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

The potential surfaces for stereomutation of cyclopropane and 1,1-difl uorocyclopropane (1) have been calculated at the GVB-PP(1)/6-31G leve l of theory, and SD-Cl/6-31G calculations have been performed at sele cted geometries. In the stereomutation of 1 the preference for coupled methylene rotation is calculated to be much larger than in cyclopropa ne, and disrotation is predicted to be favoured in 1, whereas conrotat ion is predicted for the hydrocarbon. The presence of a methyl group a t each of the terminal carbons is computed to enhance the preference f or coupled rotation in the stereomutation of 1, provided that both met hyl groups rotate 'outwards' into the sterically less encumbered posit ions. In contrast, methyl substituents are computed to reduce the ener getic preference for coupled rotation to nearly zero in cyclopropane. However, the energetic penalty for rotation of one methyl group 'inwar ds' is calculated to be much smaller in the ring opening of cyclopropa ne than of 1. These differences between the potential surfaces for ste reomutation of the hydrocarbon and fluorocarbon can be readily underst ood on the basis of orbital interaction diagrams for the two (0,0) dir adicals. Computational explorations of the general conclusions of this analysis are described.