Erbium triflate in ionic liquids: A recyclable system of improving selectivity in Diels-Alder reactions

The efficiency of Er(OTf)(3) in promoting the Diels-Alder reactions between different dienes and dienophiles in ionic liquids has been investigated. Compared with the analogous cycloadditions performed in conventional solvents shorter reaction times are required to obtain good/excellent yields. In most cases an enhancement of regio- and endo:exo selectivity was observed. The role of the ionic liquid, as a function of the cationic part, i.e. the imidazolium based or the pyridinium based, is discussed well. The ILs containing the catalyst can be readily separated from the reaction products and recovered in very high purity for direct reuse, up to six cycles. (C) 2009 Elsevier B.V. All rights reserved.