Synthesis of pillar[n]arenes (n = 5 and 6) with deep eutectic solvent choline chloride 2FeCl3

Abstract

A novel and distinct method of preparing pillar[5]arene and pillar[6]arene with high selectivity and efficiency has been achieved by condensation of 1,4-dialkoxybenzene and paraformaldehyde with the choline chloride (ChCl)/ferric chloride (FeCl₃) deep eutectic solvent in CH₂Cl₂ at room temperature. Under the optimal conditions, the yield of pillar[5]arene and pillar[6]arene is 35% and 53%, respectively. The reaction mechanism is investigated by room-temperature X-band Electron Spin Resonance (ESR), indicating that a free radical takes part in this cyclization reaction and acts as an intermediate. Our research is the first report about the application of DESs in supramolecular macrocyclic host synthesis.