Optimization of phase-transfer catalysts designed from calix[4]arene

To optimize the calix[4]arene structure for a phase-transfer catalyst, we modified the upper rim with lipophilic tert-octyl groups and the lower rim with O(CH₂CH₂O)_mMe (m=1, 2, and 3). Through two-phase solvent extraction, ¹H NMR studies on the metal-binding site, and phase-transfer catalysis we have reached conclusions that (i) lipophilic groups introduced into the upper rim effectively enhance the catalytic activity whereas (ii) to compose an effective ionophoric cavity on the lower rim, OCH₂CH₂OMe (i.e., m=1) suffices and m=2 and m=3 rather decrease the catalytic activity and cause emulsification. The optimized calix[4]arene-based phase-transfer catalyst showed the catalytic activity comparable with dicyclohexyl-18-crown-6.