Probing the helical integrity of multivicinal all-syn-fluoro alkanes

This study extends our interest in the synthesis and conformational behaviour of all-syn multivicinal fluoro alkane motifs. Specifically an all-syn 1,2,3,6,7,8-hexafluorooctane chain was assembled with a run of three fluorines, of the same stereochemical sense (syn) to the direction of the chain, on each side of an ethylene (-CH2CH2-) spacer to explore if the helical sense of the chain across the ethylene bridge is maintained. The solid state (X-ray) structure indicated a continuous helix however in solution (NMR) and by DFT computation, although the individual all-syn 1,2,3-trifluoro motifs maintain good helical integrity, the molecule is much more dynamic across the ethylene bridge. It was notable however that a low energy, non helical conformer has a high molecular dipole (= 7.15D) indicating a role for this skipped motif in soft materials such as liquid crystal design.