Crossed -catenane

The simplest class of catenane involves two interlocked rings, as depicted in Fig. la. Such singly-interlocked species are termed 2-crossing catenanes and are formally given the nomenclature 2-crossing-[njcatenane, where... 

Figure 6 Oligocenter double-stranded helices and the molecular knots and catenanes derived from them by appropriately joining of the termini in the helical strands. An integral number of turns lead to knots and additional half turns give multiply crossed catenanes.

To conclude this chapter, we will extend our investigation of metal-ion control over topology to the tying of molecular threads into knots. We commence by returning to some of our ideas about topology. The representation of a catenane in Figure 7-46 emphasises that a two-dimensional graph contains two points at which lines cross. 

Figure 7-46. A cartoon view of a catenane. The view emphasises that it is impossible to draw a catenane in two dimensions without a minimum of two points at which lines cross. The crossing points are indicated by arrows.

Figure 7-47. Two cartoon views showing the formation of a catenane from a difunctional molecular thread. The first view emphasises the approach that we discussed in Section 7.5. The second view emphasises the formation of the precursor and the catenane in terms of the number of crossing points which must be drawn in the molecular thread.

The ring topology is the potential to form unique polymer structures. Like linear polymers, cyclic polymers not only can be branched or cross-linked, but also can form non-covalently linked structures based on their loop topology. These are referred to as topological polymers, including rotaxane, catenane, threaded rings, and rings threaded by network chains. Recently, much attention has been paid to how their particular properties not only differ from linear polymers, but also how they differ from a component of an interlocked polymer system, such as polycatenanes and polyrotaxanes. 

The synthesis of the first doubly interlocked [2]-catenane, based on the three-dimensional template effect of Cu+I, has been completed in principle, helices containing an odd number of metal centers lead to even numbered crossing [2]-catenanes [94JA375]. 

Chemical catenanes are modeled by topological links. A topological link is a finite union of mutually disjoint knots (including the unknot). A knot is therefore the special case of a link with only one component. Links are nontrivial if and only if they cannot be embedded in the plane without crossings. All the links referred to in this chapter are nontrivial, but the components are usually unknots. 

Finally, it needs to be noted that while a simple [2]-catenane can be made topologically chiral by orientation of the rings - see 83, a four-crossing [2]-catenane such as 77 is automatically chiral without the need for such orientation. 

Figure 6.30 Stepwise strategy for obtaining a A-crossing-[2]-catenane of type 81 (top). The phenanthroline-containing fragments employed (centre) and three products from the reaction (bottom)...

It needs to be noted that a trefoil knot is only the first member of a potential series of related molecular knots.If the number of metal centres employed in generating the associated helix is even, then the number of crossing points will be odd, and a single, closed knotted structure will be formed (trefoil, pentafoil, heptafoil, etc.). On the other hand, if the number of metal centres is odd, then an even number of crossing points will result, giving rise to multiply-interlocked [2]-catenanes. 

Catenanes have a nonplanar molecular graph, which means that it will be impossible to draw their backbone on a sheet of paper without introducing crossing... 

Figure 2 Pass Ale types ofpolymers based upon catenane architecures A Linear mechanically-linked polymers B mechanically cross-linked polymers C stars with a flexible, potentially switchable, catenane core D combs with catenarw linkages E main chain catenanes with switchable surface properties F highly mechani-cally-crosslinked networks G poly- or [n]catenanes. Only examples of types A and F have thus far been prepared...

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