Synthesis and study of calix[4]arenes bearing azo moieties at the meta position
Graphical abstract
Introduction
Calix[4]arenes1 are very popular macrocyclic building blocks for supramolecular chemistry due to their easy preparation and almost limitless freedom in subsequent chemical derivatization. These typical features make calixarenes a perfect choice for the role of molecular scaffolds in the design and synthesis of novel molecular receptors.2
Electrophilic aromatic substitution represents the most straightforward approach to upper rim derivatization in calixarene chemistry.3 All previously reported reactions (nitration, halogenation, sulfonation, alkylation, formylation, Friedel−Crafts reactions etc.) on the calixarene skeleton, has led with no exceptions to the formation of para-substituted isomers (Fig. 1). Such intermediates are then used as starting points for the synthesis of various receptors and more sophisticated supramolecular systems. This strict regioselectivity is a consequence of the electronic effects of the substituents present on the aromatic subunits, namely, the –OH or –OR groups and the CH2 bridges. Obviously, the phenolic or ether moieties are much stronger o-/p-orienting functions which leads to the para-substitution pattern. Recently, we found that in the case of thiacalix[4]arene4 the presence of the sulfur bridges in contrast to the classical CH2 bridges influences the overall orientation outcomes of the molecule. As a consequence, the electronic effects of the sulfur atoms outweigh those of the alkoxy groups which results in the meta-substitution.5
During our on-going research on calixarene derivatization we discovered an unprecedented feature, that the mercuration of calix[4]arene immobilized in the cone conformation occurs exclusively at the meta position (Fig. 1).6 Thus, the resulting organomercury derivatives can be used for the introduction of various functional groups into this almost unutilized position of the calixarene skeleton.7 In this paper we report on the transformation of organomercurial intermediates into the corresponding azo compounds bearing functional groups at the meta position, that are inaccessible by common calixarene chemistry. Moreover, as this substitution pattern is still unique in calixarene literature, and the conformational behaviour essentially unknown, we undertook a study of the dynamic behaviour of such compounds in solution using NMR techniques.
Section snippets
Results and discussion
Tetrapropoxycalix[4]arene 1 (5.0 g scale) immobilized in the cone conformation was reacted as reported previously6 with 1.0 equiv. of mercury (II) trifluoroacetate (Hg(TFA)2) followed by reaction with aqueous HCl to yield meta substituted product 2 in 60% yield (Scheme 1). Similar reaction8 with 2.0 equiv. of Hg(TFA)2 led to a mixture of two regioisomers 3 (34%) and 4 (20%) possessing a meta, para or meta, meta arrangement of the chloromercurio groups on the distal aromatic subunits,
Conclusions
Direct mercuration and subsequent transformation of the organomercurial intermediates into corresponding nitroso derivatives enabled the construction of calix[4]arenes in the cone conformation bearing azo moieties at the position meta (to the alkoxy groups). These compounds represent a unique substitution pattern in calixarene chemistry and exhibited very unusual dynamic behaviour in solution. Temperature dependent 1H NMR spectra confirmed that the introduction of two bulky moieties (or even
General
All chemicals were purchased from commercial sources and used without further purification. Solvents were dried and distilled using conventional methods. Melting points were measured on Heiztisch Mikroskop – Polytherm A (Wagner & Munz, Germany). NMR spectra were performed on Varian Gemini 300 (1H: 300 MHz, 13C: 75 MHz) or Agilent 400-MR DDR2 (1H: 400 MHz, 13C: 100 MHz), Bruker Avance III 600 (1H: 600.1 MHz, 13C, 150.9 MHz), and Bruker Avance III 500 MHz (1H, 500.1 MHz, 13C, 125.8 MHz).
Acknowledgments
The authors wish to thank the Czech Science Foundation (GACR 16-13869S) and specific university research (MSMT No 21-SVV/2016).
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