Phenolate, rotaxane

Vogtle has developed this approach further and employed a series of anionic templates to prepare rotaxanes (instead of the neutral template in the above reaction) [65-67]. In this approach a phenolate, thiophenolate or sulfonamide anion is non-covalently bound to the tetralactam macrocycle (46) forming a host-guest complex via hydrogen bonding (see Scheme 21). 

Scheme 22 The reacting phenolate interacts with the wheel of the rotaxane by hydrogenbonding... 

Scheme 23 Synthesis of [2]rotaxane 48 employing centrepieces bearing a phenolate and to remote sites for stopper attachment...

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. 

Figure 51. Anionic template synthesis of rotaxane 118 with a bis(phenyl ether) axle. The mechanical bond is formed by reaction of the phenolate-macrocycle complex [32-115] (supramolecular nucleophile) with the semiaxle intermediate 117. 

The first problems with the new anion-tcmplated rouuane synthesis were encountered when the preparation of rotaxanes with smaller 3,5-dw-butyl phenol stoppers was attempted (Scheme 17). These rot xanca were intended for the investigation of their deslipping behavior 9 48 which should provide insight into the size complementarity of the stopper mid the wheel cavity. For this purpose, smaller stoppers are needed and a yield of 2% to 5%, which was obtained for 44 independent of the length of the axle centerpiece, was thus quite disappointing. 

The most recently discovered template effect is the one that makes use of anions. Vogtle and coworkers [12] have found that a phenolate equipped with one stopper can bind in the cavity of the tetralactam macrocycle by two strong hydrogen bonds. Then this nucleophile complex is reacted with an electrophilic semi-axle to obtain rotaxane in very high yields up to 95%. 

To overcome this difficulty, a new solution is developed that involves spatial separation of the functional group for stopper attachment from the phenolate that provides the basis for anion template. Rotaxanes can be synthesized with yields up to 45% [32] (Figure 11). 

Because earlier reviews deal explicitly with these template effects [4, 7, 12], the following discussion is restricted to a recently discovered anion template effect (Scheme 6.5.3). fn non-competitive solvents, the tetralactam macrocyde strongly binds anions such as chloride, bromide, or phenolate. Phenolate stoppers bound to the macrocycle can act as wheeled nudeophiles and react through the wheel with a semi-axle generated in situ to yield ether rotaxanes in yields of 57% to 95% [13]. Consequently, the macrocyde not only represents an anion receptor, but as a concave template simultaneously provides the correct orientation of the guest for threading the axle into the wheel. 

It should be mentioned here that very interesting constitutional and translational isomerism is observed in the series of catenanes and rotaxanes which contain phenol derivatives such as macrocycUc phenylene-crown components as well as phenolic polyether chains (see also Lehn s recently published book ). 

Polymerized [2]rotaxanes were claimed as good chemical vapour sensors [75]. In thin film form they were sensible to phenol vapours as well as to other H-bond donors such as p-nitrophenol or 2,2,2-trifluoroethanol. The observed phenomena was reversible and resulted in fluorescence quenching accompanied by a slight bathochromic shift. It was also found that the polymers were apt to metal bonding due to the presence of the tetrahedral pockets. This fact manifested itself in the appearance of an additional absorption band. The sensitivity of a given polymer thin film was proportional to the film porosity defined by steric properties of the R-substituant. The studies and applications of these molecules are just at the beginning. 

Scheme 6 Schematic representation of the anion templated synthesis of rotaxanes based on tetralactam macrocycles and a phenolate anions...

Vogtle and coworkers employed anion complexation in new strategies for high-yielding rotaxane syntheses. These workers discovered that phenolate anions, when bound to tetralactam macrocycles (Fig. Ij) such as 24. are capable of reacting with acid chlorides and therefore may be used to form ester rotaxanes. A variety of different rotaxanes were synthesized via this method, including systems with carbonate and acetyl axles. 

Fig. 2 Template syntheses of rotaxanes the Cu(I) ion binds a phenanthroline ligand inside a macrocycle (top left). A l f5-paraquat macrocycle is clipped around an axle bearing a hydroquinone center piece (top right). Hydrogen bonding permits the use of nonionic template effects for the preparation of amide-type rotaxanes (bottom left). Phenolate anions bound to the macrocycle react as a supramolecular nucleophile (bottom right).

Fig. 4 Sauvage s template composed of two molecular turns bound to a copper(I) anchor phenolic functions were reaction sites for ring closures and catenane or rotaxane formation.

In the most recent iteration of the feedback loop which relates molecular structure to solid-state device performance, the fast bistable [2]rotaxane 11" has been superseded " by the modified and improved version shown in Fig. 1.15, namely, the [2]rotaxane The impetus for this iteration was at least two-fold (1) to produce a molecular switch in which the bistable rotaxane starts off exclusively in one, and only one, of its two possible co-conformations and (2) to incorporate into this all-or-nothing switch enhanced oxidative stability by removing all of the phenolic residues from the hydrophilic stopper. The rotaxane, which was obtained (Fig. 1.15) by the now well-established templation protocol, has been incorporated by the familiar fabrication procedures into an 8 x 8 crossbar device (Fig. 1.16(a)). Fig. 1.16(b) demonstrates one of these 64-bit devices in operation with the exception of one of the wires, i.e., 56 out of the 64 bits can be switched. This device has been used to store (Fig. 1.16(c)) the letters DARPA in ASCII code the lifetime of the switch-closed state of this devices is about 10 minutes. The proposed electromechanical switching mechanism for the bistable [2]rotaxane which is outlined in Fig. 1.17, merits comparison with that already discussed in Fig. 1.16 for the bistable [2]rotaxane In the case of it is proposed that oxidation of the TTF unit to its radical cation (TTF ) occurs when a +2 V bias is applied across the bistable [2]rotaxane and the tetracationic cyclophane moves to encircle the... 

To exploit this ion pair receptor for rotaxane formation, Smith utilized a discrete potassium template bound within the crown ether group of 17 to aid the association of an anionic phenolate half-axle 19 in the adjacent isophthalamide cleft (Scheme 10.10). Successive esterification reactions between this complex, isophthaloyl dichloride (18), and a further equivalent of the half-axle 19 are necessary to form a fully stoppered axle component. The templated arrangement promotes formation of this axle within the macrocycle, yielding uncharged rotaxane 20. 

The cooperative behaviors of both the anionic and cationic templates are vital for efficient interlocked molecule formation. The anion templation component of this approach, in which the phenolate anion is held within an isophthalamide macrocycle and consumed in the reaction, was pioneered by Vogtle in 1999. " However, the macrobicycle design and presence of the bound alkali metal enabled rotaxane 20 to be synthesized in the polar solvent medium of 5 1 THF DMF in a yield of 20%. Furthermore, the direct templating action of the potassium was demonstrated by the fact that no equivalent reaction occurred in its absence or with the larger cesium cation that was not able to bind efficiently in the crown ether. 

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