Entropy- and Hydrolytic-Driven Positional Switching of Macrocycle between Imine- and Hydrogen-Bonding Stations in Rotaxane-Based Molecular Shuttles

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• 作者：Takeshi Umehara ; Hidetoshi Kawai ; Kenshu Fujiwara ; Takanori Suzuki
• 刊名：Journal of the American Chemical Society
• 出版年：2008
• 出版时间：October 22, 2008
• 年：2008
• 卷：130
• 期：42
• 页码：13981-13988
• 全文大小：644K
• 年卷期：v.130,no.42(October 22, 2008)
• ISSN：1520-5126

文摘

The construction and switching properties of a novel class of molecular shuttles 1 with imine-bonding stations for macrocyclic diamine parts are reported. Studies on dithioacetalized [2]rotaxane 4 with two hydrogen-bonding stations and a masked imine-bonding station showed that protonation of a macrocycle increases the shuttling barrier due to hydrogen-bond formation between NH₃⁺ groups and the TEG-stations. Hydrolysis of the imine-bonds of the imine-bridged molecular shuttles 1b,c with TEG-stations could exclusively give the [2]rotaxane 2b,c·2H²⁺, with the macrocycle hydrogen-bonded with the TEG-station. In contrast, 1a without TEG-stations gave an equilibrated mixture of 1a, monoimine 3a·H⁺, and 2a·2H²⁺ under similar acidic hydrolytic conditions. The equilibrium between 1b,c and 2b,c·2H²⁺ to control the position of the macrocycle could be successfully switched to either side by applying acidic hydrolytic or dehydrating conditions. Furthermore, the equilibrium was largely biased to [2]rotaxane 2b,c·2H²⁺ under acidic hydrolytic conditions and could be reversed in favor of bis-imine 1b,c just by heating. This is a successful example of a molecular shuttle exhibiting entropy-driven translational isomerism with remarkable positional discrimination. An examination of thermodynamic parameters showed that imine-bond hydrolyses and the formation of hydrogen bonds between the macrocycle and the station are thermodynamically matched processes, because both processes are enthalpically favored and accompanied by a loss of entropy. The combination of imine-bonding and hydrogen-bonding station in a rotaxane system is the key to realizing the clear entropy-driven positional switching of the macrocycle observed.