Aromatic triazole foldamers induced by C-H···X (X = F, Cl) intramolecular hydrogen bonding.

Aryl-triazole oligomers based on isobutyl 4-fluorobenzoate and isobutyl 4-chlorobenzoate were designed and synthesized. Crystal structure and (1)H-(1)H NOESY experiments demonstrate that the oligomers adopt stable helical conformation, which are induced by C(5)-H···X-C (X = F, Cl) intramolecular hydrogen bonding between triazole protons and halogen atoms. The stabilities of the folded conformations are confirmed by DFT calculations, which show that each C(5)-H···F-C planar interaction lowers the energy by ~3 kcal mol(-1) on average, and by ~1 kcal mol(-1) when C(5)-H···Cl-C bridges are formed. The hydrogen-bonding networks are disrupted in competitive hydrogen-bonding media such as DMSO, generating the unfolded oligomers.