Using Photoresponsive End-Closing and End-Opening Reactions for the Synthesis and Disassembly of [2]Rotaxanes: Implications for Dynamic Covalent Chemistry

• Published in: Journal of organic chemistry 2009-03, Vol.74 (6), p.2374-2379
• Main Authors: Tokunaga, Yuji, Akasaka, Koichiro, Hashimoto, Nobuharu, Yamanaka, Shou, Hisada, Kenji, Shimomura, Youji, Kakuchi, Suzuka
• Format: Article
• Language: English
• Subjects: Chemistry; Exact sciences and technology; Heterocyclic compounds; Heterocyclic compounds with o, s, se, te hetero atom and condensed derivatives; Hydrogen Bonding; Isomerism; Noncondensed benzenic compounds; Organic chemistry; Organic Chemistry Phenomena; Photochemical Processes; Preparations and properties; Rotaxanes - chemical synthesis; Rotaxanes - chemistry; Solvents; Stilbenes - chemistry
• ISSN: 0022-3263; 1520-6904
• DOI: 10.1021/jo8025143

We have synthesized two [2]rotaxanes, each possessing a (Z)-α-methylstilbene unit as one of its stoppers, in good yield through the photoisomerization of terminal (E)-α-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)-α-methylstilbene unit is not sufficiently bulky to prevent dissociation of the DB24C8 unit, whereas a (Z)-α-methylstilbene unit acts as a true stopper. We also synthesized these [2]rotaxanes from the (Z)-α-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 α-methylstilbene termini) in a polar solvent. These rotaxanes are stable at room temperature, but dissociate slowly to their two components at elevated temperatures.