Inclusion reactions

8 Warmuth, R., Yoon, J., Recent highlights in hemicarcerand chemistry , Acc. Chem. Res. 2001,34, 95-105. 

The robust, well-shielded cavity found in hemicarcerands offers tremendous scope for the use of these hosts as micro-reaction vessels in order to protect reactive species from bimolecular decomposition by isolating them from the outside medium. Furthermore, the unique intracavity environment with its fluid-like properties in which guest species are, formally, in a very condensed state at very high pressures, may well result in unique inclusion reactivity. Indeed, the inner volume of carcerands and hemicarcerands has been described as a new phase of matter distinct from solid, liquid and gas. A number of elegant demonstrations have been made of the potential of inclusion reactions, and there is clearly a great deal of scope for their use as molecular reaction vessels. 

One of the simplest demonstrations of the effect incarceration has on a guest s reactivity is the measurement of the basicity of included amine ligands. Solutions of pyridine in CDCI3 may be shown by H NMR spectroscopy to be readily protonated by CF3CO2D. An analogous reaction of the pyridine hemicarceplex of the open portal hemicarcerand 6.101 results in the pyridine remaining unprotonated. This means that incarcerated pyridine is a much weaker base than the free molecule. This difference is explained most reasonably by the fact that the host has only a very limited ability to solvate the pyridinium ion and will sterically inhibit the formation of pyridinium-trifluoroacetate contact ion pairs. 

The reactivity of the incarcerated cyclobutadiene was probed by warming the NMR sample to 220°C for 5 min. This resulted in the formation of free cyclooctatetraene (6.108), clearly from ejection of the cyclobutadiene from the cavity and its subsequent dimerisation via 6.107. Reaction with O2 (which is able to enter the cavity of 6.101 gave incarcerated malealdehyde (6.109). 

While extremely elegant, the preparations of reactive compounds snch as cyclobntadiene and 

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Water plays a crucial role in the inclusion process. Although cyclodextrin does form inclusion complexes in such nonaqueous solvents as dimethyl sulfoxide, the binding is very weak compared with that in water 13 Recently, it has been shown that the thermodynamic stabilities of some inclusion complexes in aqueous solutions decrease markedly with the addition of dimethyl sulfoxide to the solutions 14,15>. Kinetic parameters determined for inclusion reactions also revealed that the rate-determining step of the reactions is the breakdown of the water structure around a substrate molecule and/or within the cyclodextrin cavity 16,17). 

A recent discovery shows that the lone pair electrons of the chlorine atom can also facilitate the formation of NCS frameworks. A novel family of salt-containing, mixed-metal sihcates (CU-14), Ba6Mii4Sii2034Cl3 and Ba6Fe5Sin034Cl3, was synthesized via the BaCl2 salt-inclusion reaction [6 a]. These compounds crystallize in the NCS space group Pmcli (No. 26), adopting one of the 10 polar, non-... 

Encapsulation of organic ion-radicals markedly changes their chemical reactivity. It inhibits undesirable interactions and increases the desired properties due to shielding effects. Encapsulation is usually considered as host-guest or inclusion reactions. 

Lerchner, J., Seidel, J., Wolf G. and Weber, E. (1996) Calorimetric detection of organic vapours using inclusion reactions with coating materials. Sensors and Actuators B Chemical, 32, 71-5. 

Yoshida and Hayashi [19] determined formation constants for a-CD inclusion complexes with azobenzene derivatives 1 listed in Figure 3 that are in the range of 5.0 x 103 to 1.75 x 104 moL -dm3. The rate constants for inclusion reaction and its mechanism are dependent on the substituent group Y. For Y = C02 the reaction is very fast (milliseconds) and proceeds as one step process. When Y = As03H the reaction is slow. When Y = S02NH2 or S03, the inclusion proceeds as a two-step process. 

Yoshida, N., and Hayashi, K. (1994) Dynamic Aspects in Host-Guest Interactions. Part 2. Directional Inclusion Reactions of Some Azo Guest Molecules with a-Cyclodextrin, Perkin Trans. 2 1285-1290. 

Generally the inclusion reaction is described to take place in a thermal gradient ranging from 90°C to room temperature. The complexes formed are often insoluble and can be separated as precipitates (21, 22). Inclusion complexes such as these often form under normal food processing conditions. The complexing of free starch due to the addition of fatty acid derivatives during production to potato flakes for instant mashed potatoes is a case in point. In this case the desired effect is related to taste due to a perceptible change in texture. 

Fluorescence correlation spectroscopy analyses the temporal fluctuations of the fluorescence intensity by means of an autocorrelation function from which translational and rotational diffusion coefficients, flow rates and rate constants of chemical processes of single molecules can be determined. For example, the dynamics of complex formation between /3-cyclodextrin as a host for guest molecules was investigated with singlemolecule sensitivity, which revealed that the formation of an encounter complex is followed by a unimolecular inclusion reaction as the rate-limiting step.263... 

M) types. Irradiation of the separated crystals in the presence of O2 and a photosensitiser results in an inclusion reaction, again forming 7.36 in the solid state. Dissolution of the product crystals and measurement of the optical rotation of the sample after the TOT host has racemised gives the specific rotations shown in Scheme 7.1. This is clear evidence of transfer of the chirality of the TOT host lattice to the photooxygenation product. In making this measurement it is of course important to wait until the TOT has entirely racemised in solution to ensure that no measured optical rotation (and therefore enantiomeric excess) is due to the TOT host itself. 

Channel Inclusion Compounds, p. 223 Hydrogen Bonding, p. 658 Inclusion Reactions and Polymerization, p. 705 Isostructurality of Inclusion Compounds, p. 767 Organic Zeolites, p. 996 Polymorphism, p. 1129... 

Cyclophanes Definition and Scope, p. 414 Inclusion Reactions and Polymerization, p. 705 Macrocycle Synthesis, p. 830 Naked Anion effect, p. 939... 

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