Co-catalysis over a bi-functional ligand-based Pd-catalyst for tandem bis-alkoxycarbonylation of terminal alkynes

A bi-functional ligand (L1) containing a diphosphino fragment and sulfonic acid group (-SO₃H) enabled PdCl₂(MeCN)₂ to efficiently catalyze the tandem bis-alkoxycarbonylation of terminal alkynes to produce aryl-/alkyl-substituted succinate (α,ω-diesters). It was found that the -SO₃H incorporated in L1 indispensably assisted the Pd-catalyst in accomplishing this tandem reaction via intramolecular synergic effects. Co-catalysis over the L1-based Pd-catalyst was not due to the physical mixture of Xantphos and MeSO₃H. In situ FTIR analysis verified that the formation and stability of Pd-H active species were facilitated by the presence of L1. The formation of stabilized diacylpalladium intermediate (F) was the critical driving force for the second-step methoxycarbonylation. DFT calculation was carried out to optimize the geometric structure of F, which indicated that the developed intramolecular O⋯H hydrogen bonds were an important structural feature to stabilize F. In addition, the L1-based Pd-catalyst could be recycled successfully for at least 3 runs in the ionic liquid [Bmim]NTf₂ without obvious activity loss and detectable metal leaching.