Fluorine magnetic resonance studies. III. Fluorine-fluorine coupling over six and seven bonds in fluorostyrenes

The 19F nuclear magnetic resonance spectra of β,β-difluorostyrenes and 1-difluoromethyleneindans bearing fluorine substituents in the aromatic ring have been recorded and the long-range couplings 6JF,F and 7JF,F are discussed. In each compound the larger couplings are to the olefinic fluorine trans to the aromatic ring. The couplings are maximum in the planar indans, reducing to unobservable values when non-planarity is achieved in suitably substituted styrenes. The allylic coupling 4JH,F involving olefinic fluorines shows only slight stereochemical dependence.