where activation barriers would increase by ~20 and ~10 kcal mol -1 respectively in the absence of delocalization. This effect is corroborated by experimental results on the addition of dibenzylamine across the interbridgehead bonds of bicyclo[1.1.0]butane and bicyclo[2.1.0]pentane sulfones (Figure ), where the former affords the cyclobutylamine product at ambient temperature, whereas the latter requires heating to 80 °C to form the equivalent cyclopentane. This reactivity difference directly opposes the behavior that would be expected solely based on strain release energies (i.e., thermodynamics) alone (-40. ) for Marcus Ea values from refs. and ,