Determination of local effects for chloroaluminate ionic liquids on Diels-Alder reactions

Room temperature ionic liquids are an exciting class of solvents that have the potential to accelerate and control a vast range of reactions. The Diels-Alder reaction, paradigm in organic synthesis, highlights the advantages provided by ionic liquids as the reaction between cyclopentadiene and methyl acrylate in 1-ethyl-3-methylimidazolium tetrachloroaluminate and heptachlorodialuminate [EMIM][AlCl4] and [EMIM][Al2Cl7], respectively, has been reported to react with rates over 200 times faster and endo selectivity 10 times greater than commonly used reaction conditions. Density functional theory (DFT) calculations at the B3LYP/6-311+G(2d,p) theory level have been employed to determine the origin of the reported rate accelerations. The DFT simulations find that specific hydrogen bonding between the ionic liquid cations and the dienophile at the transition state is primarily responsible, however, the rate of reaction was found to be moderated by the solvent's hydrogen bond accepting ability (anion effect). Different anion-to-cation ratios were tested and a 1: 1 ratio was determined to give the best agreement with experimental observations. The computed DFT activation barriers were within reasonable agreement of the reported rates, however it is clear that a full microenvironment featuring hundreds of ions is necessary for proper computational treatment of the solvent effects delivered by the ionic liquids. (C) 2009 Elsevier Inc. All rights reserved.