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Catalytic Effects of Ammonium and Sulfonium Salts and External Electric Fields on Aza-Diels-Alder Reactions.

The mechanism of the aza-Diels-Alder reaction catalyzed by tetraalkylammonium or trialkylsulfonium salts is explored with density functional theory (DFT). Favorable electrostatic interactions between the dienophile and the charged catalyst stabilize the highly polar transition state, leading to lower free energy barriers and higher dipole moments. Endo selectivity is predicted for both uncatalyzed and catalyzed systems. We also computationally evaluate the effects of oriented external electric fields (EEFs) on the same aza-Diels-Alder reaction, demonstrating that very strong EEFs would be needed to achieve the catalytic strength of these cationic catalysts.