Nuclear Magnetic Resonance Spectroscopy. Conformational Properties of Substituted 1,1-Difluorocyclohexanes

Abstract

Rates of ring inversion and conformational equilibria of gem-fluorocyclohexanes carrying various substituents were determined by fluorine magnetic resonance spectroscopy. For 1 ,l-dilluorocyclohexane, 1,1,2,2-tetrafluorocyclohexane, 4-methyl-4-ethyl-l,l-difluorocyclohexan4e,,4 -dimethyl-l,l-difluorocyclohexanea,n d 4-chloro-1,l-difluorocyclohexane, the activation energies (E,) for ring inversion were found to fall between 8.0 and 11.2 kcal/mole. For 4-chloro-1 ,l-difluorocyclohexane, the difference in free energy for the conformation with chlorine equatorial and the conformation with chlorine axial was found to be -0.32 and -0.17 kcal/mole for solutions in propene and acetone, respectively, and practically independent of temperature. The corresponding free-energy difference for the conformations of 4-methyl-4-ethyl-1 ,l-difluorocyclohexane is - 0.10 kcal/mole. With 3-methyl-, 4-methyl-, and 4-t-butyl-substituted ll-difluorocyclohexanes the equilibria between the conformations is shifted so much in favor of one conformer that only qualitative conclusions could be drawn from the fluorine magnetic resonance spectra.