mainly described in this review. In contrast to small molecular systems, the chiral information of nonracemic guest molecules transfers with a significant amplification in a dynamic helical polymer, such as stereoregular poly(phenylacetylene)s bearing functional pendant groups as an excess of a single-handed helix through noncovalent bonding interaction, which provides an efficient chirality-sensing system. The chirality sensing of other oligomers (foldamers) is also briefly described.