Organic radical reactivity: activation and deactivation through alkyl substitution.

We have recently shown that alkyl substituents destabilize carbon-centered radicals, contradicting the conventional view that alkyl substitution stabilizes radicals through hyperconjugation. In this work, we show quantum chemically that these thermodynamically destabilized organic radicals are kinetically activated towards radical additions at double bonds. However, this electronic effect can be counteracted by steric repulsion in the case of sterically crowded double bonds. These findings reconcile the apparent contradiction between thermodynamic and kinetic effects.