Guest-induced chirality

This guest-induced chirality is discussed more easily with a dimer of type II (or III), that is chiral also when the direction of the carbonyl groups changes rapidly. 

Racemic diacid 79 was imprinted at an elevated temperature with an alkaloid. Quinine and quinidine that are pseudo enantiomers led to opposite enantiomers. The guest-induced chirality was preserved on cooling to rt, which was maintained even in the absence of a guest (fi/2 = 14 years). The chiral enrichment process was also reversible, allowing the diacid to be used as a chiral switch (09OL2599). 

Guest-induced chirality within the cavity of hydrogen-bonded softballs 307, 308, 359, and 360 (a racemic mixtnre of the latter is formed as a resnlt of dimerization of nonsymmetric ligand syntone 148) after encapsnlation of chiral camphor derivatives (Scheme 3.37) has been examined [39] by H NMR method. 

It has also been reported from circular dichroism (CD) studies [36] that polysaccharide-based CSPs can induce chirality in enantiomeric guests such as (4Z,15Z)-bilirubin-Ixoc (BR) (Fig. 5). Although not optically active, BR has two enantiomeric helical conformations maintained by six intramolecular hydrogen bonds between two carboxylic acid moieties and two pyrromethenone — NH— protons. These (R)- and (5)-helical conformers are in dynamic equilibrium in an achiral solution [37], but some optically active compounds can enantioselectively bind to BR to induce CD spectra in solution [38-40]. A significant induced CD... 

Jansen, J., Debrabandervandenberg, E. M. M., Meijer, E. W., Induced chirality of guest molecules encapsulated into a dendritic box. Recueil Des Travaux Chimiques Des Pays-Bas-Journal of the Royal Netherlands Chemical Society 1995, 114, 225-230. 

Supramolecular architectures are highly sensitive to chiral perturbations in general, and in systems that form liquid crystals in particular. Small amounts of enantiopure guest molecule added to a nematic host can induce a transition to a cholesteric phase, and the helical organization in the mesoscopic system is very sensitive to the structure of the guest molecule. Chiral amplification was successfully achieved in such liquid crystals, using CPL as the chiral trigger for the phase transition [183]. 

There are many host systems that are inherently chiral or attain chirality upon crystallization. When achiral molecules are included in these chiral hosts induced chirality may result. If this occurs the induced chirality of the guest reactant can be preserved when it is converted to a product [175-178]. This has been shown to be possible with several systems. Once again, the reactants are nonchiral and give racemic products in isotropic solution. Structures of a few chiral molecules that serve as hosts are listed in Sch. 35. The following examples in which l,6-Z /s(o-chlorophenyl)-l,6-diphenyl-2,4-dyine-l,6-diol and cyclodextrin serves as the host illustrate the point. 

The signs of the first and second Cotton effects in complexes with diols are opposite to those determined by the classical dibenzoate chirality rule (compare Figure 26).263 This means that the chiral guest induces an opposite chirality in the resorcarene host 129. Thus, 129 can be used as a supramolecular CD probe for the assignment of the absolute configurations of glycols and alcohols. 

The formation of these capsules is reversible, and thus, chiral discrimination (a given chiral host distinguishes between two enantiomeric guests) is superimposed by chiral induction (a certain chiral guest induces the formation of one of two possible enantiomeric host capsules). 

Although not very efficient at present, these results clearly show that chiral cyclophanes provide chiral cavities which can recognize and induce chirality in the bound guests. 

Fig. 17 Fluorescent sensors based on displacement of fluorophores. (a) Chiral boronate 56 containing a fluorophore which is quenched upon displacement (b) CdSe/ZnS quantum dot (QD) modified with a spacer and a boronic acid unit bound to a fluorophore-containing P-cyclodextrin 57, which, upon interaction with the analyte, releases the fluorophOTe, thus pievtaiting energy transfer (ET) from QD (c) guest-induced phosphorescence of CD-l-hromonaphthalene complex...

The host is not required to adopt chiral forms in the absence of a chiral guest. In a complexed state, (a) the chiral guest biases the equilibrium between enantiomeric conformations of the host and (b) the chiral guest induces the host to change in structure from achiral to chiral upon complexation and biases the newly generated equilibrium through the supramolecular transmission of chirality. 

A regio- and stereoselective Beckmann rearrangement utilized diastereose-lective host guest interactions of the inclusion complexes 225 and 228 in a solid state reaction. Initially, a 1 1 mixture of the chiral host 223 and the racemic oximes 224 and 227, respectively, was treated with ultra sound in the solid state to induce the optical resolution. Then H2SO4 was added to start the Beckmann rearrangement, the corresponding c-caprolactams 226 and 229 were isolated in 68 % and 64 % yields and ee of about 80 % and 69 % (determined by HPLC analysis on chiracel OC) (Scheme 43) [46]. 

The question that emerges at the climax of this survey relates to the possibility of using crystalline inclusion phenomena for optical resolutions of molecular species. Can this be done effectively with suitably designed host compounds The definitely positive answer to this question has elegantly been demonstrated by Toda 20) as well as by other investigators (see Ch. 2 of Vol. 140). An optically active host compound will always form a chiral lattice. Therefore, when an inclusion type structure is induced, one enantiomer of the guest moiety should be included selectively within the asymmetric environment. 

---