Channel inclusion complexes

The polymerization of trans-1,3-pentadiene, 149, in a chiral channel inclusion complex with enantiomerically pure perhydrotriphenylene affords an optically active polymer, 150 (236). Asymmetric polymerization of this monomer guest occurs also in deoxycholic acid inclusion complexes (237). 

Lahav, Leiserowitz and their coworkers have used inclusion complexes of deoxycholic acid to carry out asymmetric photochemical reactions. The authors reported that acetophenone forms a 2 5 crystalline channel inclusion complex with deoxycholic acid 22 as the host and that irradiation of the complex in th4 solid state leads to abstraction of a hydrogen atom by the acetophenone 23a from C5 of the steroid followed by coupling of the resulting radical pair to produce1 photoproduct 24 (Scheme 9) [216,219]. The authors were able to follow the course of the reaction by x-ray crystallography. Only one diastereomer of the product was obtained in the reaction. Aoyama et al. have studied the reaction of AT,N-dialkylpyruvamides in deoxycholic acid inclusion crystals (Scheme 10) [246], Solid-state irradiation of the inclusion complex of 22 and 25 gave the... 

Recrystallisation from water containing picric acid (PA) gives stable isomorphous yellowish needles, whose structure has been reported The TMA H O framework of these crystals is well defined but the striking diffuse scattering indicates that the PA molecules located in the channels are partially disordered. This inability to position the guest with respect to the host matrix is a feature common to many channel inclusion complexes and makes it difficult to draw conclusions about the details of the host-guest interaction. 

Our knowledge of the channel inclusion complexes of TTP has reached a stage of tantalising incompleteness. The overall structure of the complexes has been determined and appears rather simple and straightforward. However, the major chemical characteristic of these complexes, namely the aromatic nature of the guests, is quite without an established explanation while the structural basis for many unusual details of the diffraction patterns remains outside our grasp. It seems probable that only low-temperature diffraction studies will provide enough additional information to enable us to unravel these conundrums. 

Channel inclusion complexation of organometallics Dipolar alignment for second harmonic generation, Chem. Mater. 1 128 (1989). 

Tam. W. Eaton, D.F. Calabrese, J.C. Williams, I.D. Wang, Y., Anderson, A.G. Channel inclusion complexation of organometallics Dipolar alignment for second harmonic generation. Chem. Mater. 1989, 1, 128-140. 

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. 

X-ray diffraction studies on gramicidin commenced as early as 1949 218-219> and this early work pointed to a helical structure 220). Recent work by Koeppe et al. 221) on gramicidin A crystallised from methanol (/%) and ethanol (.P212121) has shown that the helical channel has a diameter of about 5 A and a length of about 32 A in both cases. The inclusion complexes of gramicidin A with CsSCN and KSCN (P212121) have channels that are wider (6-8 A) and shorter (26 A) than the uncomplexed dimer 221 222). Furthermore there are two cation binding sites per channel situated either 2.5 A from either end of the channel or 2.5 A on each side of its centre 222) Unfortunately these data do not permit a choice to be made from the helical models (i)—(iv) and it is not certain if the helical canals studied are the same as those involved in membrane ion transport. 

Lehn 242 243) has described a solid phase model of a K+ channel based on the crown ether 37. The crystal structure of this inclusion complex reveals stacking of the crown ethers into vertical columns, empirical formula [2 37,2 K, 3 H20]2+, linked by water and potassium ions. The counter ions, empirical formula [K, 3 Br, 4 H20]2, comprise a polymeric chain running parallel to the columns. 

When guest molecules are arranged together in the channel of a host-guest inclusion complex, intermolecular reactions of the guest compound may proceed stereoselec-tively and efficiently. An enantioselective reaction is expected when optically active host compounds are used. 

Recently, it has become clear to the author that cyclodextrin is one of the promising hosts for macromolecular recognition, with the finding that the cylindrical channel formed when it is stacked in a linear row is able to accommodate one or more long-chain guests. Thus, he and his coworkers launched a series of experiments to explore the formation of inclusion complexes between... 

It should be noted that not all host-guest phane complexes are of the type just described. Indeed, a significant number of lattice inclusion complexes also occur. In these, the host molecules stack such that a channel running between hosts is formed. Guest molecules occupy this channel. As expected, such an arrangement is usually reflected by relatively poor host-guest selectivity. 

A consequence of the occupation of more than one site in solid inclusion complexes is illustrated with a single example, although many more are available in the literature. Detailed study of arylalkyl ketones included in deoxycolic acid (DCA) channels have been carried out by Lahav, Leiserowitz,... 

Whether or not a compound can form inclusion complexes with CDs largely depends on the compound s size compatibility with the dimensions of the CD cavities. The stability of a complex also depends, however, on other properties of the guest molecule, such as its polarity. Compounds used medicinally usually are large molecules. Therefore, it is very commonly observed that the complexes form such that only certain groups or side chains penetrate into the carbohydrate channel. 

All-trans-perhydrotriphenylene (PHTP) (cf. insert in Figure 14) is the product of exhaustive hydrogenation of triphenylene. It belongs to one of ten stereoisomers of PHTP. The chiral compound of high rotational symmetry (D3 — C3 -h 3 C2) forms inclusion complexes. The stereoselective polymerization via 7-radiation of the prochiral diolelin 1,3-pentadiene within the chiral nano channels of (.R)-(-)-all-trans-PHTP led to an optically active 1,4-trans-isotactic polymer (Nattaand Farina, 1976) (cf. Figure 13). 

Poly (e- caprolactone) (PEC) forms inclusion complexes with all three CDs wherein the y-CD/PCL complex contains two side-by-side PCL chains in each y-CD channel when the molecular weight of PCL is low [26,42],... 

Crystalline inclusion complexes (IC s) have been also formed between polymers and another small-molecules, host clathrated provide a unique environment for observing the solid - state behavior of isolated polymer chains. In their IC s with small-molecule, host clathrates, such as urea (U) [1] and perhydrotriphenylene (PHTP) [57], the included polymer chains are confined to occupy narrow channels (ca. 5.4 A in diameter) where they are extended and separated from neighboring chains by the channel walls, which are composed exclusively of the host clathrate, crystalline matrix. Choi et al. [58] have been studied the behavior of isolated, extended polymer chains included in their IC s with U and PHTP by a combination of molecular modeling [59,60] and experimental observations in an effort to determine their conformations and mobilities in these well-defined, containing environments. 

Inclusion compound (or inclusion complex) — A complex in which one component, the host, forms a cavity or, in the case of a crystal, a crystal lattice containing spaces in the shape of long tunnels or channels in which... 

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