‘Metal Ion Electrophilic Catalysis’ in Ring-Opening Reactions of 1,2-Epoxides by Metal Halides in Ionic Liquids

Abstract

Metal ion electrophilic catalysis (Li⁺ > Na⁺ >> K⁺) has been found in ring-opening reactions of 1,2-epoxides 1-5 by metal halides MHal in ionic liquids. The results have been rationalized on the basis of a transition state where the cation M⁺ stabilizes the ­negative charge developing on the oxygen atom of the oxirane ring while favoring the nucleophilic attack at the adjacent carbon by the ion-paired anion Hal^-.

Key words

epoxides - vic-halohydrins - oxirane ring-opening - ionic liquids - metal ion catalysis

References and Notes

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13

In a typical procedure a solution of the epoxide (0.1 M) in the ionic liquid was reacted under stirring with 2 equiv of the appropriate salt (and 1.5 equiv of NaHCO₃ if necessary), at 25-80 °C. After completion of the reaction (TLC analysis) the mixture was extracted with MTBE (4 times) and concentrated under vacuum. The crude was purified on silica gel column (eluant EtOAc-light PE) to afford the pure product. The halohydrin was identified (¹H NMR) by comparison with the authentic sample. The residue of the ionic liquid was dissolved in CH₂Cl₂, filtered on Celite^® and, after removal of CH₂Cl₂ under vacuum, reused in subsequent runs.

16

Most of the common dipolar non-HBD (hydrogen-bond donor) solvents (e.g., MeCN) are found to be good cation-solvating media. [17]