Achiral host molecules

It was found that the title achiral host molecules (134) are arranged in a chiral form in their inclusion complex with a guest. By using this phenomenum, some rac-guests were resolved by complexation with achiral 134. 

Cucurbit[n]urils (CB[n]s) are a family of macrocyclic compounds comprising 5-10 gly-coluril units. CB[6], CB[7] and CB[8] are similar in cavity volume to a-CD, j8-CD and 7-CD, respectively. Differing from CDs, CBs are achiral host molecules of Dni, symmetry and therefore cannot provide chiral microenvironment. 

A number of well-known host molecules have achiral molecular structures. In other words, the atoms of these molecules are, or may formally be, arranged in space such that a plane of symmetry or a centre of symmetry is present. At first sight, this group of compounds may appear to be irrelevant to a discussion on chirality, but this is far from being so. 

Achiral objects can be assembled into chiral solid-state structures, and this is frequently the case for urea 1 when it encloses guests. Other compounds adopt a chiral conformation in solution and therefore may ultimately produce either chiral or achiral host structures. On the other hand, thiourea 2 forms an inclusion lattice that is achiral. This arrangement is nonetheless very effective in enclosing guest molecules. 

The role of supramolecular chemistry in materials is perhaps expressed most impressively in liquid crystals, in which slight variations of chiral content can lead to dramatic influences in the properties of the mesophases. The helical sense of these mesophases is determined not only by intrinsically chiral mesogens but also by the use of dopants which more often than not interact with achiral host LCs to generate chiral phases (Fig. 7). These phenomena are important both scientifically and technologically, most notably for the chiral smectic and cholesteric liquid crystal phases [68-71]. These materials—as small molecules and as polymers [72,73]—are useful because their order... 

From one point of view, the Cy are excellent choices as host molecules because they absorb only in the far UV. As a result, the induced CD signal in the near UV is due only to the complex and its magnitude is, in principle, directly proportional to the concentration of the complex in solution, ipi = 0. d [Cy-L]. This is really true only in the cases where L is an inherently achiral molecule. For racemic mixtures the more exact Beers law summation equation that relates the observed induced signal to the individual contributions is ... 

By inclusion complexation with a chiral host compound, achiral guest molecules can be ordered in a chiral manner, and photoirradiation of the complex gives optically active reaction products. The inclusion complexation can also be accom-... 

Achiral molecules can crystallize in a chiral space group or form inclusion crystals of achiral guest and chiral host molecules in the absence of any external source of chirality.588-590 Irradiation of such solid-state samples may then generate optically pure compounds in high chemical yields. This type of asymmetric induction is introduced later in Special Topic 6.5 and Case Study 6.21. 

Enclathration is not infrequent and inclusion compounds are becoming a large family of well characterized chemical spedes. TOT ranks among the most attractive host molecules owing to the ability of differentiating between enantiomeric guest molecules in two different enantiomorphous host lattices. Besides, the imusually high propensity of TOT tq form stable clathrates is further illustrated by no less than seven different achiral crystalline forms. 

Photoresponsive molecules are important for the fabrication and photomodulation of photoresponsive CLC materials, no matter if they are employed as chiral mesogens, achiral nematic LC hosts, or chiral/achiral guest molecules. The most important property of these molecules in photoresponsive CLC materials is the changes in molecular shape (geometry/conformation) as a result of the light-induced isomerization, which constitutes the basis for photomning of the properties of CLCs [18], The first example of using photoresponsive molecules to modulate the properties of CLCs was reported by Sackmann in 1971 [19], where azobenzene was employed as the dopant molecule. To date, various kinds of photoisomerizable molecules have been developed for this purpose (Fig. 5.3). 

Besides the use of chiral bases or catalysts in solution, a rather interesting and unique approach that belongs to the present category involves the utilization of inclusion complexes of the stabilized ylides [104]. In the solid state, an achiral stabilized ylide such as 190 is reacted with a symmetrically substituted prochiral cyclohexanone such as 189 in the presence of a chiral host molecule. The best result was obtained using the chiral host molecule 191, which gave the dissymmetric alkene 192 with up to 57% ee. 

The chirality of compounds like 1-3 is entirely based on the chirality of the derivatizing reagents. However, due to their nonplanar shape, calixarenes offer numerous additional possibilities for producing chiral host molecules, which are not based on a chiral subunit but on the absence of a plane of symmetry or an inversion center in the molecule as a whole. In other words, opening of the macrocyclic structure would lead to an achiral linear molecule. Below we attempt to present a systematic overview of such inherently chiral calixarenes. 

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. 

In 1994 we published the first chiral dendrimers built from chiral cores and achiral branches [ 1,89], see for instance dendrimer 57 with a core from hydroxy-butanoic acid and diphenyl-acetaldehyde and with twelve nitro-groups at the periphery (Fig. 21). As had already been observed with starburst dendrimers, compound 57 formed stable clathrates with many polar solvent molecules, and it could actually only be isolated and characterized as a complex [2 (57- EtO-Ac (8 H20))]. Because no enantioselective guest-host complex formation could be found, and since compounds of type 57 were poorly soluble, and could thus not be easily handled, we have moved on and developed other systems to investigate how the chirality of the core might be influencing the structure of achiral dendritic elongation units. 

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