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.
Tokunaga
Yuji
Y
Department of Materials Science and Engineering, Faculty of Engineering, University of Fukui, Fukui, 910-8507, Japan. tokunaga@matse.u-fukui.ac.jp
Akasaka
Koichiro
K
Hashimoto
Nobuharu
N
Yamanaka
Shou
S
Hisada
Kenji
K
Shimomura
Youji
Y
Kakuchi
Suzuka
S
D006860:Hydrogen Bonding; D007536:Isomerism; D055601:Organic Chemistry Phenomena; D055668:Photochemical Processes; D043862:Rotaxanes / Q000138:chemical synthesis* / Q000737:chemistry; D012997:Solvents; D013267:Stilbenes / Q000737:chemistry
D043862:Rotaxanes; D012997:Solvents; D013267:Stilbenes