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.