Concerning the Origin of Substituent-Induced Fluorine-19 Chemical Shifts in Aliphatic Fluorides: Carbon-13 and Fluorine-19 Nuclear Magnetic Resonance Study of 1-Fluoro-4-phenylbicyclooctanes Substituted in the Arene Ring

Article abstract of DOI:10.1021/jo00136a027

Several 1-fluoro-4-(para-substituted phenyl)bicyclo<2.2.2>octanes have been synthesized and their NMR spectra (13C and 19F) recorded.The reverse 19F substituent chemical shifts (SCS) were found to correlate very well with substituent parameters (?_I and ?_R⁰) by using the dual substituent parameter (DSP) equation.Linear electric field effects for several substituents were estimated from the ρ_I?_I term for the DSP correlation, and these were employed for calculating the coefficient (A) of the Buckingham equation (SCS = AE_z).The negative sign of the Avalue for linear electric field effects on C_sp3-F bonds stands in contrast to the positive value for fluorine attached to sp² carbons.It was also found that the 19F SCS correlate extremely well with the corresponding 13C SCS of the bridgehead carbon (C1) attached to fluorine and with the changes in the one-bond carbon-fluorine spin-spin coupling constants (Δ¹J_CF).These results reveal that the phenomenon of reverse 19F SCS of aliphatic fluorides is a manifestation of dominant changes in fluorine ?-electron density.The origin of exalted 19F SCS of aliphatic fluorides in trifluoroacetic acid solution has been delineated.