Rotaxanes transition metal-templated synthesis

Rotaxanes, Transition-Metal-Templated Synthesis of (Blanco, Chambron,... 

This chapter will be concerned mainly with the transition-metal-templated synthesis of rotaxanes. General features of these compounds, including stereochemical issues, will be addressed at first. The intent, however, is not to... 

Figure 2.24. Schematic representation of the principle of transition-metal-templated synthesis of [2]-rotaxanes from macrocyclic chelate (A), metal cation (black disk) and open chelate (B). The latter bears functions X at its extremities, which will be used for anchoring or constructing the stoppers (represented as diamonds). (/) Threading step (//) stoppering step (in) removal of the metal template.

PART II TRANSITION METAL-TEMPLATED SYNTHESIS OF ROTAXANES... 

The precatenate intermediate can be regarded as a threaded complex, which is thermodynamically stabilized by coordination bonds, from this simple consideration, a transition metal-templated synthesis of rotaxanes was devised, the principle of which is shown in Figure 28 [117]. The threading step (i) is a complexation reaction. 

Figure 51 Principle of transition metal-templated synthesis of a [3]-rotaxane, from two chelating macrocycles (B) and a bis-chelate-containing molecular thread (A) functionalized with reactive end groups X (same conventions as in Figure 43). (ii) Threading step, affording prerotaxane (C) construction of the porphyrin stoppers providing copper(I)-complexed [3]-rotaxane (D).

Fig. 11 Principle of transition metal-templated synthesis of a [2]rotaxane. A thick line represents a dpp chelate, a black dot represents a metal cation, a hatched diamond represents a Au(III) porphyrin and an empty diamond represents a Zn(II) porphyrin. The transition metal controls the threading of Au(III) porphyrin-pendant macrocycle (A) onto chelate (B), to form prerotaxane (C). Construction of the porphyrin stoppers at the X functions leads to the metal complex [2]rotaxane (D). Removal of the template cation forms the free rotaxane (E)...

The earliest syntheses of rotaxanes were largely based on the statistical or directed methods.2,3 Statistical methods require very precise reaction conditions, and directed methods involve numerous chemicals steps. However, the use of templates allows high control of these synthetic methods resulting in efficient and precise assemblies of rotaxanes that incorporate a wide range of chemical functionalities. Two types of interactions occur in synthetic template methods (1) purely organic and (2) transition-metal-templated. In this latter case, the template can easily be removed at the end of the synthesis, whereas in the former, the interactions between the template and the components of the final rotaxane will often be maintained. Selected examples will now illustrate the statistical, the directed, and templated strategies outlined above. The transition-metal-templated route will be developed separately. 

The scope of this chapter was to present a rationale of the synthetic methods used to construct rotaxanes, with particular emphasis on the transition metal-templated route. The key step is to assemble a molecule which can be described as composed of a macrocycle threaded onto a linear molecular component. Therefore, the problem of rotaxane synthesis was mainly the problem of threading at the molecular level. 

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