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-diethoxyethenecatalyzedby Lewis acid (LA), B(C6F5)(3) or BPh3, was examined by using & omega;B97X-D-level density functionaltheory (DFT) calculations. In the absence of LA, the reaction proceedsin a stepwise fashion to form two chemical bonds, first between theC(2) atom in tropone and the C-2 atom in etheneand then between the C-5 atom in the former and the C-1 atom in the latter. When B(C6F5)(3) is attached to the O atom in tropone, the C-5 atomin tropone is attacked preferentially by the C-1 atom inethene in the second stage. The attack of the O atom in tropone isshown to be less likely; thus, the [4 + 2] addition is favored inthe B(C6F5)(3)-catalyzed reaction.In contrast, the attack of the O atom in the BPh3-attachedtropone to the C-1 atom in ethene is preferred over theattack of the C-5 atom, indicating that the [8 + 2] cycloadditioninstead of the [4 + 2] cycloaddition proceeds in the BPh3-catalyzed reaction. Whether the C-1 atom in ethene isattacked by C-5 or by O in the second bond formation stepis shown in this study to be governed mainly by the nucleophilicityof & sigma;-lone pair electrons of the carbonyl O atom of tropone inthe presence of LA. These results are consistent with the experimentsreported by Li and Yamamoto.

Automated summary

The cycloaddition reaction between tropone and 1,1-diethoxyethene catalyzed by Lewis acid (LA) was studied using density functional theory (DFT) calculations. The presence of different Lewis acids (B(C6F5)3 or BPh3) led to different reaction pathways, favoring either [4 + 2] or [8 + 2] cycloaddition reactions, respectively. The nucleophilicity of the carbonyl O atom of tropone in the presence of LA was found to govern the selectivity of the reaction.