Rotaxanes stereochemistry

For a discussion of the stereochemistry of these compounds, see Schill, G. Catenanes, Rotaxanes, and Knots, Academic Press NY, 1971, p. 11. 

The latest findings of amide-based rotaxanes focus on their chemistry and stereochemistry. The optical properties of an axle bearing chiral glucose stoppers are significantly influenced by rotaxanation - Cotton effects are amplified and the maxima of the CD spectra are shifted bathochromically [71]. 

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.

Threading of two cyclodextrins onto a symmetrical dumbbell can occur head-to-head, head-to-tail, or tail-to-tail, defining a new class of diastereoisomeric [3]-rotaxanes, as shown schematically in Figure 2.12d. Anderson and co-workers have shown that end-capping of 4,4,-bis(diazonio)azobenzene chloride 43 with 2,6-dimethylphenol 44 in the presence of a-CDX produced the [3]-rotaxane 45 in 12% yield, in addition to the [2]-rotaxane 46 in 9% yield, and free dumbbell 47 in trace amounts (Figure 2.18).42 The stereochemistry of the [3]-rotaxane species is remarkable because the two cyclodextrin beads have their smallest rims facing to each other. Therefore the threading reaction was stereoselective. The reasons for the exclusive formation of the tail-to-tail stereoisomer are not clearly established. 

So far we discussed the application of the graph theory in studies of electronic structure of molecules. In this chapter we shall describe some topological concepts that appeared in stereochemistry in connection with the investigations of a new remarkable type of chemical compounds, catenanes, rotaxanes, knots. 

Figure 22 The directed synthesis of [2]-rotaxane (60). Cyclization of the key intermediate (56) is directed by the very carefiilly controlled stereochemistry of the ketal ca n atom and provides threaded intermediate (57) [108],...

Frisch, H.L. and Wasserman, E. (1961) Chemical topology, J. Am. Chem. Soc. 83, 3789-3794. Schill, G. (1971) Catenanes, rotaxanes and knots. Academic Press, New York. Walba, D.M. (1985) Topological stereochemistry. Tetrahedron 41, 3161-3212. Dietrich-Buchecker, C.O. and Sauvage, J.P. (1987) Interlocking of molecular threads From the statistical approach to the templated synthesis of catenands, Chem. Rev. 87, 795-810. Philp, D. and Stoddart, J.F. (1991) Self-assembly in organic synthesis, Synlett 445-458. 

Mechanically interlocked molecules (MIMs), such as catenanes and rotaxanes, are molecules with at least two components that are not covalently bound, but interlocked in such a manner that they cannot be separated without the breaking of a covalent bond. Since this physical linkage is known as a mechanical bond [24], we refer to the stereochemistry of MIMs as mechanostereochemistry [25]. MIMs have been appreciated for their synthetic challenge and aesthetic value [26] as well as their potential applications. In particular, MIMs have garnered much interest as artificial molecular switches and machines [27-31] because their internal noncovalent bonding interactions can be modulated by external stimuli to control the relative translational and/or circumrotational motions of their interlocked... 

In order to perform an integrative self-sorting between species that at the same time are differing both for the sequence and the stereochemistry of their components, the new constitutionally asymmetric axle 19 " (Scheme 30.7) constituted by two different ammonium centers was designed and tested with two different calix-wheels la and 10 [34]. In this instance, considering the possible relative orientation of the directional calix-wheels, a [1 + 2] system could give rise to a 2 x 4 = 8 homo-pseudo[3]rotaxanes and 2x4 = 8 hetero ones, with a total of 16 possible species (Fig. 30.19). Clearly, some of them are simultaneously isomeric at the sequence and stereochemical level [34]. 

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