Rotaxane reaction

Subsequently, further rotaxanation reactions, shown in Table 14, were developed, offering several advantages. A high reactivity of the functional chain end with the stopper reagent is very important because dissociation of threaded rings occurs during the reaction, especially when the reaction is performed in an organic solvent like DMF or DMSO, in which the ICs are not stable. 

In the first paper on arenediazonium salt/crown ether complexes, Gokel and Cram (1973) mention that they were not able to synthesize the rotaxane 11.14 by an azo coupling reaction of the complexed diazonium ion with Af,Af-dimethylaniline. 

More recently, Kim et al. synthesized dendritic [n] pseudorotaxane based on the stable charge-transfer complex formation inside cucurbit[8]uril (CB[8j) (Fig. 17) [59]. Reaction of triply branched molecule 47 containing an electron deficient bipyridinium unit on each branch, and three equiv of CB[8] forms branched [4] pseudorotaxane 48 which has been characterized by NMR and ESI mass spectrometry. Addition of three equivalents of electron-rich dihydrox-ynaphthalene 49 produces branched [4]rotaxane 50, which is stabilized by charge-transfer interactions between the bipyridinium unit and dihydroxy-naphthalene inside CB[8]. No dethreading of CB[8] is observed in solution. Reaction of [4] pseudorotaxane 48 with three equiv of triply branched molecule 51 having an electron donor unit on one arm and CB[6] threaded on a diaminobutane unit on each of two remaining arms produced dendritic [ 10] pseudorotaxane 52 which may be considered to be a second generation dendritic pseudorotaxane. 

Reaction (Scheme 2) of salt 5-2PF6 with an excess of 6, in the presence of the macrocydic polyether BPP34C10, provided1181 [BPP34C10-7] [PF6]4 via a threading-followed-by-stoppering approach.1231 This rotaxane incorporates a free 4,4-bipyridinium recognition site within its dumbbell-shaped compo-... 

The threading-followed-by-capping method has been recently employed by Stoddart to prepare a [2]rotaxane under thermodynamic control [60]. In this approach, the dibenzylammonium ion 28 - which is terminated by an aldehyde function - is mixed with the dibenzo[24]crown-8 ether (20) to form a threaded species. Upon addition of a bulky amine, the aldehyde-terminated template can be converted into an imine in a reversible reaction establishing a dynamic equilibrium (see 29 and 30 in Scheme 17). 

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). 

Clipping. The clipping method relies on the synthesis of the rotaxane by formation of the macrocycle (host) in the presence of a dumbbell shaped molecule. The partial macrocycle undergoes a ring closing reaction around the dumbbell shaped molecule, thereby forming the rotaxane. 

Reaction of 3-(9-julolidinyl)prop-2-en-l-al with N-( -adamantyl)-4-methyl-pyr-idinium chloride and a-CD in aqueous sodium hydroxide yielded styryl dye rotaxanes 6a and 7a as well as the free dye 8a (Fig. 5) [4, 27]. Analogously, the two rotaxane isomers 6b and 7b, and the free dye 8b were obtained from julolidine aldehyde and 4-methyl-2,6-diphenylpyridinium chloride. As compared to the hydrophobic dyes 8a, 8b, the rotaxanes 6a, 6b and 7a, 7b are highly soluble in water. The absorption/emission maxima of the rotaxanes 6a (525/710 nm) and 7a (535/718 nm) in DMSO are red-shifted compared to free styryl dye 8a... 

Squaraines 17b and 17c have terminal acetylene residues, which allowed to convert the squaraine dyes and tetralactam macrocycles into permanently interlocked rotaxane structures using copper-catalyzed and copper-free cycloaddition reactions with bulky stopper groups [58]. 

A variety of hydrophobic and hydrophilic squaraine rotaxane probes and labels such as 21a-21e c Rp and 22a-22e c Rp, containing reactive carboxylic functionalities and hydrophilic sulfo groups, are disclosed in a recent patent application [60]. It was shown that not only aniline-based squaraines 21a-21e but also heterocyclic squaraines 22a-22e can form stable pseudorotaxane complexes. The indo-lenine-based squaraine 22a forms rotaxane 22a C Rp. Importantly, also the sulfonated squaraine 22b could be successfully encapsulated in a Leigh-type, phenylene-based, tetralactam macrocycle to yield the water-soluble rotaxane 22b C Rp. Quatemized, indolenme-based squaraines do not form pseudorotaxanes probably because of sterical hindrance caused by /V-alkyl and 3,3 -dimethyl groups. On the other hand, quatemized benzothiazole (22c) and benzoselenazole (22d) squaraines could be embedded in a Leigh-type macrocycle to yield rotaxanes 22c C Rp and 22d C Rp, respectively. The hydrophilic, mono-reactive rotaxane 22e-NHS C Rp based on asymmetric squaraine, synthesized by a cross-reaction of squaric acid with the two different indolenines, was also obtained. 

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],... 

Hiibner GM, Reuter C, Seel C, Vogtle F (2000) Rotaxane synthesis via nucleophilic substitution reactions the trapping of electrophilic threads by organic anion-wheel complexes. Synthesis 1 103-108... 

In order to demonstrate the effect, it was felt that cleaner kinetic data was needed. This was secured by modification of the participants in the cycloaddition. A tertiary butyl group was introduced onto the amino nitrogen of each substrate (Fig. 7, R = t-Bu). This renders the reaction noncatalytic in the strict sense. Because the tert-hutyl substituents are too large to pass through the cavity of cucurbituril, the resulting product of cycloaddition is a stable rotaxane, i.e. the triazole cannot dissociate. However, this is desirable in that the chemically meaningful presteady-state phase becomes delineated in a way that could only be incompletely realized in the previous kinetics. As a bonus the undesirable substrate inhibition by propargylammonium ion, which also obscured earlier kinetic measurements, could be avoided as well. 

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