PREPARATION AND CONFORMATION OF MONOHALOTETRAHYDROXYCALIX[5]ARENES

The intraannular introduction of a halogen into a calixarene skeleton is described. Monoaminotetrahydroxy-p-tert-butylcalix[5]arene (2b) was diazotized by treatment with isoamyl nitrite/HCl/EtOH. Thermal dediaz oniation of the salt in chloroform yielded monochlorotetrahydroxy-p-te rt-butylcalix [5]arene (5a) and the xanthenocalix[5]arene 6. These pro ducts result from the capture of an intermediate phenyl cation derivat ive by chloride ion or intramolecularly, by a neighboring phenol ring. The product ratio 6:5a was not affected by the addition of excess ext ernal chloride, suggesting that the reacting intermediate exists as an ion pair. The dediazoniation reaction in the presence of bromide and iodide ions afforded the corresponding halocalixarenes 5b and 5c, whil e in the presence of a fluoride ion the calixindazole 8 was obtained. X-ray diffraction indicates that 5b exists in a distorted cone conform ation (cone-in) in which the extraannular edge of the halophenyl ring is tilted toward the cavity center. The halocalixarenes undergo a cone -to-cone inversion process with a barrier in the 11.3-12.5 kcal mol(-1 ) range in CDCl3. The barrier height is a function of the size of the halo substituent. Xanthenocalix[5]arene 6 crystallizes with three etha nol solvent molecules and exists in a cone-like conformation.