Fluorine NMR. Spectra of conformationally constrained Gem ‐difluorocyclohexanes

The 19 F chemical shifts and geminal coupling constants have been measured for 3,3‐dimethyl‐ (I), 3,3,5‐trimethyl‐ (II) and 3,3,5,5‐tetramethyl‐1, 1‐difluorocyclohexane (III) and 3,3‐difluorobicyclo [3.2.1]octane (IV). The Eyring parameters for the ring inversion were obtained for I and III. Representative values for the Arrhenius activation energy ( Ea ), Δ G ‡, Δ H ‡, and Δ S ‡ are: 11.0, 9.4, 10.4 kcal/mole and 4.5 e.u. for I, and 10.0, 8.3, 9.7 kcal/mole and 8.3 e.u. for III. It appears that the syn ‐axial methyl‐fluorine interaction has a negligible effect on the inversion process. However, the syn ‐axial methyl‐methyl interaction, as found in III, significantly increases the rate of inversion. Substituent effects on the 19 F shifts are marked. Introduction of methyl at C‐3 in an equatorial position leads to shielding of the equatorial and axial fluorines (+1.8 and + 1.3 ppm). Substitution in the axial C‐3 position causes deshielding of the equatorial and axial fluorines (−5.9 and −4.9 ppm).