Tada-aki Yamagishi

para-Bridged Symmetrical Pillar[5]arenes: Their Lewis Acid Catalyzed Synthesis and Host–Guest Property

Condensation of 1,4-dimethoxybenzene (DMB) with paraformaldehyde in the presence of BF3.O(C2H5)2 gave novel para-bridged pentacyclic pillar DMB (DMpillar[5]arene). Moreover, para-bridged pentacyclic hydroquinone (pillar[5]arene) was prepared. Pillar[5]arene formed 1:1 host-guest complexes with dialkyl viologen and alkyl pyridinium derivatives. However, pillar[5]arene did not form complexes with the diadamantyl viologen derivative since a bulky adamantyl group was unable to thread the cavity of pillar[5]arene.

Photoreversible switching of the lower critical solution temperature in a photoresponsive host-guest system of pillar[6]arene with triethylene oxide substituents and an azobenzene derivative.

A new water-soluble thermoresponsive pillar[6]arene with triethylene oxide groups was synthesized. The pillar[6]arene showed lower critical solution temperature behavior in aqueous solution. Its clouding point was photoreversibly switched based on a photoresponsive host-guest system. The trans form of an azobenzene guest formed a stable 1:1 complex with the pillar[6]arene. Complexation increased the clouding point. Irradiation with UV light induced a conformation change for the azobenzene guest from the trans to cis form, and dethreading occurred because of a size mismatch between the cis form and the pillar[6]arene cavity. This dethreading decreased the clouding point. The photoresponsive host-guest system was reversible, and the clouding point could be switched by alternating irradiation with UV or visible light. We demonstrated photoresponsive reversible clear-to-turbid and turbid-to-clear transitions for the solution based on the reversible switching of the clouding point using the photosensitive host-guest system.

Supramolecular polymers with alternating pillar[5]arene and pillar[6]arene units from a highly selective multiple host–guest complexation system and monofunctionalized pillar[6]arene

A highly selective multiple host–guest complexation system based on pillar[5]arene and pillar[6]arene is reported. A pyridinium cation moiety is included in the pillar[5]arene cavity, but hardly forms a stable host–guest complex with pillar[6]arene. A 1,4-diazabicyclo[2.2.2]octane cation moiety forms a host–guest complex with pillar[6]arene, but not with pillar[5]arene. A synthetic route to mono-reactive pillar[6]arene is also developed, enabling the synthesis of pillar[6]arene with a pyridinium cation group at a single position. Based on the highly selective multiple host–guest complexation system and mono-reactive pillar[6]arene, a supramolecular polymer with alternating pillar[5]arene and pillar[6]arene units is constructed.

Reversibly tunable lower critical solution temperature utilizing host-guest complexation of pillar[5]arene with triethylene oxide substituents.

A thermoresponsive macromolecule consisting of 10 outer triethylene oxide groups and a pillar[5]arene core was prepared. The macromolecule showed lower critical solution temperature behavior. Moreover, its clouding point can be reversibly tuned based on the addition of guest and host compounds; the clouding point increased upon addition of a guest didecylviologen salt and decreased when the competitive host cucurbit[7]uril was added.

Facile, rapid, and high-yield synthesis of pillar[5]arene from commercially available reagents and its X-ray crystal structure.

We monitored the progress of formation of dimethoxypillar[5]arene by size-exclusion chromatography. Surprisingly, the cyclization reaction completely finished in just 3 min. By improving the reaction conditions and purification process, we successfully obtained dimethoxypillar[5]arene in a short time and in high yield (71%) from commercially available reagents. By improving the deprotection reaction of the methoxy moieties, pillar[5]arene was isolated quantitatively. Single crystal X-ray analysis confirmed the structure of pillar[5]arene in the solid state.

Synthesis and Conformational Characteristics of Alkyl-Substituted Pillar[5]arenes

A series of pillar[5]arene derivatives with alkyl groups of different length were synthesized. The new alkyl-substituted pillar[5]arene derivatives 1,4-bis(ethoxy)pillar[5]arene (C2), 1,4-bis(propoxy)pillar[5]arene (C3), 1,4-bis(butoxy)pillar[5]arene (C4), 1,4-bis(pentyloxy)pillar[5]arene (C5), 1,4-bis(hexyloxy)pillar[5]arene (C6), and 1,4-bis(dodecanoxy)pillar[5]arene (C12) were obtained by Lewis acid-catalyzed condensation of dialkoxybenzene monomers with paraformaldehyde. The conformational characteristics of the pillar[5]arene derivatives were investigated by dynamic (1)H NMR measurements. When the alkyl substituents were bulkier than methyl groups, the rotation of phenolic units in the pillar[5]arenes was suppressed and their conformation was immobilized. As their length increased, the alkyl substituents packed at the upper and lower rims and thus lowered the conformational freedom of the pillar[5]arenes.

Planar-Chiral Macrocyclic Host Pillar[5]arene: No Rotation of Units and Isolation of Enantiomers by Introducing Bulky Substituents

Enantiomers of bulky percyclohexylmethyl-substituted pillar[5]arene (Cy-C1-Pillar) were able to be separated by chiral column chromatography, and the separated enantiomers did not racemize. Even though modified with the bulky cyclohexylmethyl-substituents at both rims, Cy-C1-Pillar was able to capture a guest molecule.

High‐Yield Diastereoselective Synthesis of Planar Chiral [2]‐ and [3]Rotaxanes Constructed from per‐Ethylated Pillar[5]arene and Pyridinium Derivatives

Planar chiral [2]- and [3]rotaxanes constructed from pillar[5]arenes as wheels and pyridinium derivatives as axles were obtained in high yield using click reactions. The process of rotaxane formation was diastereoselective; the obtained [2]rotaxane was a racemic mixture consisting of (pS, pS, pS, pS, pS) and (pR, pR, pR, pR, pR) forms of the per-ethylated pillar[5]arene (C2) wheel, and other possible types of the [2]rotaxane did not form. Isolation of the enantiopure [2]rotaxanes with one axle through (pS, pS, pS, pS, pS)-C2 or (pR, pR, pR, pR, pR)-C2 wheels was accomplished. Furthermore, pillar[5]arene-based [3]rotaxane was successfully synthesized by attachment of two pseudo [2]rotaxanes onto a bifunctional linker. [3]Rotaxane formed in a 1:2:1 mixture with one axle threaded through two (pS, pS, pS, pS, pS)-C2, one (pS, pS, pS, pS, pS)-C2 and one (pR, pR, pR, pR, pR)-C2 (meso form), or two (pR, pR, pR, pR, pR)-C2 wheels. The [3]rotaxane enantiomers and the meso form were successfully isolated using appropriate chiral HPLC column chromatography. The procedure developed in this study is the starting point for the creation of pillar[5]arene-based interlocked molecules.

Organic hybrid of chlorinated polyethylene and hindered phenol. I. Dynamic mechanical properties

Dynamic mechanical properties and microstructure of an organic hybrid consisting of chlorinated polyethylene (CPE) and 3,9-bis[1,1-dimethyl-2{β-(3-tert-butyl4-hydroxy-5-methylphenyl)propionyloxy}ethyl]-2,4,8,10-tetraoxaspiro[5,5]-undecane (AO-80) were investigated. The AO-80 clearly exhibited two second-order transitions at 6 and 69 °C in addition to the melting: the transition at lower temperature is assigned to the glass transition, and the transition at higher temperature is considered to be caused by the dissociation of hydrogen bond between the hydroxyl groups of AO-80. When blending with CPE, part of AO-80 molecules was dispersed into the CPE matrix, and most of them formed an AO-80-rich phase. As a result, a novel transition appeared above the glass-transition temperature of the CPE matrix. It was assigned to the dissociation of the intermolecular hydrogen bond between the α-hydrogen of CPE and the hydroxyl groups of AO-80 within the AO-80-rich phase. Dynamic mechanical properties and microstructure of CPE/AO-80 hybrid were controlled by the thermal treatment. It was found that the CPE/AO-80 hybrid is a good damping material and shows a shape memory effect.

Thermally responsive shuttling behavior of a pillar[6]arene-based [2]rotaxane.

A [2]rotaxane constructed from a per-ethylated pillar[6]arene as a wheel and a pyridinium derivative as an axle was prepared. The wheel segment of the per-ethylated pillar[6]arene moved from one station to another along the axle as a result of thermal stimuli.