Reactivity of chelating dicarbene metal complex catalysts, I: An investigation on the Heck reaction

Four chelating di(N-heterocyclic carbene) palladium(II) complexes have been prepared and used as catalysts in the Heck reaction of aryl bromides at very low Pd concentration (“omeopathic conditions”). They differ in the nature of the nitrogen substituent, of the bridge between the carbene units or of the heterocyclic system. The obtained results indicate that the catalysts perform well with activated aryl bromides, but less efficiently than simple palladium(II) acetate in the case of deactivated aryl bromides. Such an inhibiting effect of the dicarbene ligands can be correlated with their ability to form inactive dimers and/or bischelate, tetracarbene palladium(II) derivatives in the course of the reaction and appears to depend on their steric features, with catalytic activity increasing with ligand bulkiness.

Graphical abstractChelating dicarbene ligands exhibit an inhibiting effect in the Pd-catalyzed Heck reaction of deactivated aryl bromides, which can be correlated with their ability to form inactive dimers and/or bischelate, tetracarbene palladium(II) derivatives. Figure optionsDownload full-size imageDownload as PowerPoint slide