Masamichi Yamanaka

Encapsulated-guest rotation in a self-assembled heterocapsule directed toward a supramolecular gyroscope

The self-assembled heterocapsule 1·2, which is formed by the hydrogen bonds of tetra(4-pyridyl)-cavitand 1 and tetrakis(4-hydroxyphenyl)-cavitand 2, encapsulates 1 molecule of guests such as 1,4-diacetoxybenzene 3a, 1,4-diacetoxy-2,5-dimethylbenzene 3b, 1,4-diacetoxy-2,5-dialkoxybenzenes (3c, OCH3; 3d, OC2H5; 3e, OC3H7; 3f, OC4H9; 3g, OC5H11; 3h, OC6H13; 3i, OC8H17), 1,4-diacetoxy-2,5-difluorobenzene 4a, and 1,4-diacetoxy-2,3-difluorobenzene 4b. The X-ray crystallographic analysis of 3c@(1·2) showed that the acetoxy groups at the 1,4-positions of 3c are oriented toward the 2 aromatic cavity ends of 1·2 and that 3c can rotate along the long axis of 1·2. Thus, the 1·2 (stator) with the encapsulation guest (rotator) behaves as a supramolecular gyroscope. A variable temperature (VT) 1H NMR study in CDCl3 showed that 3a, 3b, 4a, and 4b within 1·2 rotate rapidly even at 218 K, whereas guest rotation is almost inhibited for 3h and 3i even at 323 K. In this respect, 4b with a large dipole moment is a good candidate for the rotator of 1·2. For 3c–3g, the enthalpic (ΔH‡) and entropic (ΔS‡) contributions to the free energy of activation (ΔG‡) for the guest-rotational steric barriers within 1·2 were obtained from Eyring plots based on line-shape analysis of the VT 1H NMR spectra. The value of ΔG‡ increased in the order 3c < 3d < 3e < 3f < 3g. Thus, the elongation of the alkoxy chains at the 2,5-positions of 3 puts the brakes on guest rotation within 1·2.

Urea derivatives as low-molecular-weight gelators

This paper reports an overview of low-molecular-weight gelators (LMWGs) that have a ureide moiety as a hydrogen-bonding site. Various mono-, bis-, tris-, and tetrakis-urea compounds can form supramolecular gels with organic solvents. The author developed a C3-symmetrical tris-urea molecule that can form a ubiquitous framework of LMWGs. The supramolecular organogel of the tris-urea molecule exhibited a chemical-stimuli-responsive reversible gel–sol phase transition. Supramolecular hydrogels are constructed from self-assemblies of amphiphilic urea derivatives. Sugar-connected amphiphilic tris-urea was found to form a gel with water, and the hydrogels showed chemical-stimuli-responsive gel–sol phase transitions. The potential of supramolecular hydrogels as matrices of electrophoresis has been demonstrated through the supramolecular gel electrophoresis (SUGE) of protein samples using our developed amphiphilic tris-urea LMWG.

Enantioselective Diels-Alder reactions catalyzed by chiral 1,1′-(2,2′-bisacylamino)binaphthalene-ytterbium complex

Abstract A new axially chiral ligand, 1,1′-(2,2′-bisacylamino)binaphthalene, was found to be effective in the ytterbium-catalyzed asymmetric Diels-Alder reaction between cyclopentadiene and crotonyl-1,3-oxazolidin-2-one. The Diels-Alder reaction using 15 mol% of 1,1′-(2,2′-bisbenzoylamino)binaphthalene·Yb(OTf)3·2i-Pr2NEt afforded the adduct with high enantioselectivity (97% yield, endo exo = 91 9 , >98% ee for the endo adduct).

Structural alteration of hybrid supramolecular capsule induced by guest encapsulation.

Heterofunctionalized C(2v) symmetrical cavitand 1 with 4-pyridylethynyl and 3-carbamoylphenyl groups in alternating arrangement was designed and synthesized. A 1:1 mixture of the cavitand 1 and a cis-coordinated palladium(II) or platinum(II) complex self-assembled into a hybrid supramolecular capsule via both metal-ligand coordination bonds and hydrogen bonds. Formation of the capsular assembly was confirmed by NMR spectroscopy and mass spectrometry. The hybrid capsule encapsulated the appropriate guest, the molecular sizes of which fit the size of the capsular cavity. Structural alteration of the hybrid capsule was induced by the guest encapsulation. A C(2h) structure for the encapsulation complex was assigned by 2D NMR spectra analysis. Thermodynamic and kinetic properties of the guest encapsulation were investigated. The kinetics of in/out guest exchange was strongly influenced by hydrogen bonding in the hybrid capsule.

Chloroalkane gel formations by tris-urea low molecular weight gelator under various conditions.

C3-symmetrical tris-urea low molecular weight gelator 1 was synthesized in three steps from phloroglucin. The gelation ability of 1 in four chloroalkanes, i.e., CH2Cl2, CHCl3, 1,1,2-trichloroethane, and 1,1,2,2-tetrachloroethane, was investigated under various conditions. Thermal treatment was effective in gelating 1,1,2-trichloroethane. In the presence of equimolar 1 and CuBr2, 1,1,2-trichloroethane and 1,1,2,2-tetrachloroethane formed gels. Mixtures of 1 and CHCl3 or 1,1,2,2-tetrachloroethane gave gels after ultrasound irradiation. CH2Cl2 changed into a gel in the presence of equimolar 1 and BiCl3 after sonication. Spherical particles with rough surfaces were found by SEM observation of CHCl3 gel prepared from ultrasound irradiation of 1 and Y(NO3)3.

An intriguing effect of Yb(OTf)3-TMSCl in the halogenation of 1,1-disubstituted alkenes by NXS: selective synthesis of allyl halides

A novel protocol to effect the efficient selective halogenation of 1,1-disubstituted alkenes with NXS catalyzed by Yb(OTf)3–TMSCl, which affords the corresponding allyl halides in high yield, including allyl bromide, chloride, iodide and fluoride, is described. A remarkable feature of Yb(OTf)3–TMSCl-catalyzed halogenation is that, unlike conventional radical halogenation with N-halosuccinimides, the reaction discriminates between the allylic and benzylic positions. The reaction occurs selectively at the allylic position to give allylic halides, but not at the benzylic position.

Supramolecular gel electrophoresis of acidic native proteins.

Amphiphilic tris-urea molecules self-assemble into a supramolecular hydrogel in tris(hydroxymethyl)aminomethane-glycine buffer. The supramolecular hydrogel is used as a matrix for the electrophoresis of acidic native proteins, in which proteins are separated based on their isoelectric points rather than their molecular weights. The proteins remain in their native forms during migration, and their activities are retained after electrophoresis. Glucoside substituents on the amphiphilic tris-urea molecule allow for the affinity electrophoresis of a carbohydrate-binding protein to be performed. The proteins can be efficiently recovered from the supramolecular hydrogel using a simple procedure. This is a major advantage of using this noncovalent, self-assembled material.

Self‐Assembled Boronic Ester Cavitand Capsules with Various Bis(catechol) Linkers: Cavity‐Expanded and Chiral Capsules

Two molecules of cavitand tetraboronic acid and four molecules of various bis(catechol) linkers self-assemble into capsules through the formation of eight dynamic boronic ester bonds. Each capsule has a different cavity size depending on the linker used, and shows particular guest encapsulation selectivity. A chiral capsule made up of the cavitand and a chiral bis(catechol) linker was also constructed. This capsule induces supramolecular chirality with respect to a prochiral biphenyl guest by diastereomeric encapsulation through the asymmetric suppression of rotation around the axis of the prochiral biphenyl moiety.

Development of C3-Symmetric Tris-Urea Low-Molecular-Weight Gelators

This article describes recent developments in C3 -symmetric tris-urea low-molecular-weight gelators and their applications. The C3 -symmetric tris-ureas are excellent frameworks to form supramolecular polymers through noncovalent interactions. In organic solvents, hydrophobic tris-ureas form supramolecular gels. Amphiphilic tris-ureas form supramolecular gels in aqueous media. Functional supramolecular gels were prepared by introducing appropriate functional groups into the outer sphere of tris-ureas. Supramolecular hydrogels obtained from amphiphilic tris-ureas were used in the electrophoresis of proteins. These electrophoreses results showed several unique characteristics compared to typical electrophoreses results obtained using polyacrylamide matrices.

Encapsulation-induced remarkable stability of a hydrogen-bonded heterocapsule.

Remarkably enhanced stability of the self-assembled hydrogen-bonded heterocapsule 1⋅2 by the encapsulation of 1,4-bis(1-propynyl)benzene 3 a was found with Ka =1.14×10(9)  M(-1) in CDCl3 and Ka2 =1.59×10(8)  M(-2) in CD3 OD/CDCl3 (10 % v/v) at 298 K. The formation of 3 a@(1⋅2) was enthalpically driven (ΔH°<0 and ΔS°<0) and there was a unique inflection point in the correlation between ΔH° versus ΔS° as a function of polar solvent content. The ab initio calculations revealed that favorable guest-capsule dispersion and electrostatic interactions between the acetylenic parts (triple bonds) of 3 a and the aromatic inner space of 1⋅2, as well as less structural deformation of 1⋅2 upon encapsulation of 3 a, play important roles in the remarkable stability of 3 a@(1⋅2).