Density functional study on the mechanism of the Simmons–Smith reaction

Abstract

Quantum chemical calculations based on density functional theory (DFT) including relativistic effects either through relativistic core potentials or an explicit, quasi-relativistic approach were used to study the mechanism of the Simmons–Smith cyclopropanation reaction. Under the assumption that the reactive organo-zinc–iodine species is monomeric, and neglecting solvent effects, a concerted one-step mechanistic scenario, postulated to be the most likely mechanism in the literature previously, has indeed been identified as the energetically most favourable pathway for the parent reaction of ethene with ICH₂Znl to give cyclopropane and ZnI₂. The attack of the cyclopropanating agent is electrophilic in character. Depending on the computational model applied, activation energies with respect to the educts are between 48 and 61 kJ mol^–1, while the overall reaction is predicted to be exothermic by 140–158 kJ mol^–1.