Ab Initio Molecular Orbital Calculations of Electronic Effects on the Kinetics of Cyclopropylcarbinyl Radical Ring Openings

Ab initio molecular orbital calculations have been performed on the ring-opening reactions of cyclopropylcarbinyl radical analogues containing vinyl and methoxy substituents on the ring and for vinyl, methoxy, and methyl substituents on the radical center. Barrier heights were calculated at the UHF/6-31G*, UMP2/6-31G*//UHF/6-31G*, and PMP2/6-31G*//UHF/6-31G* levels of theory. Substituent effects were analyzed using isodesmic reactions. Vinyl substituents on the ring reduce the barrier by 7−8 kcal/mol because of allylic conjugation stabilizing the transition state. Methoxy substituents on the ring reduce the barrier by ca. 2 kcal/mol, primarily as a result of hyperconjugation. Vinyl and methoxy substituents on the radical center raised the barrier 4.5 and 0.8 kcal/mol, respectively, because the stabilizing effects on the reactants are greater than the effects on the transition states. A methyl substituent decreased the barrier slightly in contrast to intuitive expectations. The computational results were compared to experimental thermochemical and kinetic data when available, and the agreements were good in an absolute sense and excellent in a relative sense. This work demonstrates that predictions of reactivity patterns and relative kinetics for radical reactions that do not involve polarized transition states can be computed without the use of supercomputers.