Origin of reversal of stereoselectivity for [4+2] cycloaddition reaction between cyclopentadiene and methyl methacrylate in the presence of the chloroloaluminate ionic liquid (1-ethyl-3-methyl-imidazolium chloride): In silico studies

The origin of stereoselectivity for [4+2] cycloaddition reaction of methyl methacrylate with cyclopentadiene was investigated with the B3LYP-D3(BJ)/6- 31+G(d)//B3LYP/6-31+G(d) level of theory in the presence of the ionic liquid 1-ethyl-3-methyl-imidazolium chloride (EMI⁺Cl^-) and its acidic chloroaluminate melt, 1-ethyl-3-methyl-imidazolium heptachlorodialuminate (EMI⁺Al₂Cl₇^-). The reaction of methyl methacrylate with cyclopentadiene was examined in the gas phase to rationalize the effect of the ionic liquid ion pairs EMI⁺Cl ^- and EMI⁺Al₂Cl₇^-. The DFT calculated results were found to be in good agreement with the experimentally observed results. The much-discussed hydrogen bonding effect of the imidazolium cation with the dienophile is less important to govern the stereoselectivity for the cycloaddition reaction. The atoms in molecules theory was used to examine the role of hydrogen bonding between the EMI⁺ cation and methyl methacrylate in the transition state geometries. The calculated activation barriers with the M062X/6-31+G(d)//B3LYP/6-31+G(d) and MP2/6-311+G(d,p)//B3LYP/6-31+G(d) levels of theory also predict the similar exo/endo-stereoselectivity trend for the cycloaddition reactions.