Eiji Yashima

Helical Polymers: Synthesis, Structures, and Functions

2.2.1. Polyisocyanates 6117 2.2.2. Polysilanes 6118 2.2.3. Polyacetylenes 6120 2.3. Foldamer-Based Helical Polymers 6124 2.3.1. Click Polymerization 6126 2.3.2. Ring-Closing Reaction 6126 2.4. Other Types of Helical Polymers 6127 2.4.1. π-Conjugated Helical Polymers 6127 2.4.2. Metallosupramolecular Helical Polymers 6128 2.5. Induced Helical Polymers 6130 2.5.1. Induced Helical Polyacetylenes 6130 2.5.2. Other Induced Helical Polymers and Oligomers 6132

Polysaccharide Derivatives for Chromatographic Separation of Enantiomers

The first resolution of enantiomers was performed 150 years ago-mechanically. Today a powerful method for carrying out this task is HPLC on polysaccharide derivatives as chiral stationary phases. Most racemates, from an analytical to a preparative scale, now appear to be resolved by this technique. As an example, the chromatogram for the enantiomeric resolution of a fullerene derivative is shown on the right.

Memory of macromolecular helicity assisted by interaction with achiral small molecules

The helicity of biological macromolecules such as DNA and proteins is largely governed by the homochirality of their components (D-sugars and L-amino acids). In polymer and supramolecular chemistry, control of helicity is an attractive goal because of possible applications in materials science, chemical sensing and enantioselective catalysis. We reported recently that macromolecular helicity can be induced in a polymer by an optically active amine. Here we show that this helicity can be ‘memorized’ when the amine is replaced by various achiral amines. Although the maintenance of helicity in the polymer is not perfect, it can ‘repair’ itself over time. Small structural changes in the achiral amines influence the efficiency of helicity retention markedly.

Polysaccharide-based chiral stationary phases for high-performance liquid chromatographic enantioseparation.

Recent developments of polysaccharide-based chiral stationary phases (CSPs) for the direct separation of enantiomers in high-performance liquid chromatography (HPLC) are mainly reviewed together with the results on mechanistic studies by means of chromatography, NMR and mass spectroscopies, and computational methods. Miscellaneous applications of polysaccharide derivatives to the newly developed, chiral dynamic high-performance liquid chromatography (DHPLC) for obtaining a nonracemic compound are also described.

Single- and Double-Stranded Helical Polymers: Synthesis, Structures, and Functions

Biological macromolecules, such as DNA and proteins, possess a unique and specific ordered structure, such as a right-handed double helix or a single alpha-helix. Those structures direct the sophisticated functions of these molecules in living systems. Inspired by biological helices, chemists have worked to synthesize polymers with controlled helicity, not only to mimic the biological helices but also to realize their functions. Although numerous synthetic polymers that fold into a single-handed helix have been reported, double-stranded helical polymers are almost unavailable except for a few oligomers. In addition, the exact structures of most helical polymers remain obscure. Therefore, the development of a conceptually new method for constructing double-stranded helical polymers and a reliable method for unambiguously determining the helical structures are important and urgent challenges in this area. In this Account, we describe the recent advances in the synthesis, structures, and functions of single- and double-stranded helical polymers from our group and others and provide a brief historical overview of synthetic helical polymers. We found unique macromolecules that fold into a preferred-handed helix through noncovalent bonding interactions with specific chiral guests. During the noncovalent helicity induction process, these guest molecules significantly amplified chirality in a dynamic helical polymer. During the intensive exploration of the helicity induction mechanism, we observed an unusual macromolecular helical memory in dynamic helical polymers. Furthermore, we found that rigid-rod helical poly(phenylacetylene)s and poly(phenyl isocyanide)s showing a cholesteric or smectic liquid crystal self-assemble to form two-dimensional crystals with a controlled helical conformation on solid substrates upon exposure to solvent vapors. We visualized their helical structures including the helical pitch and handedness by atomic force microscopy (AFM). We propose a modular strategy to construct complementary double helices by employing chiral amidinium-carboxylate salt bridges with m-terphenyl backbones. The double-stranded helical structures were characterized by circular dichroism in solution and X-ray diffraction of the crystals or the direct AFM observations. Serendipitously, we found that oligoresorcinols self-assemble into well-defined double helices resulting from interstrand aromatic stacking in water. These oligoresorcinols bound cyclic and linear oligosaccharides in water to form rotaxanes and hetero-double helices, respectively. The examples presented in this Account demonstrate the notable progress in the synthesis and structural determination of helical polymers including single- and double-stranded helices. Not only do we better understand the principle underlying the generation of helical conformations, but we have also used the knowledge of these unique helical structures to develop novel helical polymers with specific functions.

Pulsating Tubules from Noncovalent Macrocycles

Slip-Sliding Apart One versatile means of synthesizing nanometer-scale cylinders has been to start with ring-shaped molecules that stack on top of each other. Huang et al. (p. 1521; see the Perspective by Zhang and Aida) took this approach a step further by giving the rings a flexible diameter. Specifically, rings were prepared consisting of six v-shaped building blocks with hydrophobic sides that could slide back and forth along one another and thereby expand or contract the pore at the center. The rings spontaneously stacked to form tubes in dilute aqueous solution, and heating induced contraction of the whole tube in a process that was readily reversible on cooling. Nanoscale tubes expand and contract with temperature changes through loose association of their constituent building blocks. Despite recent advances in synthetic nanometer-scale tubular assembly, conferral of dynamic response characteristics to the tubules remains a challenge. Here, we report on supramolecular nanotubules that undergo a reversible contraction-expansion motion accompanied by an inversion of helical chirality. Bent-shaped aromatic amphiphiles self-assemble into hexameric macrocycles in aqueous solution, forming chiral tubules by spontaneous one-dimensional stacking with a mutual rotation in the same direction. The adjacent aromatic segments within the hexameric macrocycles reversibly slide along one another in response to external triggers, resulting in pulsating motions of the tubules accompanied by a chiral inversion. The aromatic interior of the self-assembled tubules encapsulates hydrophobic guests such as carbon-60 (C60). Using a thermal trigger, we could regulate the C60-C60 interactions through the pulsating motion of the tubules.

Chloromethylphenylcarbamate derivatives of cellulose as chiral stationary phases for high-performance liquid chromatography

A new class of eight chloromethylphenylcarbamate derivatives of cellulose was prepared by introducing both an electron-donating methyl group and an electron-withdrawing chloro group on to the phenyl moieties and their chiral recognition abilities were evaluated as chiral stationary phases (CSPs) for high-performance liquid chromatography. The superiority of these derivatives over dichloro- and dimethylphenylcarbamates of cellulose as CSPs was demonstrated for some racemic compounds. The elution order and enantioselectivity were greatly dependent on the positions of the substituents. Meta- and para-disubstituted derivatives showed higher chiral recognition than ortho- and meta- or para- disubstituted derivatives. The correlation between the chemical shifts of the NH protons of the carbamate moieties and the enantiomer-resolving abilities of the derivatives is discussed. Some of the derivatives were effective CSPs in both normal- and reversed- phase conditions and could efficiently separate some chiral drug enantiomers.

Polymerization of phenylacetylene by rhodium complexes within a discrete space of apo-ferritin.

Polymerization reactions of phenylacetylene derivatives are promoted by rhodium complexes within the discrete space of apo-ferritin in aqueous media. The catalytic reaction provides polymers with restricted molecular weight and a narrow molecular weight distribution. These results suggest that protein nanocages have potential for use as various reaction spaces through immobilization of metal catalysts on the interior surfaces of the protein cages.

3,5-Dimethylphenylcarbamates of cellulose and amylose regioselectively bonded to silica gel as chiral stationary phases for high-performance liquid chr

3,5-Dimethylphenylcarbamates of amylose (ADMPC) (1) and cellulose (CDMPC) (2) chemically bonded to 3-aminopropylsilica gel were prepared with 4

Dimethyl-, dichloro- and chloromethylphenylcarbamates of amylose as chiral stationary phases for high-performance liquid chromatography

Abstract Twelve dimethyl-, dichloro- and chloromethylphenylcarbamate derivatives of amylose were prepared and their chiral recognition abilities were evaluated as chiral stationary phases for high-performance liquid chromatography. ortho-Substituted phenylcarbamate derivatives of amylose showed high chiral recognition abilities, although cellulose phenylcarbamate derivatives with ortho substituents on the phenyl moiety showed low chiral recognition. The superiority of 5-chloro-2-methylphenylcarbamate over the corresponding dimethyl- and dichlorophenylcarbamate derivatives of amylose was demonstrated. The roles of the NH residue of the carbamate moieties and methyl and chloro groups in chiral recognition were elucidated using IR and 1H NMR spectroscopic data. The tris(5-chloro-2-methylphenylcarbamate) of amylose exhibited the highest chiral recognition ability among the amylose derivatives synthesized, and can be used for the separation of some chiral drug enantiomers.