Modeling the effect of ligands and solvation on hydrolysis variants in the Pd(II)-Catalyzed hydroxycarbonylation of pentenoic acids

We explored computationally variations in the hydrolysis step of the Pd(II)-catalyzed hydroxycarbonylation of pentenoic acids for several ligands L₂ = 1,2-bis[(di-R)phosphinomethyl]benzene where R = tert-butyl, methyl, or phenyl, referring to these ligands by DTBPX, DMPX, and DPPX, respectively. Thus far, most computational models invoked three H₂O molecules in the crucial step which can occur in either a concerted or a stepwise fashion. We used density functional calculations to study systematically the effect of the number of water molecules (H₂O)_m, m = 1–3, on the nature of the crucial step, concerted vs stepwise. Accordingly, the concerted mechanism is preferred over the stepwise mechanism for m = 1. For m = 2, the stepwise mechanism is preferred for the ligand DTBPX, and the concerted mechanism for the ligands DMPX or DPPX. For m = 3, the stepwise mechanism is preferred for all three ligands under study and, importantly, also results in the overall lowest hydrolysis barriers. An energy decomposition analysis of these hydrolysis barriers, evaluating corrections due to larger basis set, dispersion interaction, and solvation, indicates that the latter term indeed is responsible for rendering adipic acid the most likely product.