Stereoisomers topological

Singly and doubly interlocked [2]catenanes can exist as topological stereoisomers (see p. 144 for a discussion of diastereomers). Catenanes 35 and 36 are such stereoisomers and would be expected to have identical mass spectra. Analysis showed that 35 is more constrained and cannot readily accommodate an excess of energy during the mass spectrometry ionization process and, hence, breaks more easily. 

Figure 4. The trefoil knot a closed ring with a minimum of three crossing points. The rings (a), (b) and (c) are topological stereoisomers the two knots (a) and (b) are topological enantiomers.

Figure 21. The equilibrium between the helical interlaced system precursor of the trefoil knot and its face-to-face analogous complex leading to the face-to-face complexes. Interconversion between the two isomeric cyclic products is, of course, not possible. For the cyclic compounds, the total number of atoms x connecting two phenolic oxygen atoms is 16 if n=4 (pentakis(ethyleneoxy) fragment) or 19 if n = 5 (hexakis(ethyleneoxy) linker). Each knot is represented by the letter k accompanied by the overall number of atoms included in the cycle. The face-to-face complexes contain two monocycles (letter m), the number of atoms in each ring also being indicated. It can be noted that each knot has a face-to-face counterpart. For instance [Cu2(k-90)]2+ and [Cu2(m-45)2]2+ are constitutional isomers. They are by no means topological stereoisomers [34, 35].

Topological stereoisomers have identical bond connectivity, but they cannot be interconverted one into another by continuous deformation [51]. They are homeomorphic, not isotopic. Their molecular graphs are non-planar, either intrinsically or extrinsically. 

Fig. 33. a) Demetallation of the dicopper(I) trefoil knot 852 + leading to the trefoil knot 86. b) Two topological stereoisomers trefoil knot 86 and unraveled macrocycle 87... 

We believe it important to stress out that the expression topological isomers should never be used instead of constitutional isomers , since, as we will see later, the expression topological stereoisomers is used only for stereoisomers differing by extrinsic topology... 

Figure 1.17. The four classical stereoisomers of a CTV-Cgo tris-adduct conjugate with an e.e.e addition pattern, and their planar representations. Each structure corresponds to a unique topological stereoisomer.

Figure 1.18. The four classical stereoisomers of a CTV-Cgo tris-adduct conjugate with a trans-3, trans-3,trans-3 addition pattern, and their planar representations. Topologically, (/, ft)-44 is equivalent (can be deformed without breaking any bond) to (M, k)-45, and (M, fC)-44 is equivalent to (/ , fC)-45. It appears that the four classical stereoisomers can be reduced to only two different topological stereoisomers.

Figure 2.2. The trivial knot (single curve, a) and the two enantiomers of the trefoil knot (b and c). All three knots are topological stereoisomers.

Balaban, A.T. (1988a). Chemical Graphs. Part 49. Open Problems in the Area of Condensed Polycyclic Benzenoids Topological Stereoisomers of Coronoids and Congeners. Rev.Roum.-Chim.,33,699-7(n. 

The second major category of isomers and the focus of this chapter are stereoisomers. Being isomers they too have the same number and kind of building block atoms, but, unlike constitutional isomers, they have identical topologies. Stereoisomers, in turn, are divided into two groups enantiomers and diastereomers. Enantiomers are isomers that are not superimposable on their mirror images. And, by definition, diastereomers are all other stereoisomers that are not enantiomers. 

The structures XI to XIII or XIV and XV are topological stereoisomers since they have identical connectivity (homeomorphic), but no continuous deformation will allow them to interconvert (not isotopic). Furthermore, structures XII and XIII are topological enantiomers and the knots XII or XIII and the unknotted ring XI are topological diastereomers. 

Figure 2 Topological stereoisomers (a) the trivial ring and (b and c) the TrefoU knots which are closed rings with three crossing points, b and c are topological enantiomers.

How are these stereoisomers different from conventional diastereomers The circle and the knot can be infinitely deformed— bent, twisted, stretched, and compressed— but they will never be interconverted (as long as we don t cross any bonds). Conventional isomers can be interconverted by deformation, as in the case of 2-butanol in Figure 6.9. Conventional stereoisomerism depends on the precise location of the atoms in space, leading to the terms geometric or Euclidian isomerism. With topological stereoisomers, we can move the atoms all around, and retain our isomerism. 

For each case in Figure 6.14, we have stereoisomers—structures with the same connectivities but differing arrangements of the atoms in space. They are not enantiomers, so they must be diastereomers. The novelty lies in the fact that these stereoisomers interconvert by a translation or reorientation of one component relative to the other. In some ways these structures resemble conformers or atropisomers, which involve stereoisomers that interconvert by rotation about a bond. For the supramolecular stereoisomers, however, interconversion involves rotation or translation of an entire molecular unit, rather than rotation around a bond. Note that for none of the situations of Figure 6.14 do we have topological stereoisomers. In each case we can interconvert stereoisomers without breaking and reforming bonds. 

Clearly, the building of molecules displaying novel topological properties and the isolation and study of their topological stereoisomers are challenges to synthetic chemists. This is especially true for interlocked rings and knots. 

Figure 13. a) Obtention of the free trefoil knot after demetallation, b) 26 and 27 are true topological stereoisomers. 

---