Insights into the Base-Assisted Acrylate Formation from CO2/C2H4 Coupling by Pd- and Ni-catalyst: A DFT Mechanistic Study

摘要

In this work, seven base-assisted reaction paths were designed focusing on the rate-determining metallalactone deprotonation process to further promote our understanding on nickel- and palladium-catalyzed acrylate formation by CO2/C2H4 coupling in the presence of bases. The deprotonation efficiency of the bases was in accordance to the corresponding basicity, with stronger bases showing higher activity. Besides, a novel twostep beta-H elimination mechanism through directly deprotonating the beta-H agostic intermediate formed from the metallalactone cleavage proved to be most promising in both the nickel and palladium systems. Whereas the literature-suggested direct beta-H elimination of the distorted five-membered ring was relatively inferior. Meanwhile, the high H-rotation energy barriers hindered the emerging of the cation-stabilized four-membered lactone. The effects of the ligand properties appeared to be different in the nickel and palladium systems, since the dcpe (1,2-bis(dicyclohexylphosphino)ethane) ligand facilitated the Pd-catalyzed beta-H elimination yet showed minute influence in the Ni-system. In addition, the palladium center was more active than the nickel center under bulky environment during the base-assisted beta-H elimination, while the nickel center behaved better in less hindered surroundings, owning to the high sensitivity of the nickel center towards the steric factor.