Pd–benzothiazol-2-ylidene complex in ionic liquids: Efficient catalyst for carbon–carbon coupling reactions

Pd-benzothiazol-2-ylidene complex 3 catalyzes efficiently the carbon–carbon coupling reactions in the ionic liquid tetrabutylammonium bromide (TBAB) as solvent. The IL does exert a striking influence on the reactivity and the stability of the catalyst. This is probably due to the formation, by reaction with TBAB, of an anionic and more nucleophilic complex surrounded by large tetrabutylammonium cation that would impede, by imposing a Coulombic barrier for collision, the formation of clusters growing further into catalytically inactive metal particles. On the contrary, ionic liquids with planar structures and poor nucleophilic anions like the [bmims], that form tight ion pairs, are barely efficient in influencing the reactivity and stability of the catalyst.

Pd–benzothiazol-2-ylidene complex 3 does catalyze, in tetrabutylammonium bromide melt as solvent, very fast and efficient both the Heck and the carbonylation reactions of aryl halides. In the Heck reaction of the aryl halides with cinnamates, the addition of tetrabutylammonium acetate as base, in order to control the stereospecificity of this process, leads to the decomposition of 3 affording Pd-nanoclusters that efficiently catalyze a stereospecific synthesis of β-aryl cinnamates.Figure optionsDownload as PowerPoint slide