Regio and stereoselectivity in ionic cycloadditions

Acridizinium cation (ACR) and 2,3-dimethylisoquinolinium ion (DMIQ) undergo [2 + 4(+)] addition with dienophiles methyl vinyl ether (MVE) and propylene (PY) and the addition takes place across the diene unit containing the cationic centre and the dienophile acts as the electron donor. These reactions have two regiochemical paths and in each of them two possible stereo adducts could be formed. DFT calculations performed at B3LYP/6-31G(d) level have shown that the reactions pass through concerted mechanism and the TSs are highly asynchronous. Methoxy group in the dienophile can take up cis or traits orientation with respect to the double bond and in that traits orientation of the methoxy group is preferred. Calculations further show that syn 2 adduct is kinetically and thermodynamically more favoured in both the reactions in excellent agreement with the experimental observations. ACR is found to be more reactive than DMIQ as a diene and as a dienophile MVE is found to be more reactive than PY. Computed bond orders establish that the syn 2 transition states are the most 'reactant' like. Though the reactions have both electrostatic control and frontier orbital control the former dominates in the initial stages of the reaction.