Bifunctional Phosphine/Phosphonium Lewis Acid Ligands; Design, Synthesis And Application In Carbonylative Sonogashira Reaction

The co-catalysis in combination of transition metal catalysts and organocatalysts has been emerged as a powerful strategy to promote new reactions and improves the reactivity in recent years. In organocatalysis, amino catalysts, Lewis base catalysts, Br(?)nsted acid catalysts, Bronsted base catalysts, phase transfer catalysts and N-heterocyclic carbene catalysts have been exploited. However, only the examples of the co-catalysis in combination of transition metal catalysts with organic Lewis acid catalysts have been developed by using the bifunctional ligands containing phosphines and Lewis acidic boranes. Comparable to organic Lewis acidic boranes [B(Ⅲ)], organophosphonium [P(V)+] also are typical Lewis acid catalysts which can interact with Lewis base containing O, N, F or H atoms.In this dissertation, at first, the designed bifunctional ligands of L1-L5 that contain phosphines and Lewis acidic phosphoniums [P(V)+] were synthesized and characterized (Scheme 1). The corresponding ionic Pd complexe 5A ligated by L5 and Au complexes 1A-3A ligated by L1-L3 respectively were synthesized and analyzed by single-crystal X-ray diffraction analysis (Scheme 2).L1 L2 L3 L4 L5Scheme 1 Bifunctional Phosphine/Phosphonium Lewis Acid Ligands L1-L5 Pd complex Au complexes5A 1A 2A 3AScheme 2 Pd complexe 5A and Au complexes 1A-3ASecondly, the co-catalysis in combination of Pd catalyst and P(V)+ Lewis acid catalyst by using the bifunctional ligands of L1-L5 was investigated for carbonylative Sonogashira reaction. It was found that the highest TOF of 2293 h-1 were obtained over Pd-L1 catalytic system. The involved Lewis acidic P(V)+ sites play considerable role in promoting the reaction. When the strong Lewis base of F- was added to the reaction system, the catalytic activity of Pd-L1 system was deteriorated due to the quenching of the P(V)+acidic sites through acid-base pair interaction. Evidently, the secondary P(V)+-O bonding interaction which was observed in complex 1A occurred in carbonylative Sonogashira reaction and then gave rise to the stabilization of the acylpalladium intermediates, which correspond to the improved reaction rate. On the other hand, Pd-L1 catalytic system, in which L1 was a type of phosphine-functionalized phosphonium-based ionic liquids, was proved to be recoverable and recyclable in combination of room temperature ionic liquid solvent.