Calix[6]arene Functionalized at Four Bridges: Conformation and Intramolecular Guest Exchange

Reaction of the tert-butoxycarbonyl derivative of p-tert-butylcalix[6]arene (2) with excess NBS under irradiation yields a calixarene with four bromomethine bridges as the major product (3). On the basis of the number of signals in the NMR spectrum, to the product was assigned a 1,2,4,5-substitution pattern. X-ray crystallography indicates that the product possesses the rct₂ configuration and crystallized as a mixture of two conformers. The major form in the crystal exists in a 1,2,3-alternate conformation where a single tert-butoxycarbonyl group is self-included in the cavity and all bromines are located at equatorial positions. A rotational barrier of 18.4 kcalmol^-1 was measured for the intramolecular guest exchange rotational process that involves the mutual exchange between the unique self-included Boc group and the Boc group on the opposite ring (Boc = tert-butoxycarbonyl). Tetrabromocalix[6]arene 3 underoges an intramolecular guest exchange process involving the unique self-included Boc group and the Boc group on the oppiiosite ring (Boc = tert-butoxycarbonyl).