Theoretical analysis of radical addition reactions: On the anomalous behavior of CH₃ toward fluoro-substituted olefins

Although radicals usually attack the less substituted site of olefins, CH₃ adds to the more substituted end of CF₂=CFH. A theoretical study of this anomalous reaction as well as the addition of CH₃ to CH₂=CHF, which conforms to the normal orientation rule, has been done by means of UHF 3-21G calculations. In contrast with semiempirical results (MNDO), the ab initio calculations are found to account correctly for the experimental facts. The energy barriers have been analyzed by means of the energy partitioning proposed by Morokuma. From these results and a comparison with available data on fluoro-substituted ethanes the origin of the anomalous regioselectivity in the reaction of CH₃ with CF₂=CFH is rationalized.