Catenanes

Cyclodextrin catenanes were synthesized for the first time by Stoddart s group in 1993 [66, 67] (Fig. 12.7). The molecular thread consisted conceptually of three parts, a hydrophobic aromatic core bearing on either side hydrophilic arms ending with amino groups that were dipped together by reaction with terephthaloyl chloride as in 13. 

The architectural exquisiteness of the products was high but the isolated yidds of the pure compounds were very low. Not deterred by harvesting only small amounts, the group studied in detail the structural and conformational properties of these compounds and proved that [2j- and [3]catenanes were produced (with one and two CyD rings, respectively), exhibited orientational isomerism ([2]cate-nanes) and head-to-tail/head-to-taU and head-to-tad/head-to-head isomerization 

Although preparation of cydodextrin catenanes of polymethylene derivatives and polycatenanes has been daimed, no data are furnished [68]. Since 1993, the only catenane example essentially reported has been the in situ formation of a fi yclo-dextrin [2]catenane that was characterized in solution by NMR experiments (Fig. 12.9) [69]. A self-assembled macrocyle, through Pd(II)(en) complexation of two threads, forms the [2]catenane 14 with -CyD at appropriate concentrations but the [3]rotaxane 15 with the wider y-CyD. Mass spectral characterization has also been provided for both spedes but none has been isolated. 

Extensive search in the literature has not produced any other cydodextrin catenane. Apparently the difficulty of isolation of such stmctures in meaningful yields, espedally when involving native CyDs, has discouraged further attempts. 

Dodziuk, Introduction to Suprantolecular Chemistry, Wuwer, Dordrecht, 2002, Sects. 2.3 and 8.2. 

Very few examples have been reported of [2]-catenanes. The compounds consist of two interlocking rings, each incorporating a porphyrin moiety either as an integral part of the ring or as a hanging subunit.  

Interestingly, the average planes of both porphyrins in 117 are parallel to one another. However, little or no eicctronie interaetion exists between the porphyrins, as shown by their absorption speetroscoiry properties, which are consistent with a large spatial sepai ation.. Space-tilling 

An obvious choice for D and A is to use a zinc(ll) and a gold(lll) porphyrin respectively since these components have proven particularly useful for the study of photo-chemically induced electron transfer.  

Treatment of the inacrocycle 118 with Cu(CHiCN).,-(PF(,) followed by threading of the diiodide 119 results in the formation of the precatenate 120 in quantitative yield. This comirlex was combined with the zinc(ll) porphyrin 121 

The broad landscape of chemical topology and topoisomerism has been summarized in comprehensive reviews [2-5], The accomplishments of Schill, Walba, Sauvage, Stoddart, and others are landmarks in organic synthesis. This chapter describes a personal odyssey in which the focus is on statistical approaches - tinged by polymer science in their continual reference to the flexibility of chains. Some early laboratory efforts, and the technical considerations which led to them, are discussed, as is more recent activity. 

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. 

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 tetraazamacrocycle and the cobalt-alkyl-macrocycle are interlocked, but the above species is not strictly a catenane since there are cobalt-nitrogen coordinate bonds forming a chemical linkage between the two rings. 

Almost all metal template-based syntheses of catenanes use Cu , but there are a few instances of other metal ion templates, including Zn +, Pd, and Pt +. The relative substitution-lability of Pd + makes it suitable as a preparative template the substitution-inertness of Pt + has been used as the basis for a temperature-switchable catenane molecular lock [257]. This mode of switching is based on platinum-nitrogen bond dissociation at elevated temperatures. 

Chirality may be introduced into catenanes of the cyclophane-polyether type by replacement of the / -phenylene unit in the starting material LI089 with a chiral entity such as LI 139 [279] or LI 140 (Eq. 4.82) [280]. 

Early attempts to develop this particular route ran into difficulties over the problem of non-availabihty of starting materials. However it was discovered by 

Multi-ring catenanes may also be prepared by the cyclophane approach. The polyether LI 160 (Eq. 4.89), the trinaphtho-analogue of L1094, is a large ring and 

---

Another synthetic strategy is based on self-assembly driven by molecular recognition between complementary TT-donors and 7T-acceptors. Examples include the synthesis of catenanes and rotaxanes that can act as controUable molecular shuttles (6,236). The TT-donors in the shuttles are located in the dumb-beU shaped component of the rotaxane and the 7T-acceptors in the macrocycHc component, or vice versa. The shuttles may be switched by chemical, electrochemical, or photochemical means. 

Catenanes are formed when two or more closed-circular DNAs are linked together to form a chain. Catenanes were first isolated in human mitochondrial DNA and have since been identified in a number of biological systems. These stmctures often occur as intermediates during the repHcation of circular DNA molecules. 

Both catenanes and knots can bring together remote DNA sequences and may be important in transcription regulation and genetic recombination... 

Catenanes and rotaxanes including macroheterocyclic fragments 99T5265. 

Template synthesis and chirality of catenanes, rotaxanes, and pretzelanes including N-macroheterocyclic lactams and related compounds as structure components 99PAC247. 

Intramolecular mobility of metal complexes of rotaxanes and catenanes with macroheterocyclic fragments 98ACR611. 

Nonionic template synthesis of amide-linked catenanes and rotaxanes with macroheterocyclic fragments 97AG(E)930. 

Self-assembly in construction of pseudorotaxanes, rotaxanes, and catenanes with crown ether fragments 98PAC419. 

Self-assembly of [2]catenanes containing metals in their backbones 99ACR53. 

As expected, the yields of catenanes by this approach are low, which is why improved methods for the preparation of such compounds have been developed. The acyloins are often only intermediate products in a multistep synthesis. For example they can be further transformed into olefins by application of the Corey-Winter fragmentation. 

Although the number of applications of olefin metathesis to transition metal complexes is small compared to the number of applications in organic synthesis, this field is becoming increasingly important. Spectacular examples are the double RCM reactions of copper phenanthroline complexes as a synthetic route to catenanes [113] or a recently reported approach to steric shielding of rhenium complex terminated sp-carbon chains [114]. 

When the reaction of two compounds results in a product that contains all the mass of the two compounds, the product is called an addition compound. There are several kinds. In the rest of this chapter, we will discuss addition compounds in which the molecules of the starting materials remain more or less intact and weak bonds hold two or more molecules together. We can divide them into four broad classes electron donor-acceptor complexes, complexes formed by crown ethers and similar compounds, inclusion compounds, and catenanes. 

These compounds contain two or more independent portions that are not bonded to each other by any valence forces but nevertheless must remain linked. Catenanes are made up of two or more rings held together as links in a chain, while in rotaxanes a... 

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. 

For a monograph, see Schill, G. Catenanes, Rotaxanes, and Knots Academic Press NY, 1971. For a review, see Schill, G. in Chiurdoglu Conformational Analysis Academic Press NY, 1971, p. 229. 

This molecule has no chiral carbons, nor does it have a rigid shape, but it too has neither a plane nor an alternating axis of symmetry. Compound 32 has been synthesized and has, in fact, been shown to be chiral. Rings containing 50 or more members should be able to exist as knots (33, and see 37 on p. 114 in Chapter 3). Such a knot would be nonsuperimposable on its mirror image. Calixarenes, ° crown ethers, catenanes, and rotaxanes (see p. 113) can also be chiral if suitably substituted. For example, A and B are nonsuperimposable mirror images. 

For a review of chirality in Mobius-strip molecules catenanes, and knots, see Walba, D.M. Tetrahedron, 1985, 41, 3161. 

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. 

The template-directed preparation of cycloi is(paraquat-4,4 -biphenylene (a molecular square ) has been achieved the use of a macrocyclic hydroquinone-based polyether template incorporating an ester moiety in one polyether chain afforded a 1 1 mixture of two topologically stereoisomeric [3]catenanes <96CEJ877>. 

Schalley CA, WeUandt T, Briiggemann J,V6gtle F (2004) Hydrogen-Bond-Mediated Template Synthesis of Rotaxanes, Catenanes, and Knotanes. 248 141-200 Scheer M,see Balazs G (2003) 232 1-23... 

---