Chiral protic imidazolium salts with a (−)-menthol fragment in the cation: synthesis, properties and use in the Diels–Alder reaction

Abstract

New chiral protic imidazolium salts containing a (1R,2S,5R)-(−)-menthol substituent in the cation and four different anions (chloride, hexafluorophosphate, trifluoromethanesulfonate and bis(trifluoromethylsulfonyl)imide) were efficiently prepared and extensively characterized. Detailed NMR analysis was performed, and a comparison of the chemical shifts of the protons and carbons of the imidazolium cation as a function of the combined anion was discussed. The specific rotation, solubility in commonly used solvents, thermal properties including phase transition temperatures, and thermal stability were also determined. Three of the synthesized tertiary salts (Cl, PF₆ or OTf anion) were crystalline solids; 1-H-3-[(1R,2S,5R)-(−)-menthoxymethyl]-imidazolium bis(trifluoromethylsulfonyl)imide, (−)[H-Ment-Im][NTf₂] was a liquid at room temperature. The chiral protic salts were used in a Diels–Alder reaction as a test reaction, and the results were compared with those from aprotic chiral ionic liquids having the same chiral substituent in the cation (1R,2S,5R)-(−)-menthol with a bis(trifluoromethylsulfonyl)imide anion. Both protic and aprotic chiral salts, used in a Diels–Alder reaction, were pure (−)-enantiomers, which was determined by NMR with Δ-TRISPHAT tetrabutylammonium salt as a chiral shift reagent. Protic salts offered distinctly higher endo/exo ratios than aprotic ones, but an enantiomeric excess was not obtained. The stereoselectivity reached the same high level even after the fourth recycle of (−)[H-Ment-Im][NTf₂] in the reaction of ethyl-vinyl ketone with cyclopentadiene at temperature of −35 °C.