Understanding the\nElectronic\nEffects of Lewis Acid\nCatalysts in Accelerating Polar Diels–Alder Reactions

The\nelectronic effects of Lewis acid (LA) catalysts in reducing\nthe activation energies of polar Diels–Alder (P-DA) reactions\nhave been studied within Molecular Electron Density Theory. To this\nend, a quantum topological energy partitioning scheme, namely, the\nRelative Interacting Atomic Energy (RIAE) analysis, is applied to\nthe transition state structures (TSs) and the ground state of the\nreagents of two different LA-catalyzed P-DA reactions. Analyses of\nthe ξ<i>E</i><sub>total</sub><sup>X</sup> total energies of the two interacting frameworks <i>f</i>(X) show that the electronic energy stabilization of the\nelectrophilic frameworks, resulting from the global electron density\ntransfer (GEDT), is the cause of an effective decrease of the activation\nenergies. On the other hand, an in-depth analysis of the ξ<i>E</i><sub>intra</sub><sup>A</sup> intra-atomic energies of the atoms belonging to the electrophilic\nethylenic framework in the LA-catalyzed P-DA reactions of cyclopentadiene\nwith acrolein indicates that the strong electronic stabilization of\nthe carbonyl carbon, resulting from the GEDT taking place at the TSs,\nis the main factor responsible for the decrease of the activation\nenergies in these LA-catalyzed P-DA reactions. Finally, the increase\nin GEDT at the TSs of these P-DA reactions causes an increase in the\nlarger C–C distance, resulting from the stabilization of the\nelectrophilic framework, thereby decreasing the suggested <i>Pauli repulsion</i>.