Application of the methylenology principle to substitution reactions. A theoretical study

Abstract

The substitution reaction of chloride with methyl chloride, 2-chloroethyl radical and allyl chloride has been investigated with a number of different ab initio theoretical methods. Depending on the theoretical method chosen, the intrinsic barrier for the S_N2′ reaction in allyl chloride is 7–11 kcal mol^–1 higher than the barrier for the S_N2 reaction in methyl chloride. The reaction of chloride with the 2-chloroethyl radical proceeds through formation of an intermediate complex, which can best be characterized as an ethylene fragment flanked by a resonating chloride anion/chloride radical pair. The overall process has been termed ‘S_RN2^c’, as nucleophilic substitution occurs in this open shell system with overall ‘cine’ regiochemistry. The intrinsic barrier for the S_RN2^c reaction is approximately 10 kcal mol^–1 lower than that for the S_N2 reaction. The differential barrier heights in these three substitution reactions have been rationalized using the valence bond curve crossing model. The high S_N2′ barrier is due to a larger initial excitation energy as compared to the S_N2 reaction and also to a smaller transition state resonance energy. The very low barrier and the formation of an intermediate in the S_RN2^c reaction is the consequence of a low lying electronic state, in which homolytic C–Cl bond cleavage has occurred and a C–C double bond is formed. This ‘double bond’ state descends low enough to cross with the Lewis curves used to describe bond breaking and bond making in nucleophilic substitution reactions in general. Only a small initial excitation is necessary to reach the ‘double bond’ state from the electronic ground state. This small initial excitation is the origin of the low barrier for the S_RN2^c substitution process.