External Electric Field Induces a Mechanistic Crossover in the Reactivity of 1,3-Dienes with Sulfur Dioxide: Sulfolene Versus Sultine.

1,3-Dienes (1) react with SO2 to produce sulfolenes (2) and sultines (3). Previous experiments and computational studies have shown that hetero-Diels-Alder (HDA) reaction producing the sultine is kinetically favorable compared to the chelotropic (CE) reaction producing the sulfolene. In this article, DFT calculations for a series of substituted 1,3-dienes show that under the influence of a moderate oriented external electric field (OEEF) of 2.0-4.0 V nm-1 along the reaction axis, the chelotropic reaction becomes the kinetically favorable reaction. In the absence of the OEEF, the destabilizing distortion involved in bringing the 1,3-diene and SO2 to the CE transition-state (TS) exceeds than for the HDA TS. However, under the influence of the OEEF, the strongly stabilizing electrostatic interactions in the CE TS effectively overcome the structural distortion energy. The enhanced dipole moment of the CE TS vis-à-vis the HDA TS under the OEEF accounts for the stabilization of the former.