To elucidate the reaction mechanism, density functional theory (DFT) calculations were conducted on the crosscoupling reaction of substrates A1 and B1 catalyzed by Sc-3. The detailed results, including Gibbs free energy profiles and structural information, are summarized in Figure ). Subsequently, substrate B1 coordinates with E-INT3 to form E-INT4, which undergoes a 2,1-insertion via transition state E-TS2, yielding the intermediate INT5 with an energy barrier of 16.7 kcal/mol, which is 4.2 kcal/mol lower than that of the Z-isomer (Z-TS2). Structural analysis of transition states E-TS2 and Z-TS2 reveals significant steric hindrance in the Z-isomer between the phenyl ring of A1 and the methylene group (Fig. ). Next, another molecule of substrate A1 undergoes a sigma-bond metathesis reaction with E-INT5 to form E-INT6 via E-TS3, with an energy barrier of 27.8 kcal/mol, which is 4.2 kcal/mol lower than that of the Z-isomer (Z-TS3). This step is identified as the rate-determining step of the reaction, which was in agreement with the result of deuteriumlabelling experiments. Finally, the product dissociates, completing the catalytic cycle. Notably, the pathways without the participation of N,N,2-trimethylaniline were investigated as well. However, a higher free energy barrier was observed (For more details, see SI, Figure ). These outcomes also provided a rationale for the influence of Lewis base additives on the E/Z ratio of the product (Table , entries 10 and 11).