POTENTIAL SURFACES FOR CYCLOPROPANE STEREOMUTATIONS - WHAT A DIFFERENCE GEMINAL FLUORINES MAKE

The potential surfaces for stereomutation of cyclopropane and 1,1-difluorocyclopropane (1) have been calculated at the GVB-PP(1)/6-31G* level of theory, and SD-Cl/6-31G* calculations have been performed at selected geometries. In the stereomutation of 1 the preference for coupled methylene rotation is calculated to be much larger than in cyclopropane, and disrotation is predicted to be favoured in 1, whereas conrotation is predicted for the hydrocarbon. The presence of a methyl group at each of the terminal carbons is computed to enhance the preference for coupled rotation in the stereomutation of 1, provided that both methyl groups rotate 'outwards' into the sterically less encumbered positions. In contrast, methyl substituents are computed to reduce the energetic preference for coupled rotation to nearly zero in cyclopropane. However, the energetic penalty for rotation of one methyl group 'inwards' is calculated to be much smaller in the ring opening of cyclopropane than of 1. These differences between the potential surfaces for stereomutation of the hydrocarbon and fluorocarbon can be readily understood on the basis of orbital interaction diagrams for the two (0,0) diradicals. Computational explorations of the general conclusions of this analysis are described.