Design of a highly active Pd catalyst with P,N hemilabile ligands for alkoxycarbonylation of alkynes and allenes : a density functional theory study

Abstract

In palladium-catalysed methoxycarbonylation of technical propyne, the presence of propadiene poisons the hemilabile Pd(P,N) catalyst. According to density functional theory calculations (B3PW91-D3/PCM level), a highly stable π-allyl intermediate is the reason for this catalyst poisoning. Predicted regioselectivities suggest that at least 11% of propadiene should yield this allyl intermediate, where the reaction gets stalled under the turnover conditions due to an insurmountable methanolysis barrier of 25.8 kcal mol-1. Results obtained for different ligands and substrates are consistent with the available experimental data. A new ligand, (6-Cl-3-Me-Py)PPh2, is proposed, which is predicted to efficiently control the branched/linear selectivity, avoiding rapid poisoning (with only 0.2% of propadiene being trapped as Pd allyl complex), and to tremendously increase the catalytic activity by decreasing the overall barrier to 9.1 kcal mol-1.