Isolation, characterization, and conformational characteristics of p-tert-butylcalix[9-20]arenes

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 H-1 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.