Palladium-PEPPSI-NHC Complexes Bearing Imidazolidin-2-Ylidene Ligand: Efficient Precatalysts for the Direct C5-Arylation of N-Methylpyrrole-2-Carboxaldehyde

Synthesis and characterization of Pd/NHCF complexes with fluorinated aryl groups

The key problem of the instability of fluorine-containing diazadienes was addressed to perform the efficient synthesis of imidazolium salts containing fluorine substituents in the aryl groups. The subsequent reaction of fluorine-containing imidazolium compounds (NHC_F) with palladium salts under simple conditions afforded new Pd/NHC_F complexes. Computational and structural studies were performed to assess the effect of fluorine on the Pd-NHC bond and gave insight into the electronic effects in the molecule. The introduction of fluorine substituents into the aryl rings of the NHC ligands leads to a slight decrease in their σ-donor properties. At the same time, there is a slight increase in the π-acceptor capacity of NHC_F. These two effects compensate for each other, so that the Pd-NHC bonding energy remains virtually unchanged. Another observed effect is associated with a slight weakening of the trans influence of the NHC_F ligands, which is expressed in the strengthening of the Pd-Solv bond in (NHC)Pd(Solv) complexes. For the first time, a series of novel Pd/NHC_F complexes were synthesized via a straightforward approach from fluorine-containing anilines.

Direct Arylation in the Presence of Palladium Pincer Complexes

Direct arylation is an atom-economical alternative to more established procedures such as Stille, Suzuki or Negishi arylation reactions. In comparison with other palladium sources and ligands, the use of palladium pincer complexes as catalysts or pre-catalysts for direct arylation has resulted in improved efficiency, higher reaction yields, and advantageous reaction conditions. In addition to a revision of the literature concerning intra- and intermolecular direct arylation reactions performed in the presence of palladium pincer complexes, the role of these remarkably active catalysts will also be discussed.