DFT Studies on the\nMechanism of Ligand-Regulated Palladium-Catalyzed\nIodide-Assisted Hydrocarbonylation of Olefins with Formic Acid: Favored\nReaction Routes and Selectivities

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