Nucleophilic Reactions at Cationic Centers in Ionic Liquids and Molecular Solvents

We report a quantitative comparison of the rates of nucleophilic reactions in ionic liquids and molecular solvents taking place at a cationic center. Cationic sulfonium electrophiles were reacted with three amine nucleophiles (n-butylamine, di-n-butylamine, and tri-n-butylamine) in several molecular solvents (toluene, dichloromethane, tetrahydrofuran, acetonitrile, and methanol) and ionic liquids {1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide, 1-butyl-1-methylpyrrolidinium trifluoromethanesulfonate, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, and 1-butyl-3-methylimidazolium trifluoromethanesulfonate}. The solvent effects on these reactions are examined using a linear solvation energy relationship based on the Kamlet−Taft solvent scales (α, β, and π*). These correlations reveal that hydrogen-bonding interactions provide the dominant effects in determining the rate of reaction. In particular, hydrogen bonds donated by the sulfonium electrophile to the solvent are the most important controlling factor on the rate of nucleophilic reaction.