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|Título:||Electrostatic repulsion between cucurbiturils can be overcome in pseudorotaxane without adding salts|
Moreira, José A.
Costa, Ana M. Rosa da
Poblete, F. J.
|Citação:||Pessêgo, M.; Moreira, José A.; Rosa da Costa, Ana M.; Corrochano, P.; Poblete, F. J.; Garcia-Rio, Luis. Electrostatic Repulsion between Cucurbiturils Can Be Overcome in Pseudorotaxane without Adding Salts, The Journal of Organic Chemistry, 78, 8, 3886-3894, 2013.|
|Resumo:||The host−guest chemistry betweencucurbituril (CB7) and a series of bolaform (Bn) surfactants with different chain lengths, n = 12−22, was the target of our study. Pseudorotaxanes are formed when the alkyl chain of the bolaform has more than 14 carbon atoms. In these cases, two CB7 molecules can be accommodated between the two
head groups of the bolaform without addition of electrolytes to the medium. In the case of a bolaform with 12 carbon atoms,
the electrostatic repulsion between the carbonyl groups of the CB7 molecules avoids the threading of a second CB7 molecule
yielding a mixed structure formed by a pseudorotaxane and an external host−guest complex. The assembly behavior was investigated using NMR spectroscopy, isothermal titration calorimetry (ITC), and kinetic measurements.|
Cucurbit[n]urils (CBn, n = 5−8 and 10) are a family of macrocyclic host molecules composed of n glycoluril units linked by a pair of methylene groups with two identical carbonyl portals and a hydrophobic cavity.1−4 The two carbonyl portals have a considerable negative charge density, which results in a high tendency of CBn to form host−guest complexes with cationic compounds.5 Cucurbiturils have also been employed to construct polyrotaxanes and polypseudorotaxanes by differents aproaches.6−15 The pseudorotaxanes may be formed by a “click” reaction between an alkyne and an azide in the inner cavity of cucurbituril. A second general procedure is to thread polymeric alkylammoniums or polyviologens with CBn. Combinations of heteromacrocyclices such as cyclodextrins (CDs) and CBn were used to construct polypseudorotaxanes. Liu11,16,17 and Kim18 studied the movement of CBs controlled by pH or by another macrocyclic molecule in polypseudorotaxanes containing CDs. Stoddart19,20 and Du21 have reported works on supramolecular nanovalves prepared by CB-catalyzed pseudorotaxanes, which could be activated under a range of pH stimuli and competitive binding. In a recent study, we have showed that CB7 has a strong ability to complex alkyltrimethylammonium cations forming 1:1 complexes.22 In the presence of an excess of CB7 2:1 complexes have been reported, where a mixed internal and external complex has been proposed (Scheme 1).22a Macartney et al.23−25 developed an extensive investigation on the host−guest interactions between CB7 and dicationic guests containing different head groups and various aliphatic and aromatic linkers. They found the formation of 1:1 ( - pseudorotaxane, Scheme 2a) and 2:1 host−guest complexes(Scheme 2b) between dicationic guests and CB7. Our previous results indicate that binding constants for internal alkyltrimethylammonium complexes are 102 times larger than for the external complexes.22a Consequently, the formation of - pseudorotaxanes should be expected in the case of dicationic guests (Scheme 2c). However, the formation of pseudorotaxane plus an external complex instead of a pseudorotaxane can be explained by the electrostatic repulsion between the negatively charged CB7 portals that hinders the formation of pseudorotaxane structures. Traditionally, this problem has been solved by providing the medium with a high salt content, where cations dispose between the CB7 portals screening their electrostatic repulsions and allowing their approach.8,9 The aim of our study is to achieve the formation of pseudorotaxane-type structures without addition of electrolytes to the medium. Our approach is based on the use of bolaform surfactants (dicationic guests) with cationic head groups separated by an alkyl spacer. An increase in the alkyl spacer between the cationic head groups also increases the distance between the CB7 and, consequently, decreases the intensity of the repulsive forces, which facilitates the formation of the pseudorotaxane. In this study, we show that when bolaform surfactants (Bn) with n ≥ 14 are used - pseudorotaxanes form, with both hosts sharing the alkyl spacer, without electrolyte addition to the medium. For that use, we used a well-established methodology to investigate host−guest complexes in aqueous solution: isothermal titration calorimetry (ITC) as well as diffusion-ordered NMR spectroscopy (DOSY) allowed us to obtain the equilibrium binding constants as well as the stoichiometry of the host−guest complexes. The analysis of the 1HNMRchemical shifts of the guest as a function of host concentration can be used as an evidence of internal or external complexes. Moreover, we used a chemical probe in order to test the existence of internal or external complexes. In our previous work,22a the solvolysis of 4- methoxybenzenesulfonyl chloride (MBSC) proved to be a useful tool to distinguish between the formation of internal and external complexes.
|Versão do Editor:||http://pubs.acs.org/doi/pdfplus/10.1021/jo400295k|
|Aparece nas colecções:||FCT2-Artigos (em revistas ou actas indexadas)|
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|The Journal of Organic Chemistry 2013 Pessego.pdf||1,26 MB||Adobe PDF||Ver/Abrir Acesso Restrito. Solicitar cópia ao autor!|
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