Understanding the Electronic Effects of Lewis Acid Catalysts in Accelerating Polar Diels-Alder Reactions

The electronic effects of Lewis acid (LA) catalysts in reducing the activation energies of polar Diels-Alder (P-DA) reactions have been studied within Molecular Electron Density Theory. To this end, a quantum topological energy partitioning scheme, namely, the Relative Interacting Atomic Energy (RIAE) analysis, is applied to the transition state structures (TSs) and the ground state of the reagents of two different LA-catalyzed P-DA reactions. Analyses of the ξE_total^X total energies of the two interacting frameworks f(X) show that the electronic energy stabilization of the electrophilic frameworks, resulting from the global electron density transfer (GEDT), is the cause of an effective decrease of the activation energies. On the other hand, an in-depth analysis of the ξE_intra^A intra-atomic energies of the atoms belonging to the electrophilic ethylenic framework in the LA-catalyzed P-DA reactions of cyclopentadiene with acrolein indicates that the strong electronic stabilization of the carbonyl carbon, resulting from the GEDT taking place at the TSs, is the main factor responsible for the decrease of the activation energies in these LA-catalyzed P-DA reactions. Finally, the increase in GEDT at the TSs of these P-DA reactions causes an increase in the larger C-C distance, resulting from the stabilization of the electrophilic framework, thereby decreasing the suggested Pauli repulsion.