Barium(II) complexes of calixcrowns derived from p-tert-butylcalix[5]arene as potential transacylation catalysts. Regio- and stereo-selective monoacylation of the calixcrown

Abstract

Several crown ethers derived from tert-butylcalix[5]arene have been studied in the form of their barium(II) salts as turnover catalysts in the methanolysis of p-nitrophenyl acetate in MeCN–MeOH (9∶1, v/v) using UV spectroscopy and HPLC. The liberation of p-nitrophenol can be interpreted for 1,3-crown ether derivatives by a double displacement mechanism, although their catalytic activity is lower than for the corresponding tert-butylcalix[4]arene derivative. Regioselective O-acylation of ring 2 has been proved for crown-5 and crown-6 by ¹H NMR of the isolated intermediate. Whilst this monoacetyl derivative of crown-6 assumes the expected cone conformation, it is formed exclusively as the partial cone conformer from the crown-5. This observation enables the regio- and stereo-selective monoacylation of the 1,3-crown-5 derivative also on a preparative scale by reaction with p-nitrophenyl esters in the presence of one molar equivalent of Ba²⁺ ions.