Cyclophanes cavitands

The spherically shaped cryptophanes are of much interest in particular for their ability to bind derivatives of methane, achieving for instance chiral discrimination of CHFClBr they allow the study of recognition between neutral receptors and substrates, namely the effect of molecular shape and volume complementarity on selectivity [4.39]. The efficient protection of included molecules by the carcerands [4.40] makes possible the generation of highly reactive species such as cyclobutadiene [4.41a] or orthoquinones [4.41b] inside the cavity. Numerous container molecules [A.38] capable of including a variety of guests have been described. A few representative examples of these various types of compounds are shown in structures 59 (cyclophane) 60 (cubic azacyclophane [4.34]), 61a, 61b ([4]- and [6]-calixa-renes), 62 (cavitand), 63 (cryptophane), 64 (carcerand). 

Much interest has centred on the branch of cyclophanes known as calixarenes. They are polyphenol systems that can act as hosts in the formation of inclusion compounds, where a small guest molecule resides completely in a cavity within a single host they are cone-shaped cavitands . Several accounts have appeared of their history. The discovery by Baeyer of a formaldehyde/phenol resin led to Bakelite and to the work of A. Zincke and E. Ziegler, who gave to the first oligomer a tetrameric structure of a calix[4]arene. Later syntheses by Gutsche (1978) led to calixarenes with 4, 6 or 8 phenol residues.107-109... 

While the conformation of the methylene-bridged cavitands 1 is fixed (Fig. la), a number of more flexible cavitands with other bridging units (BU) have been prepared (Fig. lb) [31, 32]. Depending on the substituent R at phenolic oxygen and the group (G) attached to the wider rim, the calix[n]arenes 2 can adopt different conformations (Fig. lc, d) [33, 34]. The calix[n]arenes with more than four phenol units in the cyclophane basis are particularly flexible. 

Calixarenes, cavitands, carcerands, and cyclophanes as nanoscale molecular containers 02BCJ393. 

A different use of inclusion was realized with carceplexes [7] starting from concave cyclophanes, such as cavitands, carcerands with an inner sphere were synthesized or non-covalently assembled [ 8]. Reactive molecules, trapped in the interior and so shielded from reaction partners could be investigated by spectroscopy. Cyclobutadiene [9] and benzyne [10] are the most striking examples of highly reactive molecules isolated by this technique. 

Cyclophanes (resorcarens, cavitands, cryptands, etc.) Compounds containing aromatic groups [58]... 

Fig. 4 Exemplary cases of enforced cavity hosts and their supramolecular compounds (guests symbolized in broken rings) (a) Cleft-type (b) tweezer-type (c) spherand (d) cyclophane (monocyclic) (e) cyclophane ibicyclic) (f) cavitand (g) carcerand (h) calixarene (calix[4]arene) (i) resorcarene (j) cryptophane (k) cyclodextrin (P-cyclodextrin) and (1) cucurbituril.

Fig. 4 Representatives of typical cyclophane host families calixarene (21), resorcinarene (22), cavitands (23, 24), cdrcerand (25). hemicarcerand (26), crown ether (27), cryptand (28), spherand (29). and hemispherand (30)...

Over the last 10 years or so, numerous books (Dieder-ich and Vogtle are particularly recommended) and chapters in books were published, covering many aspects of cyclophane chemistry. Readers are pointed to a recent review on " Synthetic Receptors that also embraces many aspects of cyclophane chemistry. It is the intention of this article in the Encyclopedia to focus on more recent developments toward the end of the last century and the turn of the new millennium. The examples chosen are in no way exhaustive but rather serve as pertinent examples to emphasize the types of cavities the cyclophane receptors possess and interactions that drive complexation. Where possible, the bioinimetic nature of the receptors will be discussed because the ultimate goal in much of cyclophane work is to mimic nature s processes. It is hoped that the reader will see the cross-fertilization of cyclophane chemistry and how examples discussed in other articles on. for example, cyclodextrins, cryptophanes. cavitands, crown ethers, etc., are just as relevant to this article. 

Inclusion complexation has developed to becoming another widely exploited supramolecular interaction for the formation of supramolecular polymer networks, mostly in water [197, 198]. Several classes of macrocycles have been developed, including crown ethers [199, 200], porphyrins [201, 202], cyclophanes [203], catenanes [204], cavitands [205, 206], cryptophanes [207], calix[n]arenes [208], and carcerands [209]. Macrocyclic-based supramolecular gels can either be formed from low molecular weight precursors or from macromolecular building blocks. The following discussion focuses on the latter. 

Cavitand A general term for a host structure that has an enforced cavity large enough to accommodate guest molecular entities. Examples include cyclodextrins, rigid cyclophanes, resorcinarenes, spherands, etc. 

Rebek et al. developed a number of cavitand derived from resorcinol as ditopic catalysts featuring distinct recognition elements for the reacting and nonreacting parts of the substrates. In many of these reaction systems, choline derivatives are involved and the trimethylammonium head of the substrate is complexed in the cyclophane cavity of the catalyst. 

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