Computational Overview of a Pd-Catalyzed Olefin Bis-alkoxycarbonylation Process

A comprehensive density functional theory analysis is reported for the one-pot bis-alkoxycarbonylation reaction of olefins to form succinic acid esters by action of the catalyst (N-N)­Pd­(TFA)2 (N-N = bis­(2,6-dimethylphenyl)-2,3-dimethyl-1,4-diazabutadiene, TFA– = CF3CO2–). The selective and efficient process involves alkene (H2C=CHR), CO, methanol, and p-benzoquinone (BQ) molecules as reactants. The catalytic mechanism, previously proposed on the basis of available experimental and literature data, is critically revised here. A plethora of optimized intermediates and transition states and their correlating energy profiles allow a step by step reconstruction of the entire cycle, highlighting key mechanistic aspects, such as the role of the R substituent in the olefin. One of its effects is determined by the presence of a 2e– donor group, which, depending on its power, may affect the catalysis up to its total inhibition. As another aspect, the key diester product forms through a reductive elimination step (Pd­(II) → Pd(0) transformation) that excludes the previously proposed attainment of a Pd­(II)–hydride complex. Finally, the paper illustrates the action of the sacrificial BQ oxidant in the restoration of the original Pd­(II) catalyst, as found for other strictly related cases. The energy profile indicates that the rate-determining step occurs in the initial part of the reaction, given a +29.6 kcal mol–1 energy barrier, associated with a methoxo migration into an adjacent CO ligand. The result foreshadows a rather slow activation of the catalyst and a long duration of the cycle.