Isolation of all four conformations of p-tert-butylcalix[5]arene using bridging silyl groups

Treatment of p-tert-butylcalix[5]arene with RMeSi(NMe2)(2) (R = Me, Ph) yields both di-and monosubstituted products: calix[5](RMeSi)(2)(OH) (1a, R = Me; 1b, R = Ph) and calix[5](RMeSi)(OH)(3) (2a, R = Me; 2b, R = Ph). In all derivatives, the silicon atom bridges adjacent oxygens on the calix[5]arene. Isolation of the monosubstituted compounds requires careful control of stoichiometry using an excess of p-tert-butylcalix[5]arene. Variable-temperature NMR spectra of 2a suggest that at least two different exchange processes are occurring; the high-temperature process appears to involve complete inversion of the calixarene with an activation energy of 64.4 (1.6) kJ/mol. These compounds are isolated and structurally characterized. The four solid-state structures closely approximate the four different ideal conformations of calix[5]arenes.