DFT Studies on the Mechanism of Ligand-Regulated Palladium-Catalyzed Iodide-Assisted Hydrocarbonylation of Olefins with Formic Acid: Favored Reaction Routes and Selectivities

DFT calculations have been performed to gain insight into the mechanism of hydrocarbonylation of olefins and the origin of regio- and chemoselectivity. It is shown that the most feasible mechanism involves five steps: (i) decomposition of acetic formic anhydride, (ii) hydropalladation of olefins, (iii) CO migratory insertion, (iv) iodide-assisted acetate-formate exchange, and (v) formylation or carboxylation. Importantly, carboxylation proceeds via the decomposition of anhydride, followed by reductive elimination instead of direct hydrolysis of anhydride. For phosphine-ligated palladium catalysis, on one hand, the lower stability of the transition state leading to 1,2-hydropalladation could be attributed to H···H steric hindrance. On the other hand, the high chemoselectivity for the aldehyde is ascribed to increased π back-donation effect and ligand-substrate noncovalent interactions, which stabilize the transition state and hence reduce the energy barrier. For ferrocenyl phosphine-ligated palladium catalysis, significant C–H···π interaction between the substrate and proximal phenyl moiety of the phenylphosphine and π–π interaction between formate and phenyl moiety can facilitate the carboxylation process. This in-depth mechanistic insight can account for reactivity and selectivity at an atomistic level and have implications for designing new generations of palladium catalysts.