Electron-Induced Switching of the Supramolecular Chirality of Optically Active Polythiophene Aggregates

A chiral regioregular polythiophene (PT), poly[3-{4-((R)-4-ethyl-2-oxazolin-2-yl)phenyl}thiophene] (poly-1), forms chiral aggregates which exhibit a unique induced circular dichroism (ICD) in the ππ* transition region derived from the supramolecular chirality in the presence of various poor solvents or metal salts in chloroform. We report here that the chirality of such supramolecular aggregates can be switched (“on” and “off”) through electron transfer. We have found that upon the addition of copper(II) trifluoromethanesulfonate [Cu(OTf)2] to the chiral aggregates of poly-1 in a chloroform−acetonitrile mixture, the ICD disappears because of the oxidative doping of the poly-1 main chain, while a further addition of amines such as triethylenetetramine (TETA) induces undoping of the poly-1 which results in the reappearance of the ICDs. Therefore, the supramolecular chirality of the poly-1 assemblies was reversibly controlled by the addition or removal of an electron from the poly-1 main chain. This may be the first example of a reversible supramolecular chirality switch on chiral PT aggregates. We investigated the mechanism of the chirality switch through the doping and undoping process on the polymer main chain by means of absorption and CD spectroscopies, ESR, cyclic voltammetry, X-ray diffraction, and AFM measurements

Double Helical Oligoresorcinols Specifically Recognize Oligosaccharides via Heteroduplex Formation through Noncovalent Interactions in Water

We report on the oligosaccharide recognition through noncovalent interactions in water based on a unique supramolecular homoduplex-to-heteroduplex transformation of the oligoresorcinol nonamer as a fully artificial receptor. The oligoresorcinol forms a double helix in water, which unravels and entwines upon complexation with specific oligosaccharides with a particular chain length and glucosidic linkage pattern, thus generating the heteroduplex with an excess one-handed helical conformation that can be readily monitored and further quantified by absorption, circular dichroism, and NMR spectroscopies

Stereospecific Polymerization of Propiolic Acid with Rhodium Complexes in the Presence of Bases and Helix Induction on the Polymer in Water

Propiolic acid was directly polymerized in water with water-soluble rhodium complexes in the presence of bases such as NaOH to yield the cis−transoidal poly(propiolic acid) sodium salt (poly-1−Na) in high yields. Moreover, poly-1−Na exhibited an induced circular dichroism (ICD) in the UV−visible region in water due to the formation of a predominantly one-handed helical structure responding to the chirality of the optically active amino alcohols. The stereoregularity of the polymer decreased when the poly-1−Na was converted into the acid form, poly(propiolic acid). However, poly(propiolic acid) also showed ICDs in the UV−visible region in water in the presence of optically active amino alcohols

Dual Catalyst System for Asymmetric Alternating Copolymerization of Carbon Dioxide and Cyclohexene Oxide with Chiral Aluminum Complexes: Lewis Base as Catalyst Activator and Lewis Acid as Monomer Activator

Optically active aluminum complexes such as Schiff base, binuclear β-ketoiminate, and bisprolinol complexes were found to promote asymmetric alternating copolymerizations of carbon dioxide and cyclohexene oxide. The aluminum Schiff base complexes–tetraethylammonium acetate afforded isotactic poly­(cyclohexene carbonate)­s with low enantioselectivities. Lewis bases having two coordinating sites were utilized to enhance activity and selectivity based on the binuclear structure of the aluminum β-ketoiminate clarified by X-ray crystallography. [<b>2g</b>AlMe]₂–bulky bisimidazole produced the alternating copolymer with high enantioselectivity (62% ee). The polymerization is considered to preferentially proceed at more crowded, enantioselective site owing to coordination of bulky Lewis bases to aluminums in less enantioselective sites. <b>3</b>₂AlMe–2-picoline also exhibited a high enantioselectivity (67% ee). Methylaluminum bis­(2,6-di-<i>tert</i>-butyl-4-methylphenoxide) was applied to perform faster and more enantioselective copolymerizations at low temperature (82% ee). The asymmetric copolymerizations were found to be significantly dependent on size of epoxide, temperature, and kind/amount of activators

Metal-Induced Chirality Induction and Chiral Recognition of Optically Active, Regioregular Polythiophenes

Metal-Induced Chirality Induction and Chiral Recognition of Optically Active, Regioregular Polythiophene

Dual Catalyst System for Asymmetric Alternating Copolymerization of Carbon Dioxide and Cyclohexene Oxide with Chiral Aluminum Complexes: Lewis Base as Catalyst Activator and Lewis Acid as Monomer Activator

Optically active aluminum complexes such as Schiff base, binuclear β-ketoiminate, and bisprolinol complexes were found to promote asymmetric alternating copolymerizations of carbon dioxide and cyclohexene oxide. The aluminum Schiff base complexes–tetraethylammonium acetate afforded isotactic poly­(cyclohexene carbonate)­s with low enantioselectivities. Lewis bases having two coordinating sites were utilized to enhance activity and selectivity based on the binuclear structure of the aluminum β-ketoiminate clarified by X-ray crystallography. [2gAlMe]2–bulky bisimidazole produced the alternating copolymer with high enantioselectivity (62% ee). The polymerization is considered to preferentially proceed at more crowded, enantioselective site owing to coordination of bulky Lewis bases to aluminums in less enantioselective sites. 32AlMe–2-picoline also exhibited a high enantioselectivity (67% ee). Methylaluminum bis­(2,6-di-tert-butyl-4-methylphenoxide) was applied to perform faster and more enantioselective copolymerizations at low temperature (82% ee). The asymmetric copolymerizations were found to be significantly dependent on size of epoxide, temperature, and kind/amount of activators

Supramolecular Control of Unwinding and Rewinding of a Double Helix of Oligoresorcinol Using Cyclodextrin/Adamantane System

The double helix of the oligoresorcinol nonamer formed in water was unwound by β-cyclodextrin (β-CD), and the resulting single strands of the nonamer threaded the β-CD to form a twisted [3]pseudorotaxane with a controlled helicity. Upon the addition of an adamantane, the single strand of the oligoresorcinol nonamer was expelled out of the β-CD wheels, thus regenerating the double helix. This supramolecularly controlled, reversible unwinding and rewinding of the double helix is unique and can be readily monitored by spectroscopic techniques

Chiral Stimuli-Responsive Gels: Helicity Induction in Poly(phenylacetylene) Gels Bearing a Carboxyl Group with Chiral Amines

Poly(phenylacetylene) gels (gel-1-H and gel-2-H) bearing a carboxy pendant were synthesized either by the copolymerization of (4-carboxyphenyl)acetylene (gel-1-H) with a bis(phenylacetylene) derivative as the cross-linking reagent using a rhodium complex ([Rh(cod)2]BF4:  cod = 1,5-cyclooctadiene) as the catalyst or by the cross-linking of poly[(4-carboxyphenyl)acetylene] with diamines (gel-2-H). The obtained gels were found to swell in DMSO and exhibited an induced circular dichroism (ICD) in the long absorption region of the main chain in the presence of optically active amines. These results indicate that a predominantly one-handed helix can be induced in the polymer network of the gels through chiral acid−base interactions. The swelling properties and the Cotton effect intensities of the gels depend on the cross-linking ratio and the chiral amines. Gel-1-Na and gel-2-Na prepared from gel-1-H and gel-2-H, respectively, also significantly swelled in water and showed ICDs characteristic of chiral amino alcohols and free amino acids in water

Chiral Stimuli-Responsive Gels: Helicity Induction in Poly(phenylacetylene) Gels Bearing a Carboxyl Group with Chiral Amines

Poly(phenylacetylene) gels (gel-1-H and gel-2-H) bearing a carboxy pendant were synthesized either by the copolymerization of (4-carboxyphenyl)acetylene (gel-1-H) with a bis(phenylacetylene) derivative as the cross-linking reagent using a rhodium complex ([Rh(cod)2]BF4:  cod = 1,5-cyclooctadiene) as the catalyst or by the cross-linking of poly[(4-carboxyphenyl)acetylene] with diamines (gel-2-H). The obtained gels were found to swell in DMSO and exhibited an induced circular dichroism (ICD) in the long absorption region of the main chain in the presence of optically active amines. These results indicate that a predominantly one-handed helix can be induced in the polymer network of the gels through chiral acid−base interactions. The swelling properties and the Cotton effect intensities of the gels depend on the cross-linking ratio and the chiral amines. Gel-1-Na and gel-2-Na prepared from gel-1-H and gel-2-H, respectively, also significantly swelled in water and showed ICDs characteristic of chiral amino alcohols and free amino acids in water

Synthesis and Chiroptical Properties of Optically Active, Regioregular Oligothiophenes

A series of regioregular oligothiophenes bearing chiral oxazoline residues have been synthesized in high yields by a stepwise synthesis using the Stille cross-coupling reaction. The chiroptical properties of the oligothiophenes up to the octamer were investigated with UV−visible and CD spectroscopies. The octamer showed split-type Cotton effects in chloroform as a good solvent in the presence of a poor solvent such as acetonitrile, while the lower molecular weight oligomers exhibited almost no induced CD