A DFT computational study of the mechanism of butadiene carbonylation catalyzed by palladium complexes

A theoretical DFT study of the mechanism of the butadiene carbonylation catalysed by Pd(II) complexes has been carried out. The Pd(PH3)2Cl2 species has been chosen as a model compound to emulate the catalyst. Even if Pd(PH3)2Cl2 can behave, in principle, as catalytic species, leading to both observed products, i.e., methyl 3-pentenoate (linear ester) and methyl 2-methyl-3-butenoate (branched ester), the experimentally observed selectivity for the linear product (about 90%) is not explained by these results. It has been found that the reaction channels involving Pd(PH3)(CO)Cl2 and Pd(CO)2Cl2 as active catalytic species (these species are likely to form at the experimental conditions of high CO pressure) are favored since they require much lower activation barriers. Also, for both species the path leading to the linear product is highly favoured with respect to the path affording the branched ester. This aspect is particularly evident for Pd(CO)2Cl2, which seems to be the real active catalytic species of the process.