Rotaxane formation

With this separation of the hydrogen bond-acceptor template and the sites for the stopper attachment, the steric effects due to shielding by the wheel are avoided making the synthesis of the rotaxane 48 more efficient (in some of the systems the yields of rotaxane formation increase from 5 to 30% when the functionalised centre-piece is added). 

Active template methodology. In this method, recently explored by Leigh et al. [16, 17], the rotaxane is formed via synthesis of the guest molecule in the presence of the macrocyclic host, where the guest ions or organic molecules play an active template role in promoting rotaxane formation. 

Analysis of the available literature data on host-guest complexes based on cyanine and styryl dyes with CDs shows that rotaxane formation in general... 

The clipping reaction used in [52, 53, 55] to synthesize tetralactam-based squaraine rotaxanes such as 14 and 15 afforded only moderate yields (ca. 28-35%) of the rotaxanes, possibly because of the unavoidable presence of nucleophiles that react with the chemically unstable squaraines during the reaction. The slippage approach [62] minimizes the squaraine dye s contact with nucleophiles during the rotaxane formation process and therefore can be used to efficiently encapsulate a squaraine dye such as 23 in a macrocycle such as 25 [63],... 

The corresponding macrocycle, intended to be a host for carbon dioxide, was not detected, but was synthesized in 1996 by an elegant strategy via rotaxane formation (see Section 8.3, Amide-Based Rotaxanes) [25]. 

Figure 20. No rotaxane formation is detected when the building blocks 46, 47, and 3 are subjected to a clipping process.

Depending on the functionality of the stopper (amine 52 or phenol 71), the reaction affords rotaxanes with urea bonding pattern 72-74 or with carbamate units 75. In each instance the temperature had to be raised to 40°C to ensure rotaxane formation and, interestingly, the free axle 76 was the only one to be isolated. This indicates that the low solubility of the reactants and the dumbbells can be the limiting factor in rotaxane synthesis when diisocyanates are incorporated. Molecular recognition is still effective, and again the decisive pattern for molecular aggregation seems to be the initial amide bond. Furthermore the macrocycle confers solubility on the thread. 

In contrast to this, the enlargement of the macrocycle by methylene groups (homologation) has a more dramatic effect on rotaxane formation [4d,e]. When the CH2-expanded macrocycle 81 was reacted with 56m and 52 the [2]rotaxane 82 was obtained in 17% yield (Figure 31) [38, 53]. 

Another approach to test a template mechanism is to use appropriate control compounds that can be expected to fail to yield the rotaxanes or catenanes, if the tentative mechanism is indeed operative. In order to gain further support for the template mechanism of amide rotaxane formation (Scheme 10), it is desirable to have additional evidence for the... 

SCHEME 14 AXLE CENTERPIECES 30 AND 31 THAT WERE USED AS CONTROL COMPOUNDS FOR A MORE DETAILED INVESTIGATION OF THE MECHANISM OF AMIDE ROTAXANE FORMATION. 

Figure 10.3 Examples of [2]rotaxane formation via hydrogen bond-templated macrocyclization reported by Leigh and coworkers.

Cantrill SJ, Rowan SJ, Stoddart JF (1999) Rotaxane formation under thermodynamic control. OrgLett 1 1363-1366... 

Other cyclic molecules can be also used for rotaxane formation. Figure 3.20 shows a rotaxane created via the molecular recognition of an ammonium salt by a crown ether. Cyclophane-based rotaxane has also been reported (Fig. 3.21). 

The authors also studied pset/do-rotaxane formation of CDs with aliphatic polyesters, polydiene, polysiloxane, and polysilane, as summarized in Table 2. CDs were found to form inclusion complexes with aliphatic polyesters, such as PTA, PBA, PEA [107], and P(e-CL) [108,109]. a- and y-CDs formed complexes with these polyesters in high yields, although p CD gave complexes in moderate yields. Although the yields of the complexes of a-CD with PTA and... 

Ohata showed that the yield of rotaxane formation could be increased up to 19% when the smaller a-CDX was used [17], Employing shorter (Cg and C o) or longer (Cl4) methylene chains lowered the 5delds stoppering was hampered for steric reasons. When long chains were used, steric crowding was the consequence of threading of more than one CDX unit. 

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