Synthesis of a -Catenane

The assembly of the pseudo-tetrahedral intermediate 2 is clearly the key to the successful formation of the catenane 4. The process is based on the well documented tendency of copper(I), with its symmetrical d configuration, to adopt a tetrahedral co-ordination geometry. Indeed, the copper(I) complex of the parent ligand, 1,10-phenanthroline, is a stable classical tetrahedral species that was first investigated in 1933.  

Studies of the above type have been extended to the preparation of other singlemetal catenanes. These include the synthesis of the nickel(I) and nickel(II) analogues of the above copper(I) catenane 4 both nickel species were characterised by X-ray diffraction. An electrochemical investigation of the nickel(II)/nickel(I) couple indicated strong stabilisation of the nickel(I) state. This has been attributed to the ready adoption of a favourable tetrahedral geometry by the monovalent nickel cation - a geometry not especially favoured by nickel(II) which is most commonly octahedral in its high spin state. 

Larger rings have also been employed for the formation of catenanes. The 56-membered macrocycle 5, incorporating two 2,9-bis(p-biphenylyl)-1,10-phenanthroline fragments, has been demonstrated to form a catenated mixed-ligand complex 6 of type [CuLL ]+ where L = 5 and L is the smaller ring 3.  

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The acyloin condensation was used in an ingenious manner to prepare the first reported catenane (see p. 113). ° This synthesis of a catenane produced only a small ield and relied on chance for threading the molecules before ring closure. 

I was introduced to interlocked rings in 1956 by M.S. Newman, a seminar speaker at Harvard. In informal discussion after the talk he described the proposal of a graduate student at Ohio State, L. Friedman, for a many-step synthesis of a catenane. The final reaction was cleavage of the two bonds connecting the linked and chemically bound rings. 

Fig. 27. Principle of the template synthesis of a catenane. The U fragment is a metal-chelating ligand and the black dot is a transition-metal ion with a pseudo-tetrahedral coordination geometry...

Directional synthesis. In April of 1964 in the laboratory of the Freiburg University G. Schill and A. Liittringhaus succeeded in the directional synthesis of a catenane according to the following chart ... 

In the case of catenanes, the most rationale synthetic strategy is the clipping of a macrocycle onto a preformed one (Fig. 8). A double-clipping procedure can also be used [46]. Figure 9 shows the synthesis of a catenane templated by electron donor/acceptor interactions [45]. Reaction of a... 

It is now more than 15 years ago that our group published the first templated synthesis of a catenane [60-62], following the elegant work previously reported... 

The first templated synthesis of a catenane was published 20 years ago. Since then, synthetic strategies based on various template effects were proposed by several groups. Therefore, modem template strategies give satisfying yields and make these molecules accessible from a preparative viewpoint. How important template... 

In 1994 J. E Stoddart and co-workers, then at the University of Birmingham (England), achieved a remarkable synthesis of a catenane containing a linear array of five interlocked rings. Because the rings are interlocked in the same way as those of the Olympic symbol, they named the compound olympiadane. 

Figure 5.8 A single enchainment stereo isomer is obtained in the acetylcholine templated synthesis of a [2] catenane.

I was intrigued. Mulling over the problem later that evening, and considering the need both to form and to detect catenanes, a statistical approach seemed attractive. Synthesis of a C30 ring from a linear precursor in the presence of an inert C20 cyclic compound should enable threading of some of the smaller rings before cyclization. A C50 product would indicate the presence of an interlocked system. 

The successful synthesis of various catenanes by the strategy depicted in Figure 12 soon brought us to imagine a molecular trefoil knot synthesis by extending the former synthetic concept from one to two copper ions. As shown in Figure 14, two bis-chelating molecular threads (A) can be interlaced on two transition... 

Figure 17. Synthesis of [2]catenane 45 which is distinguished by a solvent-dependent translational isomerism.

Figure 6. Solid-support-based protocol for the synthesis of a quadrilateral. Beginning with the support containing a closed junction, alternate cycles of restriction and ligation are performed, always at the position indicated as 1 . Selection of the target product (triangle, quadrilateral, pentalateral,...) is determined by the point at which one chooses to restrict at site 2, exposing a sticky end complementary to that exposed by restriction at site 1. This action corresponds to a strand switch (eliminating a zero node), of the sort shown in Figure 8, below. This is emphasized by the lines of different thickness with which the square catenane is drawn.

Scheme l Synthesis of a catenate based on a Cu(I)-mediated template effect. The catenane can be liberated by demetallation with cyanide. 

There are two distinct approaches to catenane synthesis the statistical approach, and approaches relying on self-assembly, so-called directed synthesis . The statistical approach relies on the small chance that macrocyclisation may occur while a linear precursor is threaded through a macrocyclic component. Because this is a rather unlikely eventuality, it naturally results in low yields of interlinked product and is chiefly of historical interest. It was this kind of statistical approach that resulted in the first synthesis of a [2] catenane by Wasserman in 1960 (10.64), from cyclisation of the long-chain diester 10.65 while threaded through the annulus of a deuterated C34 cycloalkane 10.66 (Scheme 10.11), 57 Although the overall yield of the catenation reaction was less than 1 %, the existence of the catenane was firmly established. The relatively polar [2] catenane product, along with other polar macrocyclisation reaction products and... 

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