The Origin of the Halogen Effect on Reactivity and Reversibility of Diels–Alder Cycloadditions Involving Furan

Origin and generality of the halogen effect in Diels–Alder reactions were explored with high-accuracy CBS–QB3 computations. Halogen substitution increases cycloaddition exergonicities and decreases activation barriers. This effect is attributed to the energetic preference for electronegative halogen substituents to be attached to a more highly alkylated, and therefore more electropositive, carbon framework.