Thermoregulated aqueous biphasic catalysis of Heck reactions using an amphiphilic dipyridyl-based ligand

Abstract

A neat water-based thermoregulated system for Pd-catalyzed Heck reactions is described. It uses a thermo-responsive ligand L that enables (1) the transfer of the catalyst in the organic phase upon heating thus allowing the reaction to take place in one phase, and (2) the separation/recovery of the catalyst in the water phase upon cooling and hence catalyst reuse. The amphiphilic ligand L with an inverse temperature-dependent solubility in water is prepared by covalent attachment of 2,2′-dipyridylamine to the tip of a nonionic polyoxyethylene surfactant (decyloctaethyleneglycol C₁₀E₈). Spectroscopic studies reveal that the amphiphilic dipyridyl-based ligand L forms a 1 : 1 Pd complex (PdLCl₂) in organic solvent and a 1 : 2 Pd complex in water. Cross-coupling reactions of iodobenzene with ethyl acrylate and styrene are achieved, without any organic solvent, at 100–120 °C with sodium carbonate as a base using 0.1 to 0.5 mol% of Pd catalyst generated in water from L and Na₂PdCl₄. The reaction products, trans-cinnamic acid and stilbene, are readily isolated at room temperature, and the catalyst is recovered in the aqueous phase, without the need to add any organic solvent. The aqueous phase can be used in three catalytic runs with an almost constant catalytic activity for the coupling of iodobenzene with ethyl acrylate.