Theoretical Study on the BF₃‑Catalyzed Meinwald Rearrangement Reaction

The mechanisms of the BF₃-catalyzed Meinwald rearrangement reactions of five epoxides in dichloromethane solution have been studied at the M062X/6-311++G­(2df,2pd) level. Accordingly, the Lewis acid–epoxide complex can react through several alternative pathways, though three phases (ring opening, C–C bond rotation, and hydrogen or alkyl group migration) are required in any path. In some cases, a concerted pathway (involving all three successive phases) is found. Otherwise, the reaction takes place through a reaction mechanism involving a zwitterion or a BF₃ addition compound (formed by fluoride transfer from the BF₃ moiety to the incipient carbocationic center generated by C–O bond rupture) or both as reaction intermediate(s). The BF₂-bound fluorohydrin yields the reaction product through a concerted process involving fluoride transfer from the C–F bond to the OBF₂ group and hydrogen or alkyl group migration, as first demonstrated in this work. Effects of a number of features (solvent effects, concurrent hydrogen/alkyl group migration, carbocation substitution, benzylic conjugation) are also discussed