- Do commercially available metal salts mediate calixarene formation via hydrogen-bonded dimers?
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In this article we report the first example of a Lewis acid promoted, one-pot, Bronsted acid free, high-yielding synthesis of the calixarene macrocycle from the "monomer" p-tert-butylphenol. We report that when a commercially available metal salt (Lewis acid) is incorporated within the calixarene-forming reaction, a certain amount of control over the size of the calixarenes produced can be gained. Although a detailed mechanistic rationale on how the macrocycle is assembled is unclear, what is evident fro this work is that the metal cation, the counteranion, and the oxidation state of the salt employed are all important contributors to the outcome of the reaction process. Indeed, evidence to date suggests that a subtle "symbiotic" relationship exists between the metal cation, its oxidation state, and the anion that allows the efficient transformation of the "monomeric" p-tert-butylphenol into linear oligomers and, ultimately, into macrocyclic calixarenes. Athough the metal salt mediated process described herein is efficient and high-yielding, what is also fundamentally important is that a comprehensive mechanistic understanding of how the calixarenes are assembled be accrued. Searching for possible indicators or clues, we propose that oligomeric methylene-linked phenolic entities are initially formed and that these, we tentatively suggest, generate metal and/or anion hydrogen-bonded supramolecular intermediates. It is possible that the preorganization of the linear polyphenolic oligomers allows the formation of hydrogen-bonded structures which, critically, result in the formation of supramolecular assemblies that are subsequently "stitched" together, generating the p-tert-butylcalix[n] arenes (n = 4-9) in excellent yields. Substantiating the possibility that hydrogen-bonded entities are generated (and that these subsequently afford metal-templated assemblies), we make reference to a seldom cited 1962 Nature publication that reported the propensity of polyphenolic linear oligomers to form "well-defined intramolecularly hydrogen-bonded conformations".
- Bew, Sean P.,Sharma, Sunil V.
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supporting information; experimental part
p. 7076 - 7083
(2011/10/18)
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- Ferrocenium salts mediate para-tert-butylcalixarene synthesis
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Ferrocenium salts mediate high yielding one-pot and convergent syntheses of para-tert-butylcalixarenes in mild non-Lewis or Bronsted acidic reaction conditions; EPR indicates complex formation between the s-trioxane and the ferrocenium salt. The Royal Soc
- Bew, Sean P.,Cheesman, Myles R.,Sharma, Sunil V.
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body text
p. 5731 - 5733
(2009/04/13)
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- An expedient one-pot synthesis of para-tert-butylcalix[8]- and [9]arene
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We report the first efficient synthesis of para-tert-butylcalix[8]arene and -[9]arene via an exceptionally straightforward innovative protocol that takes place at ambient temperature, employing readily available tin(iv) chloride and s-trioxane. The Royal Society of Chemistry.
- Bew, Sean P.,Sharma, Sunil V.
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p. 975 - 977
(2007/12/31)
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- Interaction of triplet C60 with p-tert-butyl-calixarenes and their complexes with pyridine derivatives
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The interaction of triplet excited C60 with p-tert-butyl-calix[n]arenes (BCXn, n = 4, 6 or 8) and with their 2,4,6-trimethylpyridine (TMP) and pyridine complexes has been studied with laser flash photolysis experiments. It has been found that in polar solvents 3C60 is quenched by BCX6 via electron transfer, producing C60-? radical anion with a yield of 0.45 ± 0.07 in benzonitrile. In contrast, no reaction occurs in nonpolar media or with BCX4 and BCX8. Upon the addition of TMP, the rate of quenching is enhanced considerably due to the formation of TMP-BCXn complexes. The significant deuterium isotope effect indicates that the electron movement from the calixarene to the triplet excited C60 is coupled to the proton displacement from the calixarene to the base. In the presence of pyridine, which has weaker hydrogen-bonding power, the enrichment of the solvate shell in the proton acceptor molecules may play an important role in promoting triplet C60 quenching.
- Poor, Benedek,Biczok, Laszlo,Kubinyi, Miklos
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p. 2047 - 2052
(2007/10/03)
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- Acid-catalyzed formation of hexahomooxacalix[3]arenes
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The role of acid catalysis in oxacalix[3]arene synthesis has been investigated. A range of acids were used to optimize the yield of tert-butyloxacalix[3]arene, the most efficient being p-toluenesulfonic acid, which, with local symmetry complementary with that of the lower rim of the calixarene, provides a templating effect.
- Miah, Mijan,Romanov, Nikolai N.,Cragg, Peter J.
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p. 3124 - 3126
(2007/10/03)
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- ipso-substitution reaction in the convergent stepwise synthesis of calix[8]arene with regioselectively functionalized upper rim
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The acid-catalyzed multi-step synthesis of calix[8]arene 1, which contains a regioselectively functionalized upper rim, has been investigated through the careful separation of the resulting by-products. A total of 11 types of cyclic and acyclic by-products were isolated and identified by spectral and microanalytical data. Semiempirical molecular-orbital calculations demonstrated that the synthetic process involves two types of reaction mechanism, one of which leads to the favorably constructed framework, while the other results in undesirable fragmentation reactions via ipso-substitution. A Comparison of the theoretical predictions and the observed by-products revealed that steric hindrance, as well as the regioselectivity of the reaction, is also an important factor for determining the reaction pathway. A possible rationale is presented to explain the overall formation process leading to calix[8]arene 1 along with the by-products.
- Tsue, Hirohito,Enyo, Kazutaka,Hirao, Ken-Ichi
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p. 849 - 859
(2007/10/03)
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- Isolation, characterization, and conformational characteristics of p-tert-Butylcalix[9-20]arenes
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p-tert-Butylcalix[n]arenes in which n = 9-20 (designated as "large" calixarenes) have been isolated from both the base-induced and the acid-catalyzed condensation of p-tert-butylphenol and formaldehyde. The acid-catalyzed process, previously thought to form linear oligomers as the major or even exclusive products, has been optimized to produce calixarenes in almost quantitative yield, providing a better source of these "large" calixarenes than the base-induced process. Central to the discovery of the calix[9-20]arenes and the optimization of the acid-catalyzed condensation was the development of an HPLC assay that allows baseline resolution of the 17 cyclic oligomers as well as their linear counterparts. Using this assay for guidance, procedures for the obtention of ponderable amounts of p-tert-butylcalix[7-20]arenes from the acid-catalyzed process have been developed. Data are adduced that bear on possible mechanistic pathways for calixarene formation under these conditions. A study measuring the amount of each of the linear and cyclic oligomers formed at various stages during an acid-catalyzed condensation suggests that the calixarenes are probably formed from linear oligomers containing the same corresponding number of aryl residues. However, the possibility of ipso substitution as a cyclization route was also indicated by an experiment involving the presence of benzyl cations. The conformational mobilities of calix[n]arenes (n = 4-20), measured by dynamic 1H NMR spectroscopy in CDCl3 solution, show a periodicity in which calixarenes with 4, 8, 12, 16, and 20 aryl residues are more stable than their immediate neighbors, ascribed to particularly efficient hydrogen bonding and molecular packing in these conformers.
- Stewart, Donald R.,Gutsche, C. David
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p. 4136 - 4146
(2007/10/03)
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- Perforated Monolayers: Design and Synthesis of Porous and Cohesive Monolayers from Mercurated Calixarenes1
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Mercuration of a series of O-alkylated calixarenes (produced via reaction of tetrahydroxycalixarene, pentahydroxycalixarene hexahydroxycalixarene, and heptahydroxycalixarene with n-bromobutane and with n-bromohexadecane) affords an homologous series of calixarene-based surfactants that form stable monolayers at the air-water interface.Surface pressure-area isotherms, measured for each calixarene, yield limiting areas that are in excellent agreement with values predicted from space-filling models, if it is assumed that the base of each calixarene is parallel and the alkyl chains are perpendicular to the water surface.Introduction of malonic acid to the aqueous subphase results in a substantial increase in the coheresiveness of films derived from calixarene-, calixarene-, and calixarene-based surfactants, as judged by changes in surface viscosity.X-ray photoelectron spectroscopic analysis of a Langmuir-Blodgett film, produced from a malonic acid stabilized calixarene monolayer, shows a carboxylate/mercury ratio of 0.9.This value is in excellent agreement with a model that assumes that all of the bound malonate acts as a bridge between adjacent calixarene subunits in an hexagonally packed array.Water evaporation studies, carried out at the air-water interface, reveal that each of the calixarene-, calixarene-, and calixarene-based monolayers, in the presence and in the absence of malonic acid, maintains a porous structure that offers little resistance to the permeation of water, relative to a densely packed aliphatic monolayer of surfactant.
- Markowitz, Michael A.,Janout, Vaclav,Castner, David G.,Regen, Steven L.
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p. 8192 - 8200
(2007/10/02)
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