Hydrogen-Bond-Mediated Template Synthesis of Rotaxanes, Catenanes, and Knotanes

Topics in Current Chemistry (2004) 248: 141–200 DOI 10.1007/b99913 © Springer-Verlag Berlin Heidelberg 2004 Hydrogen-Bond-Mediated Template Synthesis of Rotaxanes, Catenanes, and Knotanes Christoph A. Schalley ( ) · Torsten Weilandt · Jens Brüggemann · Fritz Vögtle Kekulé-Institut für Organische Chemie und Biochemie der Universität, Gerhard-Domagk-Strasse 1, 53121 Bonn, Germany [email protected] 1 Introduction . 142 1.1 The Art of Template Design . 142 1.2 Molecules with Mechanical Bonds . 144 2 Cationic Templates: Secondary Alkyl Ammonium Ion/Crown Ether Complexes 146 2.1 Pseudorotaxanes through Ammonium Ion/Crown Ether Complexes . 146 2.2 Multivalency and Oligo-/Polypseudorotaxanes . 153 2.3 Rotaxane and Catenane Syntheses Involving Ammonium/Crown Ether Complexes . 157 2.4 Controlling Mechanical Motion Through Protonation/Deprotonation . 161 3 Neutral Templates: The Amide Template Effect . 163 3.1 Tetralactam Macrocycles as Hosts for Carbonyl Compounds . 163 3.2 Synthesis and Structural Variability of Amide Catenanes and Rotaxanes . 167 3.3 Topological Chirality . 171 3.4 Molecular Machines Based on Amide Rotaxanes . 174 3.5 Trefoil Dodecaamide Knotanes . 176 4 New Topologies through Catenation along the Seam of Hydrogen-Bonded Calixarene Capsules . 181 5 Anionic Templates: The Role of Charged Hydrogen Bonds . 182 5.1 Anion-Templated Synthesis of Rotaxanes and Their Structural Variability . 182 5.2 Deslippage as a Precision Tool for the Measurement of Steric Size . 186 5.3 Anion-Induced Mechanical Motion . 189 6Epilogue . 191 References . 192 Abstract The synthesis of rotaxanes, catenanes, and trefoil knotanes can only be efficiently accomplished, when a suitable template effect is used that provides a means either to thread a string-like molecule through a macrocycle or to wrap an open macrocycle around a string. This overview is devoted to the template effects for the synthesis of mechanically bound species which are mediated by hydrogen bonding. Three major classes can be distinguished by the charges involved: (i) cation-mediated template effects utilizing the formation of crown ether complexes with secondary ammonium ions, (ii) recognition of neutral amides by 142 C. A. Schalley et al. oligolactam macrocycles, and (iii) threading through hydrogen bonding to anions. The basic idea which we put forward in this chapter is that none of the template effects found so far and discussed here is “traceless”in the sense that they would not leave functional groups behind in the final rotaxane, catenane, or knotane structure. Consequently, the variability of the structures available through these template syntheses is limited in this respect. Also, the properties of the mechanically bound species are intimately interconnected with the template effect used for their synthesis. However, what appears here as a drawback may also be an advantage, when the functional groups used for templating can be utilized to control the properties of the final product which hopefully can in future be exploited for the devel- opment of new materials.We aim at providing sufficient examples in order to strengthen this point in throughout the article. Among the properties and functions discussed are the realization of molecular machines, topological chirality, and the stability of rotaxanes as determined from deslipping experiments. Keywords Catenanes · Hydrogen bonding · Knots · Mechanical bond · Molecular machines · Molecular recognition · Rotaxanes · Template synthesis · Topology 1 Introduction 1.1 The Art of Template Design It is almost impossible to give a concise definition of the term “template”which accounts for all chemistry to which it has been related [1–6]. Templates span the whole range from biochemistry with its complex apparatus for DNA replication [7] to biomineralization and the formation of structured inorganic materials [8] to the covalently templated synthesis of macrocycles [9] to the preparation of supramolecular catalysts [10] or host-guest complexes [11] and to metal- organic reactions for the synthesis of small ring systems [12]. Nevertheless, all these have in common that a template must serve different purposes. (i) It organizes reaction partners in such a way that a desired product is formed that would not form as easily in the absence of the template. Thus, a template con- trols reactivity through organizing the reaction partners in a favorable arrangement producing form. (ii) In order to achieve this, the template needs to bind to the reaction partners. Molecular recognition is thus a necessary prerequisite for template syntheses and the binding sites of the components must be com- plementary to each other. (iii) The control of reactivity and the recognition of the reaction partners imply that information is stored in the template and transferred to the product of the reaction. The third important aspect is thus information transfer. In fulfilling these three tasks, templates help to tame molecular complexity and provide access to otherwise inaccessible products. There are different ways to categorize templates (Fig. 1). One could, for example, try to distinguish template effects according to the (non-)covalent interactions involved. This classification has the disadvantage to be ambiguous Hydrogen-Bond-Mediated Template Synthesis 143 Fig. 1 Schematic classification of templates according to their topology. The template is shown in gray,the reaction partners in black. Dotted lines represent non-covalent bonds, gray sticks are new covalent bonds formed between the reaction partners in the templation step for templates which operate via several different types of forces. The template effect could, for example, involve hydrogen bonds and p-donor p-acceptor interactions making it difficult to decide to which category it should be assigned. Maybe, a better way is to classify the template according to its topography. The early template syntheses of crown ethers utilized a central metal ion around which a macrocycle formed with a certain size selectivity from wrapped- around open-chain precursors [13]. Such templates could be called convex, because of the convex surface of the cation mediating the template effect. In contrast, a receptor which binds two reaction partners inside a cavity and brings them to reaction is concave. This is true for many templates leading to mechanically interlocked species. One of the most prominent natural templates, i.e. single-stranded DNA, could be called a linear template according to this classification and a surface on which molecules self-assemble into an ordered array [14] may be coined a planar template. A beautiful example of a planar template is a recently reported electropolymerization using a liquid crystalline phase as the template [15]. Another important point is to distinguish between a reactant, a template, and a catalyst [16]. A strict definition would imply that the template must be removable after successfully mediating a certain reaction. Instead, a reactant becomes part of the product in one or another form. However, we will see that definitions become blurred, when discussing the synthesis of rotaxanes, catenanes, and knots. Many of the methods for the preparation of interlocked molecules are based on macrocycles which bind part of the other component in a pseudorotaxane fashion. This part is then reacted so that the desired species, e.g., a catenane or a rotaxane, is formed. The macrocyclic template thus finally becomes part of the product and might consequently be considered as a reactant rather than a template. Nevertheless, these methods are widely accepted as template syntheses in the chemical literature, although the use of removable 144 C. A. Schalley et al. transition metal ions for the synthesis of mechanically interlocked molecules [17–20] is the only true template synthesis in the strict sense of above. It is similarly difficult to separate templates from catalysts: On one hand, there are templates which do not promote catalytic reactions, because the template is bound too strongly to generate turnover. They have to be removed from the product by separation techniques and need to be used in stoichiometric amounts. On the other hand, there are catalysts which do not organize the reactants in space but rather change their intrinsic reactivity, e.g., general acid or base catalysis. Thus, they cannot be regarded as templates. These are the clear-cut cases. However, mixed forms exist, where a template is bound re- versibly to the product or where a catalyst organizes the reactants with respect to their geometry. All definitions of “template” necessarily suffer from this continuous transition from reactants to templates and to catalysts and we should keep in mind that a distinction between these categories is not always straightforward. We therefore put forward a more abstract definition of a template which is based on its ability to organize the reactants appropriately through covalent or non-covalent bonding interactions. In our definition, a template is not a defined parti- cle, i.e., an atom or molecule, but rather the sum of all connections between the species reacting with each other which are involved in geometrically controlling the reactivity in the desired way. It is the array of interactions and their spatial arrangement that count. This brings us to one of the central ideas of this account. Once we accept that templates may not be “traceless”, but leave behind a memory of their action in that functional groups remain in the final product

Load more