Clathrates host-guest reactions

The most extensive study in the field of host-guest reactions in clathrates has been that of Lahav, Leiserowitz, and co-workers (56,241,243) on the choleic acids. The results of these combined chemical and crystallographic investigations are of possible importance for stereoselective steroid functionalization. In these studies potentially reactive guests were activated thermally or photochemically to produce species that attacked the walls of the channel at specific sites determined by the proximity, orientation, and reactivity of the host molecules at the wall relative to the activated guest species. 

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). 

A simple possibility for the synthesis of esters, the reaction of an acid chloride with an alcohol, was used by Schrage and Vogtle [58] for a two-step synthesis of the macrocycle 63 from the alcohol 61 and the acid chloride 62. Compound 63, an example from the field of host/guest chemistry, forms a cavity, as studied with CPK-models, which could include planar, aromatic guests. Crystals obtained from benzene/ -heptane point to a 1 2 stoichiometry of 63 and benzene according to NMR-spectroscopic data. However, whether this is a molecular inclusion complex or just a clathrate is not yet known. 

Irradiation in air of the deoxycholic acid (DCA, 157) complex of indanone leads to oxidation of both the steroid and the guest, yielding 5- 3-hydroxy-DCA, 158, and optically active 3-hydroxyindanone (241). In the presence of air, irradiation of the DCA clathrates of isochromane, 159, and indene, 161, leads to reaction with oxidation of the host and of the allylic position of the guest to a keto group (e.g., 159 — 160 and 161 — 162).The detailed mechanisms of these oxidations remain to be elucidated. 

Clathrates are crystalline inclusion compounds formed by the physical reaction between host molecules and low molecular weight gases as guests. In the clathrate structure there are two different types of cages which can entrap guest molecules into the network of host molecules. The three-dimensional clathrate structures can be determined by X-ray diffraction method. 

Finally, we wish to comment briefly on a recent development in inclusion polymerization. As already discussed, this reaction can be carried out on the pure clathrate or in the presence of an excess monomer. Consequently, the vapor pressure of a volatile monomer during polymerization ranges from the decomposition pressure of the clathrate to the vapor pressure of the saturated solution of the host in the guest, which is generally very close to that of the pure liquid monomer. For example, the vapor pressure... 

As a natural development of the successful molecular inclusion concept, which involved electrostatic as well as van der Waals forces between the interacting host and guest entities, an increasing interest has been shown in the systematic study of lattice-inclusion type systems. A considerable effort has been devoted to the design of new hosts for the formation of stable crystaUine clathrates and the improvement of selective complexations with potential guests. Suitable examples of clathrates studied in recent years include hosts such as Dianin s compounds , perhydrotriphenylene , cyclotriveratrylene triphenylmethane hexakis-(arylthio) and -(arylthiomethyl)benzenes tri-o-thymotide (TOT) and choleic acids (cf. Fig. 1 in Ch. 1 of Vol. 140). Selected series of such clathrate inclusion systems have particularly been useful in research of photochemical reactions in the solid state and of selective molecular complexation that is central to biological phenomena... 

Another class of hydrates is gas hydrates. Unlike the stoichiometric inorganic salt hydrates, gas hydrates are nonstoichiometric clathrate soUd crystalline substances consisting of a lattice formed by water molecules (host) and entrapped gas molecules (guest). They are stable under high pressure and low temperature. The hydrate formation reaction can be described by the following general equation ... 

Consider the simplest case of a clathrate (C) with only one type of cavity. The inclusion of a guest (G) into the host (H) can be described by the process empty cavity -I- G occupied cavity. This can be described as a reversible chemical reaction of binding the guest into the host,... 

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