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 H-1-H-1 NOESY experiments demonstrate that the oligomers adopt stable helical conformation, which are induced by C-5-H center dot center dot center dot 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 center dot center dot center dot F-C planar interaction lowers the energy by similar to 3 kcal mol(-1) on average, and by similar to 1 kcal mol(-1) when C-5-H center dot center dot center dot Cl-C bridges are formed. The hydrogen-bonding networks are disrupted in competitive hydrogen-bonding media such as DMSO, generating the unfolded oligomers.