Facile and efficient formation and dissociation of a pseudo[2]rotaxane by a slippage approach using pillar[5]arene-based cyclic host liquid and solvent.

A pillar[5]arene-based pseudo[2]rotaxane was synthesized using a slippage method by only heating an axle with bulky valine derivative ends in a cyclic host liquid (CHL), which is liquid at room temperature. The pseudo[2]rotaxane synthesized in a CHL can be converted back to the starting components, i.e., the pillar[5]arene wheel and the axle, by heating in a solvent.

The template effect of solvents on high yield synthesis, co-cyclization of pillar[6]arenes and interconversion between pillar[5]- and pillar[6]arenes.

We have synthesized a pillar[6]arene in high yield and a co-pillar[6]arene using chlorocyclohexane as a solvent.

Achiral guest-induced chiroptical changes of a planar-chiral pillar[5]arene containing one π-conjugated unit.

A planar-chiral pillar[5]arene derivative containing one π-conjugated unit was prepared. Chiroptical changes were observed upon addition of the achiral guest 1,4-dicyanobutane.

Förster resonance energy transfer by formation of a mechanically interlocked [2]rotaxane.

A [2]rotaxane has been constructed from a di-pyrene appended pillar[5]arene wheel, a pyridinium axle, and a perylene stopper. It shows efficient Förster resonance energy transfer from pyrene to perylene by formation of a mechanically interlocked [2]rotaxane.

Cyclic host liquids for facile and high-yield synthesis of [2]rotaxanes.

We developed "cyclic host liquids (CHLs)" as a new type of solvent. The CHLs are a nonvolatile liquid over a wide temperature range, are biocompatible and recyclable, have high thermal stability, and are miscible with many organic solvents. Compared with typical complexation systems, the CHL system is extremely efficient for maintaining host-guest complexation because an additional solvent is not required. Based on the efficient host-guest complexation in the CHL system, we demonstrated synthesis of [2]rotaxanes in pillar[5]arene-based CHL. High yields were obtained for [2]rotaxanes capped by cationization (yield 91%) and Huisgen reaction (yield 88%) between the axle and the stopper components in the CHL system, while the association constants between the axles and wheels were quite low (10-15 M(-1)) in CDCl(3). The CHL system provides a new powerful approach for synthesis of mechanically interlocked molecules (MIMs) even with unfavorable statistical combinations of host-guest complexes.

Clickable di- and tetrafunctionalized pillar[n]arenes (n = 5, 6) by oxidation-reduction of pillar[n]arene units.

We report a new route for the selective synthesis of di- and tetrafunctionalized pillararenes via oxidation and reduction of the pillararene units. Hypervalent-iodine oxidation of perethylated pillar[5]arene afforded pillar[5]arene derivatives containing one benzoquinone unit and two benzoquinones at the A,B- and A,C-units. A pillar[6]arene derivative containing one benzoquinone unit was also synthesized. Reduction of the benzoquinone units yielded position-selective di- and tetrahydroxylated pillararene derivatives. This methodology avoids the generation of many constitutional isomers and overcomes the isolation problem of numerous constitutional isomers. From these hydroxylated pillararenes, Huisgen reaction-based clickable di- and tetraalkynylated pillar[5]arenes were prepared. Because of the highly selective and reactive nature of Huisgen alkyne-azide cycloaddition, these pillar[5]arenes can serve as key compounds for a large library of di- and tetrafunctionalized pillararenes. Based on these di- and tetrafunctionalized pillar[5]arenes as key compounds, fluorescent sensors were created by the modification of di- and tetrapyrene moieties via Huisgen-type click reactions.

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.

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.

Photoreversible transformation between seconds and hours time-scales: threading of pillar[5]arene onto the azobenzene-end of a viologen derivative.

Photoswitching of the transformation between seconds and hours time-scales is demonstrated using the threading of per-hydroxylated pillar[5]arene onto the azobenzene-end of a viologen derivative. When the azobenzene moiety was in the trans form, the threading of per-hydroxylated pillar[5]arene quickly took place at 25 °C and could not be monitored directly. The exchange rate (k) and half-life time (t(1/2)) examined by 2D EXSY NMR spectroscopy were found to be 0.209 ± 0.013 s(-1) and 3.33 ± 0.21 s, respectively. In contrast, the cis form of the azobenzene moiety required very long time (k = 2.14 ± 0.27 × 10(-5) s(-1), t(1/2) = 9.13 ± 1.2 h) to thread per-hydroxylated pillar[5]arene at 25 °C. Photoisomerization from the trans to the cis form generated the following increment of free energy of activation at 25 °C: ΔG(in)(‡) (cis form) - ΔG(in)(‡) (trans form) = 22.8 ± 0.24 kJ mol(-1), which led to the time-scale transformation. The tuning of the threading was also accomplished by heating/cooling: the rate constants increased on heating and decreased on cooling.