Quantum Chemical Study of the Cycloaddition Reaction of Tropone with 1,1-Diethoxyethene Catalyzed by B(C₆F₅)₃ or BPh₃

Cycloaddition reaction of tropone with 1,1-diethoxyethene catalyzed by Lewis acid (LA), B(C₆F₅)₃ or BPh₃, was examined by using ωB97X-D-level density functional theory (DFT) calculations. In the absence of LA, the reaction proceeds in a stepwise fashion to form two chemical bonds, first between the C² atom in tropone and the C² atom in ethene and then between the C⁵ atom in the former and the C¹ atom in the latter. When B(C₆F₅)₃ is attached to the O atom in tropone, the C⁵ atom in tropone is attacked preferentially by the C¹ atom in ethene in the second stage. The attack of the O atom in tropone is shown to be less likely; thus, the [4 + 2] addition is favored in the B(C₆F₅)₃-catalyzed reaction. In contrast, the attack of the O atom in the BPh₃-attached tropone to the C¹ atom in ethene is preferred over the attack of the C⁵ atom, indicating that the [8 + 2] cycloaddition instead of the [4 + 2] cycloaddition proceeds in the BPh₃-catalyzed reaction. Whether the C¹ atom in ethene is attacked by C⁵ or by O in the second bond formation step is shown in this study to be governed mainly by the nucleophilicity of σ-lone pair electrons of the carbonyl O atom of tropone in the presence of LA. These results are consistent with the experiments reported by Li and Yamamoto.