In general, two main strategies have been explored to synthesize rotaxanes (Scheme 14.1 ): the fi rst one is the double -stoppering approach. After threading of a string -like fragment through a ring using non -covalent interactions, a double -stoppering reaction produces the desired rotaxane. Usually, this method leads to the formation of rotaxanes with modest or low yields, especially if the precursor is not very stable under the reaction conditions. To limit the unthreading reaction leading to dissociation of the precursor, a reasonably effi cient strategy has been used which consists in a stepwise approach. It comprises three steps: (i) attachment of a fi rst stopper to the string, followed by (ii) the threading reaction, and, fi nally, (iii) fi xation of the second stopper to the thread. This procedure is well adapted when the pre -rotaxane is sensitive under the relatively harsh reaction conditions used to attach the stoppers, since a mono -stoppered precursor is less prompted to unthreading than a non -stoppered one. On the other hand, the double -stoppering approach can be very effi cient in case of stable and robust pre -rotaxanes or mild reaction conditions which do not cause detrimental unthreading.
Until recently, azides were relatively rare precursors of interlocked molecules and most of the time they were meant to be linked to alkynes using 1,3 -dipolar cycloaddition. In this review, we will restrict ourselves to their use in attachment of stoppers to pseudorotaxanes and present a selection of the most representative papers. They can be divided into purely organic molecular entanglements and into interlocked molecules prepared by transition metal templated approaches.
Huisgen ' s cycloaddition between azides and alkynes 43,44 is typically carried out at high temperature, at which unfortunately, labile molecules may not survive. Another reason for the scarcity of azides in rotaxane and catenane synthesis is the lack of regiospecifi city of this reaction, which provides both 1,4 -and 1,5 -substituted 1,2,3 -triazoles. Despite these drawbacks, a few interlocked molecules were prepared by Stoddart and co -workers by this uncatalyzed strategy using a symmetrical bulky alkyne (see Section 14.2.2 ). By contrast, the catalyzed versions of 1,3 -dipolar cycloaddition are highly regiospecifi c and benefi t from mild reaction conditions and excellent yields. The use of cucurbituril as catalyst will be presented in Section 14.2.1 . The so -called ' click chemistry ' reaction, 45,46 based on Cu(I) -catalyzed cycloaddition, discovered independently by Meldal 47,48 and Sharpless, 49 represents a promising possibility as stoppering reaction of the preliminary prepared pseudo -rotaxane. A limited number of such reactions has been recently reported, 32,33,50 -58 a particularly impressive case being that of a copper -complexed rotaxane for which the copper(I) center is used both as template and catalyst. 32,33 Scheme 14.1 Schematic representation of possible strategies to prepare rotaxanes : (i) double -stoppering approach; (ii) stepwise approach. The dotted lines represent attractive interactions between given fragments belonging to the ring and to the thread respectively