A pH-Switchable Electrostatic Catalyst for the Diels-Alder Reaction: Progress toward Synthetically Viable Electrostatic Catalysis.

Density functional theory calculations at the SMD/M06-2X/6-31+G(d,p)//M06-2X/6-31G(d) level of theory have been used to computationally design and test a pH-switchable electrostatic organocatalyst for Diels-Alder reactions. The successful catalyst design, bis(3-(3-phenylureido)benzyl)ammonium, was studied for the reaction of p-quinone with range of cyclic, heterocyclic, and acyclc dienes and also the reaction of cyclopentadiene with maleimide and N-phenylmaleimide. All reactions showed significant enhancements in catalysis (10-32 kJ mol-1 in barrier lowering) when the catalyst was protonated, consistent with electrostatic stabilization of the transition state. Electrostatic effects were found to diminish in polar solvents but were predicted to remain significant in nonpolar solvents.