Co-catalysis over a tri-functional ligand modified Pd-catalyst for hydroxycarbonylation of terminal alkynes towards α,β-unsaturated carboxylic acids

An amphiphilic tri-functional ligand (L1) containing a Lewis acidic phosphonium cation, a phosphino-fragment and a hydrophilic sulfonate anion (-SO₃^-) enabled Pd(OAc)₂ to efficiently co-catalyze the hydroxycarbonylation of terminal alkynes towards α,β-unsaturated carboxylic acids. These incorporated functional groups synergistically promoted the reaction, which proved more effective than the ligands lacking -SO₃^- and/or phosphonium and the mechanical mixtures of the individual functional groups independently. The molecular structure of Pd-L1 indicated that -SO₃^- in L1 served as a secondary O-donor ligand with reversible coordinating ability, cooperating with the phosphino-fragment to stabilize the Pd-catalyst. The in situ FT-IR analysis verified that the formation and stability of Pd-H active species in charge of hydroxycarbonylation were dramatically facilitated by the presence of L1. It was believed that, over the L1-based Pd-catalyst, H₂O was cooperatively activated by the Lewis acidic phosphonium via "acid-base pair" interaction (H₂O → P(v)⁺) and by the hydrophilic SO₃^-via hydrogen bonding (SO₃^-⋯H₂O), giving rise to the formation of dimeric and mono-nuclear Pd-H species driven by reversible SO₃^--coordination. In addition, the L1-based Pd-catalyst could be immobilized in the ionic liquid [Bmim]NTf₂ for six-run recycling uses without obvious activity loss and detectable metal leaching.