SOLVENT EFFECTS ON STRUCTURE AND REACTION-MECHANISM - A THEORETICAL-STUDY OF [2+2]POLAR CYCLOADDITION BETWEEN KETENE AND IMINE

The effects of aqueous solvent on structures and mechanism of the [2 2] cycloaddition between ketene and imine were investigated by using correlated MP2 and MP4 levels of ab initio molecular orbital theory in conjunction with the dielectric continuum Generalized Conductor-like Screening Model (GCOSMO) for solvation. We found that reactions in the gas phase and in aqueous solution have very different topology on the free energy surfaces but have similar characteristic motion along the reaction coordinate. First, it involves formation of a planar trans-c onformation zwitterionic complex, then a rotation of the two moieties to form the cycloaddition product. Aqueous solvent significantly stabi lizes the zwitterionic complex, consequently changing the topology of the free energy surface from a gas-phase single barrier (one-step) pro cess to a double barrier (two-step) one with a stable intermediate. El ectrostatic solvent-solute interaction was found to be the dominant fa ctor in lowering the activation energy by 4.5 kcal/mol. The present ca lculated results are consistent with previous experimental data.