190651-92-4Relevant academic research and scientific papers
Do commercially available metal salts mediate calixarene formation via hydrogen-bonded dimers?
Bew, Sean P.,Sharma, Sunil V.
, p. 7076 - 7083 (2011)
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".
Ferrocenium salts mediate para-tert-butylcalixarene synthesis
Bew, Sean P.,Cheesman, Myles R.,Sharma, Sunil V.
body text, p. 5731 - 5733 (2009/04/13)
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
An expedient one-pot synthesis of para-tert-butylcalix[8]- and [9]arene
Bew, Sean P.,Sharma, Sunil V.
, p. 975 - 977 (2007/12/31)
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.
Isolation, characterization, and conformational characteristics of p-tert-Butylcalix[9-20]arenes
Stewart, Donald R.,Gutsche, C. David
, p. 4136 - 4146 (2007/10/03)
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.
