Solvent-free synthesis of the smallest rotaxane prepared to date
Solvent-free synthesis of the smallest rotaxane prepared to date
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DOI：
10.1002/anie.200803056
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发表时间：
2008-01-01
影响因子：
16.6
通讯作者：
Chiu, Sheng-Hsien
中科院分区：
文献类型：
--
作者：
Hsu, Chi-Chieh;Chen, Na-Chia;Chiu, Sheng-Hsien
[2] Rotaxanes—supermolecules comprising interlocked macrocyclic and dumbbell-shaped components—are fascinating materials for the construction of molecular devices because of the machinelike movement of their constituent parts.[1] The development of efficient, convenient, and environmentally friendly methods for the synthesis of these functional interlocked molecules has progressed tremendously in the past decade.[2] We became interested, however, in answering the following fundamental question: What is the smallest [2] rotaxane that can be synthesized, either in terms of molecular weight or the number of constituent atoms? We identified the crown ether/secondary dialkylammonium ion pair, which can be simplified into a few repeating CH2CH2O units that encircle a threadlike component as small as a dimethylammonium (CH3NH2+ CH3) ion, as the simplest and smallest recognition system for preparing [2] rotaxanes. Herein, we report a new and efficient solvent-free reaction which involves ball-milling of the [2] pseudorotaxane formed from dipropargylammonium tetrafluoroborate and the crown ether [21] crown-7 (21C7) on SiO2 with 1, 2, 4, 5-tetrazine. This led to the isolation in high yield (81%) of the smallest [2] rotaxane reported to date (Scheme 1). Although it has been postulated for some time that macrocycles possessing 21 or more atoms in their ring will be able to accommodate an alkyl chain,[3] it was only recently reported that a secondary dialkylammonium ion could be threaded through a 21-membered ring macrocycle, namely benzo [21] crown-7 (B21C7).[4] In addition, a phenyl group can act as the stopper that prevents the unthreading of the interlocked ring-shaped and linear components when this small macrocycle is used. We proposed that Diels–Alder reactions of 1, 2, 4, 5-tetrazine [5] with the terminal alkyne units of a 21C7-based [2] pseudorotaxane would produce pyridazine end groups, which are slightly less bulky than phenyl groups, and might also function as stoppers in a 21C7-containing [2] rotaxane. We chose the dipropargylammonium ion (1-H+) as the alkyne-terminated linear component in the small [2] pseudorotaxane precursor, expecting its small CH2NH2+ CH2 unit to reside within the cavity of the crown ether 21C7, stabilized through N+ÀH··· O and CÀH··· O hydrogen bonds. The alkyne termini are available for functionalization (Scheme1) through Diels–Alder reactions with 1, 2, 4, 5-tetrazine to generate small, but nevertheless sufficiently bulky, pyridazine rings for stoppering the pseudorotaxanes under solvent-free conditions. The 1HNMR spectrum (Figure1b) of an equimolar (5 mm) mixture of 21C7 and 1-H· BF4 in CD3CN at room temperature shows the chemical shifts of the protons of the complex are significantly different from those of its free components. The appearance of broad signals for both the free and complexed thread 1-H· BF4 in the 1H NMR spectrum (Figure 1c) of a 1: 2 molar ratio mixture of 21C7 and 1-H· BF4 in CD3CN suggested that the rates of exchange during the complexation and decomplexation processes were slow on the 1H NMR spectroscopic timescale at 400 MHz under these conditions, but not sufficiently slow to provide the sharp signals required to obtain an accurate value for the association constant through the single-point method.[6] Instead, we used isothermal titration calorimetry (ITC)[7] to determine an association constant of (14000Æ 1300) mÀ1 for the formation of the [2] pseudorotaxane from 21C7 and 1-H· BF4 in CH3CN at 258C.[8]Concentration of an equimolar solution of the macrocycle 21C7 and the threadlike ion 1-H· BF4 gave a sticky liquid, which we presumed to …
