Benzylic Oxidation of Pillar[5]Arene: From Homo- to Cross-Conjugated Macrocyclic Systems

Selective benzylic oxidations of pillar[5]arene have been carried out to generate all the possible oxidized derivatives containing from one up to five carbonyl subunits. By using a mild oxidizing agent, namely DDQ, the conditions have been optimized to preferentially generate the monoketone derivative. The corresponding polyketones have been obtained by oxidation of the pillar[5]arene with SeO₂ under microwave irradiation. Six out of the seven oxidation products have been characterized by x-ray crystal structure analysis. Effects resulting from the homo- and the cross-conjugation within this series of compounds have been investigated and analyzed with the help of DFT calculations. Finally, binding studies have been also carried out with the pillar[5]arene and the pentaketone derivative in order to evaluate the effect of the C═O subunits on the affinity of the macrocyclic system for guest molecules.