Lewis base phase transfer catalyst promoted solvent-free nucleophilic reactions in ionic liquids: Quantum chemical analysis for cooperative enhancement in SN2 reaction rates

Abstract

We present quantum chemical analysis of Lewis base phase transfer catalyst (PTC) facilitated solvent-free nucleophilic reactions in green solvent/catalyst ionic liquids (ILs). We adopt two nucleophilic processes: First, we examine whether and how the reaction rate of 18-crown-6-promoted S_N2 reaction is further enhanced by the IL [Bmim]OMs. Second, we examine the promotion of S_N2 cynation by the IL-integrated aza-crown ether, and third, the S_N2 fluorination facilitated by [2.2.2]-cryptand in the IL. Interactions between the IL, PTC and the substrates are shown to give rise to lower Gibbs free energies of activation, with the IL anion (OMs⁻) acting as an additional Lewis base promoter on the counter-cation M⁺, (M = K, Cs), thereby further mitigating the latter's disadvantageous influence on the nucleophile. We thus propose that the ILs may be used to facilitate the nucleophilic processes as solvent/ organocatalysts instead of environmentally malicious organometallic compounds.