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Carcerands cyclobutadiene

Shielding and Stabilization. Inclusion compounds may be used as sources and reservoirs of unstable species. The inner phases of inclusion compounds uniquely constrain guest movements, provide a medium for reactions, and shelter molecules that self-destmct in the bulk phase or transform and react under atmospheric conditions. Clathrate hosts have been shown to stabiLhe molecules in unusual conformations that can only be obtained in the host lattice (138) and to stabiLhe free radicals (139) and other reactive species (1) similar to the use of matrix isolation techniques. Inclusion compounds do, however, have the great advantage that they can be used over a relatively wide temperature range. Cyclobutadiene, pursued for over a century has been generated photochemicaHy inside a carcerand container (see (17) Fig. 5) where it is protected from dimerization and from reactants by its surrounding shell (140). [Pg.75]

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). [Pg.45]

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. [Pg.92]

The same approach can be applied to organic chemistry where very labile species can be encapsulated. In this way, species that usually would rapidly decompose or polymerize even at low temperatures can be stored at room temperature, especially when the guest has no chance to escape from its host. To achieve this goal, hemicarcerands have been mostly used, leading to the preparation of cages containing highly reactive species such as 1,3-cyclobutadiene, cycloheptatetraene, or benzyne. This subject is presented in detail in a specially dedicated chapter (Reactions inside Carcerands, Ralf Warmuth). [Pg.269]

We have defined the terms carcerand, carceplex, hemicarcerand, and hemicarceplex The formation of carcer-ands an carceplexes has led to some important information regarding templation by neutral molecules Similarly, the properties of hemicarcerands and hemicarceplexes has led to some interesting host-guest properties, including the storage of highly reactive species such as cyclobutadiene or benzyne... [Pg.192]

Figure 12 Encapsulation and isolation of cyclobutadiene in caUx[4]arene-based carcerand. Figure 12 Encapsulation and isolation of cyclobutadiene in caUx[4]arene-based carcerand.

See other pages where Carcerands cyclobutadiene is mentioned: [Pg.59]    [Pg.199]    [Pg.54]    [Pg.28]    [Pg.418]    [Pg.194]    [Pg.74]    [Pg.216]    [Pg.31]    [Pg.2]    [Pg.241]    [Pg.418]    [Pg.32]    [Pg.184]   


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