ABSTRACT
We briefly review the recent experimental and theoretical developments of organocatalysis by the derivatives of ionic liquids. We focus on the efficiency of rate enhancement and mechanistic features of SN2 reactions exhibited by ionic liquid derivatives. Roles of various moieties in the ionic liquid—oligoethylene glycol complexes, specifically, their synergistic collaboration for organocatalysis, are described in detail. Two alternative mechanisms are presented for SN2 reactions promoted by ionic liquid derivatives. When [mim- t OH][OMs], in which the ionic liquid [mim][OMs] and t-butyl alcohol are fused by covalent bonding, is used as a promotor, the metal salt MX reacts as a contact ion-pair (CIP). The oxygen atoms in the promotor bind to the counterion M+ to alleviate its Coulombic influence on the nucleophile X−, enhancing the nucleophilicity. For SN2 reactions promoted by [hexaEGim][OMs], in which the ionic liquid is combined with a hexaethylene glycol side chain, two mechanisms are more or less equally feasible: the CIP mechanism and the “naked” nucleophile mechanism. In the latter mechanism, the metal salt MX is essentially “separated” into M+ and X−, producing the “naked” nucleophile that is free from the retarding influence of the counterion M+. The promotor [dihexaEGim][OMs] is the first organic agent to exhibit the capacity to completely (>99%) dissociate the metal salts into M+ and X− for extremely efficient SN2 reactions. The origin of the experimentally observed trend of reactivity (F− > Cl− > Br−) of the halide nucleophiles under the influence of [dihexaEGim][OMs], which is in direct contrast with the reactivity (F− < Cl− < Br−) usually observed for SN2 reactions in polar aprotic solvents, is also discussed.
