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 1H–1H NOESY experiments demonstrate
that the oligomers adopt stable helical conformation, which are induced
by C5–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 C5–H···F–C
planar interaction lowers the energy by ∼3 kcal mol–1 on average, and by ∼1 kcal mol–1 when C5–H···Cl–C bridges are formed.
The hydrogen-bonding networks are disrupted in competitive hydrogen-bonding
media such as DMSO, generating the unfolded oligomers.