Guests, arrangements

Observation of the Norrish Type II reaction presents some difficulty in that generation of the biradical intermediate 12 requires a six-membered transition state and this is in conflict with the linear guest arrangement normally expected in the channel. However, as noted earlier, accommodation of planar six-membered rings in urea inclusion complexes has been observed 38. It appears that in this case the necessary six-membered transition state can be produced in the channel without destruction of the crystal structure. 

In the following figures, when there are two guest molecules arranged above and below the crown, one of them will be shown with circles or the ellipses drawn by the computer and the other will be represented by the characters of the formula. This is done to clarify the host/guest arrangements and to save space. 

Figure 7.19 Possible guest arrangements in PHTP inclusion channels. While the. .. DA... interaction is by far the most likely, the occurrence of a greater number of. ..DD... defects than. ..AA... results in overall crystal polarity.

L= l,2-bis(2-pyrimidinylthio)ethane), the large macrocyclic size leads to the interpenetration of the diamondoid nets and a host-guest arrangement of the ligands within the cycles. 

Structure-Reactivity Relationship in Deoxycholic Acid Complexes.—The three-channel motifs offer a variety of host-guest arrangements that may be exploited for the performance of solid-state reactions. Two kinds of reagents were occluded (a) peroxides, hydroperoxides, and peresters, which were activated thermally or by irradiation, (b) ketones, which were activated photochemically. 

Further support for the topochemical nature of this reaction is provided by 2 1 DCA-diethylketone which also crystallizes in the a form. The change in the ketone structure, relative to acetone, is sufficient to induce a different host-guest arrangement (Figure 20). Irradiation under argon led to addition at 6, only (Scheme 15), whereas... 

It is not easy to control the steric course of photoreactions in solution. Since molelcules are ordered regularly in a crystal, it is rather easy to control the reaction by carrying out the photoreaction in a crystal. However, molecules are not always arranged at an appropriate position for efficient and stereoselective reaction in their crystals. In these cases inclusion chemistry is a useful technique, as it can be employed to position molecules appropriately in the host-guest structure. Chiral host compounds are especially useful in placing prochiral and achiral molecules in suitable positions to yield the desired product upon photoirradiation. Some controls of the steric course of intramolecular and intermolelcular photoreactions in inclusion complexes with a host compound are described. 

Fig. 14. Stereoview of the 1 1 9 benzene clathrate, showing a channel-type arrangement of the benzene guest species 38)...

Furthermore, all the examples shown above in Sects. 4.1-4.3 emphasize the significance of both steric and functional features in selective crystallizations. The latter are needed not only for binding between the individual host and guest constituents, but also for effecting a continuous and relatively unflexible pattern of intermolecular arrangement in the crystal lattice. This observation appears to be a very useful one in the systematic design of novel clathrate-based synthetic receptors. 

A similar behavior is found in the 1 2 inclusion of 26 with formic acid 71 (Fig. 20, type II). We notire a H-bonded dimer of 26 and one of the formic acid molecules binding the host dimer in a pseudo-dimeric arrangement via the free —COOH groups of the host dimer. The second guest molecule is also placed into an interstitial tunnel of the dimeric host/bound-guest matrix. Here the 1 2 stoichiometry is due to the small size of the guest partner. 

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