Helix-sense-selective polymerization of phenylacetylene having two hydroxy groups using a chiral catalytic system

Helix-Sense-Selective Memory Polymerization of Biphenylylacetylenes Bearing Carboxy and Amino Groups in Water

We report the helix-sense-selective memory polymerization (HSMP) of achiral biphenylylacetylenes bearing carboxy and amino pendant groups in the presence of basic and acidic chiral guests in water, respectively. The HSMP proceeds in a highly helix-sense-selective manner driven by noncovalent chiral ionic interactions between the monomers and guests under kinetic control, producing the one-handed helical polymers with a static memory of helicity in one-pot during the polymerization in a very short time, accompanied by amplification of asymmetry. The carboxy-bound helicity-memorized polymer self-assembles into a cholesteric liquid crystal in concentrated water, in which a variety of basic achiral fluorophores further co-assembles to form supramolecular helical aggregates that exhibit an induced circularly polarized luminescence in a color tunable manner.

Redox-Triggered Reversible Switching between Dynamic and Quasi-static α-Helical Peptides

We report the reversible transformation between a singly stapled dynamic α-helical peptide and a doubly stapled quasi-static one through redox-triggered dithiol/disulfide conversions of a stapling moiety. This process allows the rate of interconversion between the right-handed (P) and left-handed (M) α-helices to be altered by a factor of approximately 103 before and after the transformation. An as-obtained doubly stapled α-helical peptide, which is composed of an achiral peptide having an l-valine carboxylic acid residue at the C-terminus, a disulfide-based reversible staple, and a biphenyl-based fixed staple, adopts an (M)-rich form as a kinetically trapped state. The (M)-rich helix was subsequently transformed into the thermodynamically stable (P)-rich form in 1,1,2,2-tetrachloroethane with the half-life time (t1/2) of approximately 44 days at 25 °C. Reduction of the doubly stapled peptide with tri-n-butylphosphine in tetrahydrofuran/water (10/1, v/v) produced the corresponding singly stapled dynamic α-helical peptide bearing two thiol groups at the side chains, which underwent solvent-induced reversible helicity inversion. The resulting dithiol of the singly stapled peptide could be reoxidized to form the original doubly stapled form using 4,4'-dithiodipyridine. Furthermore, the P/M interconversion of a doubly stapled peptide with two flexible hydrocarbon-based staples is considerably more rapid than that with more rigid staples.

Helix-Sense-Selective Permeation of Racemic Helical Oligoacetylenes through One-Handed Helical Channels in Polymer Membranes

Helix-sense-selective permeation (HSSPerm) of racemic helical oligoacetylenes through one-handed helical channels has been realized. The one-handed helical channels were created in the one-handed helical polyacetylene membranes by the helix-sense-selective decomposition (HSS-SCAT) of the corresponding racemic helical polyacetylene membranes, followed by removing the formed oligomers. Since the HSS-SCAT reaction proceeds with just circularly polarized visible light with no reagents, no catalysts, no solvent, and high selectivity, the chiral channel-containing membrane with high purity was obtained easily. This membrane could separate racemic helical oligoacetylenes enantioselectively in up to 30%ee. To our knowledge, this is the first example of HSSPerm.

Synthesis of Stereocontrolled Degradable Polymer by Living Cascade Enyne Metathesis Polymerization

A stereocontrolled degradable polymer was synthesized via living cascade enyne metathesis polymerization. Highly stereodefined N,O-acetal-containing enyne monomers were prepared using the Pd-catalyzed hydroamination of alkoxyallenes and ring-closing metathesis. The resulting chiral polymer exhibited a narrow dispersity window. Block copolymers were prepared not only by sequentially adding nondegradable and degradable monomers but also by using enantiomerically different monomers to produce stereocontrolled blocks. Owing to the hydrolyzable N,O-acetal moiety in the backbone structure, the resulting polymer could degrade under acidic conditions generated using various acid concentrations to control the degradation. Additionally, the aza-Diels-Alder reaction modified the polymer without losing the stereochemistry.

Novel highly efficient absolute optical resolution method by serial combination of two asymmetric reactions from acetylene monomers having racemic substituents

For general optical resolution, an optical resolution agent is necessary, and the best agent should be selected for each racemic compound. In this study, we will report that a novel optical resolution method by circularly polarized light (CPL) without any optical resolution agents has been developed by serially connecting two enantioselective reactions. These reactions we developed are the enantiomer-selective helix-sense-selective polymerization (ES-HSSP) and helix-sense-selective highly selective photocyclic aromatization (SCAT) by CPL (HS-SCAT). Since this significantly unique EPHS method (EPHS = ES-HSSP + HS-SCAT) does not need any optical resolution agents, any cocatalysts, and solvents for the selective decomposition reaction (HS-SCAT), this process is quite simple and convenient. Since this process does not include any decomposition of the target racemates themselves, both enantiomers could be obtained. The optical yields for isolated compounds that were enantiomerically separated by the EPHS method were very high, for example, 78%ee, 93%ee, and 85%ee for menthol, phenethyl alcohol, and 2-butanol, respectively. In addition, their chemical yields were around 85% to 94%. Therefore, the EPHS method was found to show an excellent performance and can be applied to actual optical resolution for a wide range of racemic compounds. This is the first absolute optical resolution by CPL showing high optical and chemical yields and expected to become a practical optical resolution method.

Saccharide-Functionalized Poly(Zn-salphen)-alt-(m- and p-phenyleneethynylene)s as Dynamic Helical Metallopolymers

The study of metallopolymers with controllable helical sense remains in its infancy. We report arabinose-functionalized (Zn-salphen)-based conjugated polymers that display mirror-image circular dichroism spectra for L- and D-sugar sidechains respectively, signifying ordered (helical) coiling of the polymer backbone with opposite screw-sense preferences. The observation of different spectroscopic behavior and Cotton effects for a variety of solvents (in a reversible manner) and temperatures, ascribed to changes in the extent of intrachain (Zn⋅⋅⋅O(salphen) and π-stacking) interactions between Zn-salphen moieties, thus indicate the flexible, responsive and dynamic nature of the folded helical conformation in these systems. An application study signifying that activity can be governed by the structure and helical sense of the polymer is described.

Rhodium-Catalyzed Stitching Polymerization of Alkynylsilylacetylenes

Polymers possessing a silicon-bridged π-conjugated repeating unit constitute an important class of compounds for their potential utility as optoelectronic materials. Herein we developed a rhodium-catalyzed stitching polymerization of nonconjugated and readily prepared alkynylsilylacetylenes for the synthesis of new π-conjugated polymers with ladder-type silicon-bridged repeating units. The polymerization proceeded smoothly by employing a Rh/tfb complex as the catalyst, and not only diynes but also triynes and tetraynes could be polymerized in a stitching manner to give polymers that are inaccessible by existing methods. The solubility of the polymers in different types of solvents could be controlled by introducing appropriate functional groups on the silicon atoms, and sequence-controlled functionalized polyacetylenes could be accessed by protodesilylation of the stitched polymers. Physical properties of the obtained polymers were also investigated to understand their characteristic features.

Emergence of Highly Enantioselective Catalytic Activity in a Helical Polymer Mediated by Deracemization of Racemic Pendants

Any polymers composed of racemic repeating units are obviously optically inactive and hence chiral functions, such as asymmetric catalysis, will not be expected at all. Contrary to such a preconceived notion, we report an unprecedented helical polymer-based highly enantioselective organocatalyst prepared by polymerization of a racemic monomer with no catalytic activity. Both the right- and left-handed helical poly(biarylylacetylene)s (PBAs) composed of dynamically racemic 2-arylpyridyl-N-oxide monomer units with N-oxide moieties located in the vicinity of the helical polymer backbone can be produced by noncovalent interaction with a chiral alcohol through deracemization of the biaryl pendants. The macromolecular helicity and the axial chirality induced in the PBAs are retained ("memorized") after complete removal of the chiral alcohol. Accordingly, the helical PBAs with dual static memory of the helicity and axial chirality show remarkable enantioselectivity (86% ee) for the asymmetric allylation of benzaldehyde. The enantioselectivity is slightly lower than that (96% ee) of the homochiral PBAs prepared from the corresponding enantiopure (R)- and (S)-monomers, but is comparable to that (88% ee) of the helical PBA composed of nonracemic monomers of ca. 60% ee.

Optical-Dissymmetry Phase Transitions in an Achiral Helical-Spring Polymer through Controlled Noncovalent Interactions

Noncovalent chemistry may offer diversity in the functions and applications for artificial polymers by allowing various ordered-disordered phase transitions in a precisely controlled manner. To verify this notion from a fundamental perspective, we examined an achiral poly(phenylacetylene) derivative with an α-helical structure as a helical-spring polymer for revealing phase changes through control of intramolecular hydrogen bonding with the chiral solvent and temperature. When an amine capable of hydrogen bonding was used as the chiral solvent, either an irreversible helix-helix or a reversible helix-coil phase change occurred in an optically dissymmetric manner according to the amount of the chiral solvent added and ambient temperature. Considering the hydrogen-bonding strength values of the solvent mixture and the thermodynamic parameters, we could predict if the optical-dissymmetry phase changes would occur and, if so, how they occur. Our results were similar to those see for the denaturation of proteins, induced by solvent and temperature, based on helix-coil phase transition.

Microporous Hyper-Cross-Linked Polymers with High and Tuneable Content of Pyridine Units: Synthesis and Application for Reversible Sorption of Water and Carbon Dioxide

New hyper-cross-linked porous organic polymers (POPs) with a high content of pyridine segments (7.86 mmol pyridine g^-1 ), and a micro/mesoporous texture are reported. The networks are achieved by the chain-growth homopolymerization of 2,6- and 3,5-diethynylpyridines. The pyridine segments form links interconnecting the polyacetylene main chains in these networks. The content of pyridine segments in the networks can be tuned by copolymerizing diethynylpyridines with 1,3-diethynylbenzene. The pyridine rings in the networks serve as base and hydrophilic centers for the sorption of CO₂ and water. The homopolymer pyridine networks are highly efficient in the low-pressure adsorption/desorption of CO₂ . This sorption mode is promising for the postcombustion removal of CO₂ from the fuel gas. The poly(3,5-diethynylpyridine) network exhibits high efficiency in capturing and releasing water vapor (determined capacity 376 mg g^-1 at 298 K and relative humidity (RH) = 90% is one of the highest values reported for POPs) and is a promising material for the cyclic water harvesting from air. The reported networks are characterized by ¹³ C cross-polarization magic angle spinning NMR, thermogravimetric analysis, and N₂ adsorption/desorption and their efficiency in CO₂ and H₂ O capturing is discussed in relation to the content and type of incorporated pyridine segments.

Unprecedented Application of Covalent Organic Frameworks for Polymerization Catalysis: Rh/TPB-DMTP-COF in Polymerization of Phenylacetylene and Its Functional Derivatives

Covalent organic frameworks (COFs) are applied widely in organic catalysis; however, no precedent has been reported in polymerization catalysis. Herein, we report the new application of COFs for polymerization catalysis. Different amounts of homogeneous Rh catalyst are incorporated into the COF via post-treatment to give a series of TPB-DMTP-COF-X wt % Rh (b-e) containing varying amounts of Rh from 2.74 to 11.38 wt %. In contrast to the known Rh catalysts, TPB-DMTP-COF-X wt % Rh (b-e) display an uncommon synergistic effect and exceptional steric confinement effect of nanochannels. Therefore, they possess the advantages of both homogeneous catalysts in high activity and selectivity and heterogeneous catalysts in stability and recyclability with extremely high activity up to 1.3 × 10⁷ g·mol_Rh^-1·h^-1 and cis-selectivity up to 99% and can be readily recycled and reused five times in the polymerization of phenylacetylene and its derivatives, affording cis-transoidal polyphenylacetylene and its derivatives having helical structures, aggregation-induced emission properties, or fluorescence properties with narrow molecular weight distributions.

Racemic Monomer-Based One-Handed Helical Polymer Recognizes Enantiomers through Auto-Evolution of Its Helical Handedness Excess

A racemic monomer-based optically inactive polyacetylene folds into a one-handed helix assisted by a nonracemic alcohol, which can separate various enantiomers as a chiral stationary phase in chromatography. The chiral-resolving power is virtually identical to that of the enantiopure monomer-based one-handed helical polyacetylene. Because of its unique static memory of the induced helicity, the original racemic polyacetylene expresses an auto-evolution of its helical handedness over time, and at the same time, chirality of the nonracemic alcohol is discriminated accompanied by successive enhancement of its optical purity enantioselectively adsorbed on the helical polyacetylene owing to the chiral filter effect as directly monitored by NMR, which contributes to further enhancing the helix-sense-excess of the helical polyacetylene.

Chiral amines as initiators for ROP and their chiral induction on poly(2-aminoisobutyric acid) chains

Chirality induction by chiral amines in poly(amino acid)s by ring opening polymerization.

Synthesis of Cis-Cisoid or Cis-Transoid Poly(Phenyl-Acetylene)s Having One or Two Carbamate Groups as Oxygen Permeation Membrane Materials

Three new phenylacetylene monomers having one or two carbamate groups were synthesized and polymerized by using (Rh(norbornadiene)Cl)2 as an initiator. The resulting polymers had very high average molecular weights (Mw) of 1.4-4.8 × 106, with different solubility and membrane-forming abilities. The polymer having two carbamate groups and no hydroxy groups in the monomer unit showed the best solubility and membrane-forming ability among the three polymers. In addition, the oxygen permeability coefficient of the membrane was more than 135 times higher than that of a polymer having no carbamate groups and two hydroxy groups in the monomer unit with maintaining similar oxygen permselectivity. A better performance in membrane-forming ability and oxygen permeability may be caused by a more extended and flexible cis-transoid conformation and lower polarity. On the other hand, the other two new polymers having one carbamate group and two hydroxy groups in the monomer unit showed lower performances in membrane-forming abilities and oxygen permeabilities. It may be caused by a very tight cis-cisoid conformation, which was maintained by intramolecular hydrogen bonds.

Facile and Versatile Synthesis of End-Functionalized Poly(phenylacetylene)s: A Multicomponent Catalytic System for Well-Controlled Living Polymerization of Phenylacetylenes

A rhodium-based multicomponent catalytic system for well-controlled living polymerization of phenylacetylenes has been developed. The catalytic system is composed of readily available and bench-stable [Rh(nbd)Cl]₂ , aryl boronic acid derivatives, diphenylacetylene, 50 % aqueous KOH, and PPh₃ . This system offers a method for the facile and versatile synthesis of various end-functionalized cis-stereoregular poly(phenylacetylene)s because components from aryl boronic acids and diphenylacetylene were introduced to the initiating end of the polymers. The polymerization reaction shows a typical living nature with a high initiation efficiency, and the molecular weight of the resulting poly(phenylacetylene)s can be readily controlled with very narrow molecular-weight distributions (M_w /M_n =1.02-1.09). The experimental results suggest that the present catalytic system has a higher polymerization activity than the polymerization activities of other rhodium-based catalytic systems previously reported.

Self-supported rhodium catalysts based on a microporous metal–organic framework for polymerization of phenylacetylene and its derivatives

Highly efficient, cis-selective, stable, recyclable and reusable heterogeneous single-site MOF-supported rhodium catalysts polymerize PA and its derivatives, producing cis-transoidal PPAs and their functional derivatives having a helical configuration or AIE properties.

The construction of photoresponsive polymer particles with supramolecular helicity from achiral monomers by helix-sense-selective polymerization

Optically active azobenzene-containing polymer particles successfully prepared from achiral monomers for the first time by helix-sense-selective dispersion polymerization, also known as asymmetric helix-chirogenic polymerization.

Chiral helical polymer materials derived from achiral monomers and their chiral applications

Helix-sense-selective polymerization (HSSP) of achiral monomers and chiral post-induction of racemic helical polymers provide two alternative approaches for constructing chiral helical polymer materials.

Helix-Sense-Selective Polymerization of 3,5-bis(hydroxymethyl)phenylacetylene Rigidly Bearing Galvinoxyl Residues and Their Chiroptical Properties

Four kinds of newly synthesized achiral phenylacetylenes bearing a phenylhydrogalvinoxyl residue at 4-position were polymerized by using a chiral rhodium catalyst system, [Rh(nbd)B(C6H5)4] or [Rh(nbd)Cl]2 catalysts in the presence of chiral (R)-(+)- or (S)-(-)-1-phenylethylamine ((R)- or (S)-PEA) cocatalysts. Poly(m-HGDHPA) and poly(m-HGTHPA) in THF showed Cotton signals at the absorption regions of the main chain and hydrogalvinoxyl in the circular dichroism (CD) spectra. It indicated that excess of one-handed helical polyacetylene backbone was induced by helix-sense-selective polymerization (HSSP) under the asymmetric conditions despite the achiral monomer, and the hydrogalvinoxyl moieties were also arranged to form one-handed helical structure. However, there was no Cotton effect for poly(p-HGDHPA) and poly(p-HGTHPA) because the intramolecular hydrogen bonding did not act well to stabilize the helical conformation. The hydrogalvinoxyl units of poly(m-HGDHPA) and poly(m-HGTHPA) were converted to the corresponding galvinoxyl radicals after treatment with PbO2. In the CD spectra of the polyradicals, the Cotton effects decreased depending on their static stability of helical conformation, suggesting that reversal conformation of the polymer chain arose.

New Synthetic Methods of Novel Nanoporous Polycondensates and Excellent Oxygen Permselectivity of Their Composite Membranes

Two kinds of novel nanoporous polycondensates (sc(Rf)) have been synthesized by two new preparation methods consisting of polycondensation and highly selective photocyclicaromataization of 1/3 helical cis-cis polyphenylacetylenes with polymerizable groups. By the original methods, new well-defined sheet polymers having nanopores or nanospaces have been synthesized for the first time. Their composite membranes, containing small amounts (1.0 wt%) of sc(Rf), had ultrahigh oxygen permeability (Po2 > 1000 barrer), and their plots were beyond the Robeson's upper bound line in the graph of oxygen permselectivity (α = Po2/PN2) versus Po2. Both α and Po2 values were enhanced by adding only small amounts (1.0 wt%) of sc(Rf). One of the sc(Rf)s synthesized on the base membrane surface showed the best performance, i.e., Po2 = 5300 barrer and α = 2.5. The membrane surface was effectively covered by sc(Rf), judging from the contact angle values. It is thought that nanopores and nanospaces created in and between sc(Rf) molecules played an important role for the enhancement of both α and Po2/PN2.

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 does not 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-Sense-Selective Polymerization of Phenylacetylenes Having a Porphyrin and a Zinc-Porphyrin Group: One-Handed Helical Arrangement of Porphyrin Pendants

Newly synthesized two kinds of achiral phenylacetylenes having a free-base- or a zinc-porphyrin (1 and Zn1, respectively) were polymerized by using a chiral rhodium catalyst system, Rh⁺(nbd)[(η⁶-C₆H₅)B⁻(C₆H₅)₃] catalyst and (R)-(+)- or (S)-(⁻)-1-phenylethylamine ((R)- or (S)-PEA, respectively) cocatalyst. Poly(1) and poly(Zn1) in THF showed a Cotton signal at the absorption region of the porphyrin and the main chain in the circular dichroism (CD) spectra. This result suggests that poly(1) and poly(Zn1) exist in a conformation with an excess of one-handed helix sense and the porphyrin moiety arranged in chiral helical fashion. The one-handed helical structure of poly(1) could be sustained in a mixture of THF/HMPA (10/2, v/v) due to stabilizing by stacking effect of porphyrin moieties along the main chain. This is the first example about helix-sense-selective polymerization by using Rh⁺(nbd)[(η⁶-C₆H₅)B⁻(C₆H₅)₃] catalyst. Additionally, poly(Zn1) showed about 10 times larger CD intensity in comparison with poly(1). This result suggests the regularity of arrangement of the porphyrin in poly(Zn1) is higher compared with poly(1). Spatial arrangement of porphyrins was achieved by utilizing a one-handed helical poly(phenylacetylenes) as a template.

Proposed Mechanism for the High-Yield Polymerization of Oxyethyl Propiolates with Rh Complex Catalyst Using the Density Functional Theory Method

In this study, poly(oxyethyl propiolate)s (POP)s featuring various oxyethylene derivatives are synthesized using a [Rh(norbornadiene)Cl]₂ catalyst. In particular, POPs featuring the normal oxyethylene chain in the side-chain exhibit excellent yields and high molecular weights in methanol and N,N-dimethylformamide at 40 °C, compared with poly(n-alkyl propiolate)s (PnAP)s. The high reactivity of the oxyethyl propiolate (OP) monomers is clarified by considering the time dependences of the polymerization yields of OPs and alkyl propiolates (Aps). Furthermore, the monomer structure and intermediate conformation of the Rh complex are optimized using Density Function theory (DFT) methods (B3LYP/6-31G** and B3LYP/LANL2DZ) and a polymerization mechanism is proposed.

Polymerization of phenylacetylenes by binuclear rhodium catalysts with different para-binucleating phenoxyiminato linkages

A binuclear rhodium catalyst 3b with a 2,5-phenyloxydiiminato linkage and an nbd ligand exhibits cooperative effects in terms of enhanced catalytic activity in the polymerization of phenylacetylene and its functional derivatives.