Mechanism of SN2 Alkylation Reactions of Lithium Organocuprate Clusters with Alkyl Halides and Epoxides. Solvent Effects, BF3 Effects, and Trans-Diaxial Epoxide Opening

Abstract

The B3LYP density functional studies on the mechanism of the SN2-substitution reaction of methyl halides and epoxides with lithium organocuprates(I), (CH3)2CuLi·LiCl and [(CH3)2CuLi]2, revealed the energetics and the geometries of important transition states and intermediates along the reaction pathway. In the absence of solvent coordination on the copper atom, the reaction takes place in a single step through rate determining cleavage of the C−X bond (X = leaving group) involving nucleophilic participation of the CH3−Cu bond composed of the copper 3dz2 orbital and carbon 2s+2p orbitals. Consideration of solvent polarity and coordination of an explicit (CH3)2O molecule to a lithium atom in the cuprate cluster lowers the activation energy to <20 kcal/mol, which is a reasonable value for the reaction taking place below 0 °C. Solvation of the copper atom does not change much the geometry or the energy of the transition state, but modifies the pathway afterward. The origin of the “trans-diaxial opening of cyc...