A p-tert-butylcalix[6]arene capped with a triethanolamine-derived triple bridge
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Introduction
Calix[n]arenes are a class of [1n]metacyclophanes consisting of phenolic units linked by methylene groups. They have been studied intensely as three-dimensional molecular receptors in host–guest chemistry and for the construction of supramolecular assemblies.1 The conformational flexibility of these macrocycles is a major criterion for their ability to act as receptor molecules. Although calixarenes have been reported to change conformation during their complex formation with a suitable guest molecule and therefore can be considered as induced-fit type host molecules, it may be helpful in other cases to restrict the conformational mobility in order to obtain molecular geometries and receptor sites of well-defined size and shape.2
Due to their higher conformational flexibility as compared to calix[4]arenes, calix[6]arenes are still limited in their application as molecular receptors.3 The complex dynamics of phosphoryl derivatives of p-tert-butylcalix[6]arene 1 has been elucidated only recently.4 The introduction of appropriate bridges at the lower or upper ring of calix[6]arenes is suited to lowering the conformational freedom or to completely immobilizing the conformation of the macrocyclic skeleton. Today, we know molecular architectures in which phenol functions of calix[6]arenes such as 1 and 2 are connected by one bridge (A,D-5., 5.(b), 5.(c), 5.(d), 5.(e), 5.(f), 5.(g), 6. and A,C-bridging,6 where the letters A–F in this study denote the six phenol units), by two bridges (A,C; D,E7), and by three bridges (A,E; B,C; F,G).5g Bridging of the upper or lower rim with a multipodal cap is a particularly suitable strategy to suppress the ring inversion of the calixarene skeleton. The connection of the six phenol groups of 1 by two triply bridging phosphate groups (A,B,C; D,E,F) was the first example of a calix[6]arene with immobilized conformation.8 Other examples include upper-rim bridging with a tripodal cap,9 and lower-rim capping with C3-symmetrical tripodal caps to give A,C,E-bridged systems such as 3,10 4,6 and 511 (Scheme 1) as well as calix[6]cryptands which are A,C,E- or A,B,D-bridged by a EtC(CH2OCH2CH2OCH2CH2)3 unit.12 A lower-rim A,B,D,E-bridged system with a C4-symmetrical cap has also been reported.13
In this communication, we present a novel calix[6]arene which is bridged at the lower rim with a tripodal cap derived from triethanolamine.14
Section snippets
Results and discussion
The triply bridged calixarene 6 was obtained by deprotonation of calix[6]arene 215 with sodium hydride and subsequent triple alkylation with alkyl chloride 7 (Scheme 2). After column chromatography, 6 was obtained in 25% yield. The 1H NMR spectrum (500 MHz) at 303 K shows two signals for the tBu groups (δ=0.88 and 1.37, 1:1 intensity), one signal for the OMe groups (δ=2.96), an AB quartet (δ=3.45 and 4.40) for the ArCH2Ar methylene protons, and two sharp singlets for the aromatic protons. These
Conclusion
With calixarene 6, we have added a novel example to the so far small set of calix[6]arenes which are rigidified by C3-symmetrical A,C,E-bridging at the lower rim and we have provided the first solid-state structure for this structural type. The temperature-dependent NMR spectra in solution do not give evidence for ring inversion in the calixarene part of the molecule, but they indicate a positional equilibrium for the protons of the methoxy groups: they point away from the calixarene cavity at
Calixarene 6
5,11,17,23,29,35-Hexa-tert-butyl-37,39,41-trimethoxy-38,40,42-trihydroxycalix[6]arene15 (2) (507 mg, 0.49 mmol) was dissolved in anhydrous THF (50 mL) and anhydrous DMF (10 mL). The solution was cooled at 0°C, and sodium hydride (80%, 200 mg, 3.5 mmol) was added. The solution was brought to rt, stirred for 30 min, then heated to reflux for 10 min. A solution of tris(chloromethylcarbonyloxyethyl)amine22 (7) (150 mg, 0.63 mmol) in DMF (10 mL) was added dropwise, and the reaction mixture was refluxed for 16
Acknowledgements
Dr Y. Zhang thanks the Alexander von Humboldt Foundation for a Humboldt Research Fellowship.
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