Hemicarcerand

Carceplexes and hemicarceplexes, host-guest complexes in which carcerands and hemicarcerands, in particular those including heterocyclic fragments, are hosts 99CRV931. 

The structures and conformational properties of a simple hemicarcerand, created earlier by Cram, see <96JA5590>, as well as the complexation and decomplexation with guest molecules have been computationally studied <96JA8056>. 

K. N. Houk, K. Nakamura, C. Sheu, A. E. Keating, Gating as a Control Element in Constrictive Binding and Guest Release by Hemicarcerands , Science 1996, 273, 627-629. 

Keywords Constrictive Binding m Crown Ethers m Hemicarcerands a Host-Guest Chemistry a Noncovalent Interactions aRotaxanes a Self-Assembly a Slippage a Supramolecular Chemistry... 

Scheme 5. The synthesis of hemicarcerand 16 is temp-lated around a molecule of DMA, leading to the production of hemicarceplex 16-DMA.

Figure 7. Guest molecules egress from hemicarcerands (Host) through conformational motions known as gating. Gate-opening occurs by (a) French door and (b)...

D. J. Cram, J. M. Cram, Hemicarcerands and Constrictive Binding , In Container Molecules and Their Guests (Ed. J. F. Stoddart), The Royal Society of Chemistiy, Cambridge, 1994, pp. 149-169, and references cited therein. 

Thus, the hemicarcerands differ161 from the related carce-rands, which are similar in shape and constitution, but from which, guests cannot escape without the breaking of covalent bonds. The mechanically bound complexes formed between carcerands and their guests are termed carceplexes. 

Kinetically stable superarchitectures can be assembled by relying upon both noncovalent bonds and mechanical coercion. Thus, at elevated temperatures, rotaxane-like complexes (a) are generated when macrocycles slip over the stoppers of chemical dumbbells, while hemicarceplexes (b) are created when guests squeeze through the portals of hemicarcerands. 

Scheme6.n0 Structure of the hemicarcerands 547 that Warmuth and Marvel used to prepare isolated molecules of 1,2,4,6-cycloheptatetraene (5) and its 5-methyl derivative (10) to make them observable at room temperature and to study their reactions with hydrogen chloride and triplet oxygen.

Development of ditopic receptors 58 for both an anion and a cation is a logical extension [19b]. It should be stressed that the recognition by cyclodextrins, calixarenes, hemicarcerands, and many other systems does not involve specific recognition sites, whereas tetrahedral recognition by 53 and 54, the adenine 56 selective binding, and multiple recognition like the one shown in formula 59 [20] require the presence of specific sites in receptor capable of directional interactions. 

As stated above, systematic names of macrocyclic host molecules were absurdly complicated for routine discussions [22]. Therefore Vogtle proposed the name coronand for crown ethers, and that of coronates for their complexes while cryptand complexes were called cryptates . The corresponding noncyclic analogues are podands such as 64 [23] and podates, respectively. The cumbersome name podando-coronands (and correspondingly podando-coronates ) was proposed for lariat ethers [24] having at least one sidearm like 65. Examples of hemispherands 66 [25], cavitands 25 [26] and those of some other hosts are discussed in Chapter 7 in some detail, whilst the exceptional stability of fragile guests 4 [2a] and 67 [27] in the hemicarcerand 5 cavity are discussed in Chapters 1 and Section 7.3. 

In addition to macrocyclic hosts discussed above, many other molecules capable of selective complexation have been synthesized. They belong to so-called macrocyclic chemistry [30] encompassing crown ethers discussed in this Chapter, cryptands 61-63 [21], spherands 70 [31], cyclic polyamines 71 [32], calixarenes 18 [5], and other cyclophane cages such as 72 [33] to name but a few. Hemicarcerand 5 [2b] discussed in Chapter 1 and Section 7.3 also belongs to this domain. Typical macrocyclic host molecules are presented in Chapter 7. 

The Complexes Involving Induced Fit and Without It Endohedral Fullerene, Hemicarcerand and Soft Rebek s Tennis Ball-Like Hosts... 

Carcerands, Hemicarcerands and Novel Molecular Flasks Enabling Preparation and Stabilization of Shortlived Species [1]... 

Hemicarcerand 259 (X = CH2CH2Ph) differing from5 in the number of CH2 bridges, 4 vs. 3, and their lengths was applied by Warmuth to the photochemical... 

It should be stressed that calixarenes, hemispherands and spherands, carcerands and hemicarcerands, and some other molecules discussed elsewhere in this book, belong to the cyclophane group of compounds. 

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