Unravelling the mechanism for the mono-alkoxycarbonylation of butadiene: what is the role of the base?

The mechanism of catalytic alkoxycarbonylation of alkenes has been extensively studied and it is generally accepted that it elapses through three elementary steps: (i) alkene insertion into a PdH bond, (ii) carbonylation, and (iii) alcoholysis. In the case of butadiene methoxycarbonylation catalyzed by [Pd(d^tbpx)H]⁺, we have identified the formation of a highly stable Pd(η³-allyl) intermediate by Density Functional Theory (DFT) calculations and Nuclear Magnetic Resonance (NMR). This intermediate makes the apparent kinetic barrier of the mechanism incompatible with experimental observations (ca. 200 kJ·mol⁻¹). Upon coupling DFT calculations and experimental analysis, we proposed a revised mechanism founded on a “base assisted” pathway where the mono-alkoxycarbonylation of dienes exhibits a significantly decreased free energy of activation (ca. 100 kJ mol⁻¹). Hence, we proposed that the base coming from the Pd precursor is non-innocent and participates in the reaction upon acting as proton shuttle during the alcoholysis step. By considering various acid/base pairs, we further refined these findings and demonstrated that an optimum balance between the acid and the base must be found, regarding strengths and concentrations, to maximize the catalytic activity.