Synthesis and Conformational Characteristics of Alkyl-Substituted Pillar5arenes

A series of pillar5arene derivatives with alkyl groups of different length were synthesized. The new alkyl-substituted pillar5arene derivatives 1,4-bis(ethoxy)pillar5arene (C2), 1,4-bis(propoxy)pillar5arene (C3), 1,4-bis(butoxy)pillar5arene (C4), 1,4-bis(pentyloxy)pillar5arene (C5), 1,4-bis(hexyloxy)pillar5arene (C6), and 1,4-bis(dodecanoxy)pillar5arene (C12) were obtained by Lewis acid-catalyzed condensation of dialkoxybenzene monomers with paraformaldehyde. The conformational characteristics of the pillar5arene derivatives were investigated by dynamic (1)H NMR measurements. When the alkyl substituents were bulkier than methyl groups, the rotation of phenolic units in the pillar5arenes was suppressed and their conformation was immobilized. As their length increased, the alkyl substituents packed at the upper and lower rims and thus lowered the conformational freedom of the pillar5arenes.