Ab Initio calculations of the potential surfaces for rearrangement of methylenecyclopropane and 2,2-difluoromethylenecyclopropane. Why do the geminal fluorines have little effect on lowering the activation energy?†

Abstract

(4/4)CASSCF and CASPT2 calculations with the 6-31G* basis set have been performed in order to understand the experimental observation that the geminal fluorines in 2,2-difluoromethylenecyclopropane (2) have only a small effect on lowering the activation energies for its degenerate and non-degenerate methylenecyclopropane rearrangements, relative to the activation energy for the rearrangement of the hydrocarbon (1). As expected from previous experimental and computational studies, the geminal fluorines are calculated to destabilize the three-membered ring in 2 thermodynamically. The small amount of kinetic destabilization of 2 is shown to be due to a nearly equal destabilization of the transition structures for its rearrangements. The high energies of the transition structures are attributed to the strong preference of a CF₂ group for a pyramidal geometry. This preference is found to destabilize the transition structures both for forming a σ bond to the fluorinated carbon in the degenerate methylenecyclopropane rearrangement of 2 and for making a π bond to this carbon in the non-degenerate rearrangement of 2 to (difluoromethylene)cyclopropane (4).