Youji Shimomura

Using photoresponsive end-closing and end-opening reactions for the synthesis and disassembly of [2]rotaxanes: implications for dynamic covalent chemistry.

We have synthesized two [2]rotaxanes, each possessing a (Z)-alpha-methylstilbene unit as one of its stoppers, in good yield through the photoisomerization of terminal (E)-alpha-methylstilbene units of dialkylammonium salts in the presence of the crown ether dibenzo[24]crown-8 (DB24C8). The synthesis relies on the formation of pseudorotaxane intermediates through hydrogen bond-guided self-assembly and subsequent end-closing photoisomerization. An (E)-alpha-methylstilbene unit is not sufficiently bulky to prevent dissociation of the DB24C8 unit, whereas a (Z)-alpha-methylstilbene unit acts as a true stopper. We also synthesized these [2]rotaxanes from the (Z)-alpha-methylstilbene-terminated axle-like salts though thermodynamic covalent chemistry by taking advantage of the reversibility of the photoisomerization. To dissociate the components of the [2]rotaxanes, we performed the reverse end-opening process under UV irradiation (i.e., Z-to-E isomerization of the alpha-methylstilbene termini) in a polar solvent. These rotaxanes are stable at room temperature, but dissociate slowly to their two components at elevated temperatures.

Effect of Pressure on [2]Pseudorotaxane Formation and Decomplexation and Their Corresponding Activation Volumes

In this study, we investigated the effect of pressure on the formation and decomplexation of [2]pseudorotaxanes. High pressure accelerated the formation of [2]pseudorotaxanes in an aprotic nonpolar solvent (CDCl(3)/CD(3)CN) via the slipping approach when using two crown ether/secondary ammonium salt systems: dibenzo[24]crown-8/bis(cyclohexylmethyl)ammonium salt (1a/2a) and tetrabenzo[24]crown-8/dibenzylammonium salt (1b/2b). The influence of pressure on the rate constants for the formation of the [2]pseudorotaxanes 3a and 3b revealed activation volumes (DeltaV(double dagger)) of -2.5 and -4.6 cm(3) mol(-1), respectively, at 303 K and zero pressure. We also investigated the effect of pressure on the decomplexation of the [2]pseudorotaxanes 3a and 3b in a polar solvent (DMSO-d(6)/CDCl(3)), obtaining activation volumes of -0.9 and -0.4 cm(3) mol(-1), respectively, at 303 K and zero pressure. Moreover, we calculated the activation parameters for the decomplexation processes on the basis of transition state theory at each pressure.

Selective recognition of rotaxanes for an alkali metal ion

Rotaxanes possessing a crown ether as a wheel group and oligo ethylene glycol units in the dumbbell group were synthesized using hydrogen bonding guided self-assembly and diphenyldiazomethane ester forming end-capping of pseudorotaxanes. The selective recognition of the lariat-crown-like rotaxanes for lithium ion was discovered. That recognition depended on the number of ethylene glycol units.

A rotaxane synthesis based on stilbene photoisomerization. A photoswitchable catch and release process.

A [2]rotaxane, having (Z)-alpha-methylstilbene as a stopper, is (1) synthesized in good yield by using a (E)- to (Z)-stilbene photoisomerization process, and (2) dissociated by reverse photoisomerization from (Z)- to (E)-stilbene.