The Origin of Catalysis and Regioselectivity of Lewis Acid‐Catalyzed Diels‐Alder Reactions with Tropone**

Abstract We have studied the uncatalyzed and Lewis acid (LA)‐catalyzed cycloaddition reaction between tropone and 1,1‐dimethoxyethene using dispersion‐corrected relativistic density functional theory (DFT). The LA catalysts BF 3 , B(C 6 H 5 ) 3 , and B(C 6 F 5 ) 3 efficiently accelerate both the competing [4+2] and [8+2] cycloaddition reactions by lowering the activation barrier up to 12 kcal mol −1 compared to the uncatalyzed reaction. Our study reveals that the LA catalyst promotes both cycloaddition reaction pathways by LUMO‐lowering catalysis and demonstrates that Pauli‐lowering catalysis is not always the operative catalytic mechanism in cycloaddition reactions. Judicious choice of the LA catalyst can effectively impart regiocontrol of the cycloaddition: B(C 6 H 5 ) 3 furnishes the [8+2] adduct while B(C 6 F 5 ) 3 yields the [4+2] adduct. We discovered that the regioselectivity shift finds its origin in the ability of the LA to absorb distortion by adopting a trigonal pyramidal geometry around the boron atom.