Chiral macrocycles rotaxanes

Two higher homologues of the [2]-rotaxane are represented the [3]-rotaxane and the [4]-rotaxane. Whereas [3]-rotaxanes are relatively common [56-60], this is not the case for the linear [4]-rotaxane [61] and its branched or dendritic analogue [62]. Very important in the recent developments of rotaxane chemistry is the [2]-rotaxane whose dumbbell contains two separated sites for interaction of the threaded macrocycle. In this molecule, the macrocycle may be moved between two sites of the dumbbell, in a degenerate manner, if the sites are identical [63-65], or in a controlled manner, thanks to an external trigger, if the sites are different [28-31]. A chiral [2]-rotaxane, whose chirality arises from the mechanical bonding, is obtained if the macrocycle is oriented (by an appropriate substitution pattern) and the dumbbell made non-symmetrical, for example by attaching two different stoppers [66],... 

In a study of chiral dipeptide [2]rotaxanes it was found that the presence of an intrinsically achiral benzylic amide macrocycle near to the chiral center could induce an asymmetric response in the aromatic ring absorption bands [62], This induced circular dichroism (ICD) effect was stronger in apolar solvents (Fig. 9), where intercomponent interactions are maximized, showing a direct relationship to the tightness with which the macrocycle binds the chiral thread. Computer simulations showed that chirality is transmitted from the amino acid asymmetric center on the thread via the achiral macrocycle to the aromatic rings of the achiral C-terminal stopper. 

Undoubtedly, the most important stereochemical feature is chirality. The [2]-rotaxane depicted schematically in Figure 2.12a is made with a macrocycle and a dumbbell that are not themselves chiral but oriented by appropriate substitution patterns on these components. Vogtle and co-workers described cycloenantiomeric rotaxanes that are based on the amide templating functionality (Figure 2.13).36 Racemic [2]-rotaxane 34 was synthesized in 20% yield by reaction of unsymmetrical acid dichloride 32 with trityl aniline 33 in the presence of macrocycle 31. The object and its mirror image are created by different sequences of the amide and sulfonamide linkers in both the dumbbell and the macrocyclic component. If the order of the atoms within the dumbbell is examined, in one enantiomer the sulfonamide nitrogen atom is... 

Figure 2.12. Different stereochemistries of rotaxanes that have been realized (a) Chiral rotax-ane (b, c, and d) diastereomeric rotaxanes. Arrows mark macrocycle orientation. The cone-shaped bead represents a cyclodextrin.

Topics of relevance to the content of this chapter which have been reviewed during the year include photoactive [2]rotaxanes and [2]catenanes, photochemical synthesis of macrocycles, phototransformations of phthalimido amino acids, photoaddition reactions of amines with aryl alkenes and arenes, photoreactions between arenenitriles and benzylic donors, photostability of drugs, polycyclic heterocycles from aryl- and heteroaryl-2-propenoic acids, photoreactions of pyrroles, photoamination reactions in heterocyclic synthesis, switching of chirality by light, photochromic diarylethenes for molecular photoionics and solid state bimolecular photoreactions. 

Let us assume that a rotaxane is formed from an axle with two different stoppers and a wheel that bears a directionality defined by the sequence of atoms in the macrocycle. Both of these subunits are achiral, both are identical to their mirror images. Nevertheless, the corresponding rotaxane exists in two enantiomeric forms, although no chiral center is present in the molecule (Fig. 4). It is the molecular topology that makes these species chiral, and consequently, this phenomenon was coined "topological chirality.. It also exists for catenanes, knots, and other mechanically bound molecules. Shown in Fig. 4 is a detailed structure of such a rotaxane. The wheel contains one sulfonamide group that provides the directionality of... 

A rotaxane-based molecular shutde can have chirality regulated by the position of a macrocycle. Movement of the macrocycle between stations is restricted by the introduction of a bulky group. Asymmetric benzoylation of racemic rotaxane, which has two stations and one hydroxy group on the symmetric position of the shaft molecule, catalysed by a chiral nicotinamide derivative gives enantiomericaUy enriched rotaxane with a 67 33 distribution of enantiomers by trapping the macrocycle on one side (Scheme 5.49) [129]. 

Figure 3.25 Schematic diagrams of (a) a [2]-rotaxane, (b) a [2]-pseudorotaxane and (c) a [3]-rotaxane. (d) An enantiomeric pair of chiral rotaxanes using an asymmetric thread and a macrocycle in which directionality can be defined.

Another example of the application to molecular machines is a molecular shuttle that functions as a chiropti-cal switch (Figure 14a). The bistable rotaxane 24 consists of a chromophoric macrocycle and a UV-weak thread with a photoisomerizable olefin on one side and a chiral center on... 

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