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Synlett 2006(9): 1335-1338  
DOI: 10.1055/s-2006-941562
LETTER
© Georg Thieme Verlag Stuttgart · New York

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

Cecilia Bettia, Dario Landini*a, Angelamaria Maia*b
a Dipartimento di Chimica Organica e Industriale, Università degli Studi di Milano, Via Venezian 21, 20133 Milano, Italy
Fax: +39(02503)14159; e-Mail: dario.landini@unimi.it;
b Istituto CNR-ISTM, Via Golgi 19, 20133 Milano, Italy
e-Mail: angelamaria.maia@istm.cnr.it;
Further Information

Publication History

Received 8 February 2006
Publication Date:
22 May 2006 (online)

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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

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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]