Synthesis and conformational characteristics of alkyl-substituted pillararenes.
ABSTRACT A series of pillararene derivatives with alkyl groups of different length were synthesized. The new alkyl-substituted pillararene derivatives 1,4-bis(ethoxy)pillararene (C2), 1,4-bis(propoxy)pillararene (C3), 1,4-bis(butoxy)pillararene (C4), 1,4-bis(pentyloxy)pillararene (C5), 1,4-bis(hexyloxy)pillararene (C6), and 1,4-bis(dodecanoxy)pillararene (C12) were obtained by Lewis acid-catalyzed condensation of dialkoxybenzene monomers with paraformaldehyde. The conformational characteristics of the pillararene 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 pillararenes 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 pillararenes.