Electric Fields Interrupt with Lewis Acidity and Reverse Back to Lewis Base Function.

External electric fields (EFs) are shown to have a significant impact on various aspects of chemical reactivity. It alters the electron transfer in the pristine reaction mechanism. The Lewis acid (LA)- catalysed Oxa Diels-Alder (ODA) reaction between cyclopentadiene (Cp) and formaldehyde (HCHO) is studied here using density functional theory (DFT) calculations. EF is a vector that catalyses the reaction in one direction and inhibits the reaction in the opposite direction. However, an increased inhibiting EF also promoted a shift toward catalytic activity. The EF value that brings maximum inhibition in the reaction is referred to as the electrostatic resistance point (ERP). Stronger EF overpower the conventional LA catalysis through the Normal Electron Demand (NED) pathway and bring Lewis base (LB) catalysis through the Inverse Electron Demand (IED) mechanism. The LAs attached to the HCHO control the ERP values depending upon the resistance power that the functional group provides against the EF. Thus, EF serves as a reactivity modulator, a mechanistic switch between Lewis acid-base character and a probe for complex electron shift in reactions.