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.