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

Abstract

AbstractWe 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 BF3, B(C6H5)3, and B(C6F5)3efficiently accelerate both the competing [4+2] and [8+2] cycloaddition reactions by lowering the activation barrier up to 12 kcal mol−1compared to the uncatalyzed reaction. Our study reveals that the LA catalyst promotes both cycloaddition reaction pathways byLUMO‐lowering catalysisand demonstrates thatPauli‐lowering catalysisis not always the operative catalytic mechanism in cycloaddition reactions. Judicious choice of the LA catalyst can effectively impart regiocontrol of the cycloaddition: B(C6H5)3furnishes the [8+2] adduct while B(C6F5)3yields 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.