Rotaxanes ammonium salts

As will be discussed later, ammonium salts have also been used as hydrogen bond-donor templates to favour the formation of macrocycles (yielding pseu-dorotaxanes and rotaxanes). 

The binding of ammonium ion, NH4+, by 18-crown-6 was demonstrated early in the history of macrocycle chemistry. The O-H-N in crowns or N-H-N interaction in azacrowns has been characterized by a variety of techniques, including NMR, calorimetry, and X-ray crystal structure analysis. Recent studies in this area have shown that both quaternary and secondary ammonium salts can form complexes with crowns. In the latter case, a rotaxane molecule was prepared by treatment of a dibenzylammonium salt with dibenzo-24-crown-8 and other macrocycles, including pyrido-24-crown-8. The solid-state structure of the pseudo-rotaxane structure obtained with pyrido-24-crown-8 is shown in the left panel of Figure 23. 

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). 

In this type of rotaxane, the intramolecular hydrogen-bonding interaction between ammonium salt and crown ether is too strong to neutralize the ammonium part with usual bases. In fact, Takata et al. found that the rate of proton-exchange between [2]rotaxane and water was far slower than that between pyrrole and water, suggesting the extremely lowered acidity of the ammonium moiety [74]. However, the ammonium group can be quantitatively acylated by the excess amount of electrophile in the presence of excess triethylamine. Some oligorotaxanes can be prepared by this process (Scheme 12). 

Dipolar cycloaddition reaction of propargyl ammonium salt and 2-azidoethylammonium salt is greatly enhanced in the presence of cucurbituril because the hydrophobic cavity of cucurbituril is an excellent reaction field for the cyclocondensation [175]. Mock et al. obtained [2]rotaxane consisting... 

Stoddart and co-workers reported the attempt to construct a supramolecu-lar poly[2]rotaxane (pseudopolyrotaxane) in 1998 [211]. They synthesized a self-complementary (plerotopic) monomer consisting of a 24-crown-8-ether moiety and a secondary ammonium salt group (Scheme 41). Upon recrystal-... 

Synthesis of polypseudo[3]rotaxane was first achieved by Gibson s group (Scheme 9). Since pseudorotaxane formation between 24-membered crown ether wheel and dibenzylammonium axle is an equilibrium process, the process is essentially controllable by several parameters such as concentration, temperature, and solvent polarity. They showed that bis(24-crown-8-ether) and bis(dibenzyl-5ec-ammonium salt) formed a 1 1 cyclic dimer predominantly... 

Rotaxanes are mechanically interlocked molecules constituted of a trapped macrocycle around a suitable sterically hindered molecule at both ends. Thus, the macrocycle 34 can be easily prepared from 33 by RCM, but in the presence of ammonium salt 35, the same RCM reaction affords 73% of the rotaxane 36 in which the ammonium ion with two bulky end groups is trapped into the macrocycle (Scheme 16, left-hand route) [46]. 

On a simpler scale the Rybak-Akimova group found that when 4/-(amino-methylene)benzo[18]crown-6 was treated with an acid salt, mass spectrometric evidence indicated that dimers were the most stable species [13]. This was backed up by an X-ray crystal structure of the compound crystallized from methanol which revealed pairs of mutually interlocked crowns. The amine termini had become pro-tonated and the resultant ammonium group, illustrated in Fig. 8.4, formed a complex with a second crown. Although yet to be attempted, it would be intriguing to react the complex with a linear component containing a second amine and a bulky stopper group to generate a metal-free, interlocked pair of rotaxanes with two amine stations . The contraction and extension of the complex could then be controlled as a function of pH. 

---