The BF 3 Lewis acid (LA) catalyzed [3 + 2] cycloaddition (32CA) reaction of 1-pyrroline-1-oxide (PNO) with 2,3-dihydrofuran (DHF) has been studied within the molecular electron density theory (MEDT) at the ωB97X-D/6-311G(d,p) computational level. Electron localization function (ELF) characterizes PNO and the corresponding PNO : BF 3 complex as zwitterionic species. Conceptual DFT indices allow classifying PNO and DHF as marginal electrophiles and strong nucleophiles, while the PNO : BF 3 complex is a strong electrophile. Consequently, while the noncatalyzed 32CA reaction is non-polar, the BF 3 catalyzed one has a high polar character, revealed by the analysis of the global electron density transfer (GEDT) at the transition state structures. The BF 3 catalyzed 32CA reaction, which is classified as of reverse electron density flux (REDF), presents high exo stereoselectivity, and total ortho regioselectivity. The presence of the BF 3 catalyst decreases the activation enthalpy of the 32CA reaction in dichloromethane by 7.5 kcal mol À 1 , as a consequence of the increase of the polar character of the reaction, in complete agreement with the experimentally observed acceleration.