NHC-Pd complex based on 1,3-bis (4-ethoxycarbonylphenyl) imidazolium chloride: synthesis, structure and catalytic activity in the synthesis of axially chiral benzophenone hydrazone

Palladium N-Heterocyclic Carbene-Catalyzed Aminations: An Outline

Amination reactions play a pivotal role in synthetic organic chemistry, facilitating the generation of nitrogen-containing scaffolds with broad applications in drug synthesis, material production, polymer formation, and the generation of amino acids and peptides. Amination offers the potential to fine tune the properties of natural products and produce functional materials for various applications. Palladium N-heterocyclic carbene (Pd-NHC) emerges as an innovative and highly effective catalyst in this context. Under favorable reaction conditions, this robust and simple catalyst efficiently facilitates the synthesis of a diverse range of compounds with varying complexity and utility. Pd-NHC complexes exhibit significant σ-electron donating potential, enhancing the ease of the oxidative addition process in their mechanistic pathway. Their steric topography further contributes to a rapid reductive elimination. These complexes demonstrate remarkable stability, a result of the strong Pd-ligand bond. The wide variety of Pd-NHC complexes has proven highly efficient in catalyzing reactions across a spectrum of complexities, from simple to intricate. The domain of aminations catalyzed by Pd-NHC has undergone significant diversification, presenting new opportunities, particularly in the realms of material chemistry and natural product synthesis. This review outlines the advancements in Pd-NHC-catalyzed amination reactions, covering literature up to date.

Synthesis and a combined experimental/theoretical structural study of a comprehensive set of Pd/NHC complexes with o-, m-, and p-halogen-substituted aryl groups (X = F, Cl, Br, CF3)

Pd/NHC complexes (NHCs - N-heterocyclic carbenes) with electron-withdrawing halogen groups were prepared by developing an optimized synthetic procedure to access imidazolium salts and the corresponding metal complexes. Structural X-ray analysis and computational studies have been carried out to evaluate the effect of halogen and CF₃ substituents on the Pd-NHC bond and have provided insight into the possible electronic effects on the molecular structure. The introduction of electron-withdrawing substituents changes the ratio of σ-/π-contributions to the Pd-NHC bond but does not affect the Pd-NHC bond energy. Here, we report the first optimized synthetic approach to access a comprehensive range of o-, m-, and p-XC₆H₄-substituted NHC ligands, including incorporation into Pd complexes (X = F, Cl, Br, CF₃). The catalytic activity of the obtained Pd/NHC complexes was compared in the Mizoroki-Heck reaction. For substitution with halogen atoms, the following relative trend was observed: X = Br > F > Cl, and for all halogen atoms, the catalytic activity changed in the following order: m-X, p-X > o-X. Evaluation of the relative catalytic activity showed a significant increase in the catalyst performance in the case of Br and CF₃ substituents compared to the unsubstituted Pd/NHC complex.

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.

An NHC-CuCl functionalized metal-organic framework for catalyzing β-boration of α,β-unsaturated carbonyl compounds

A microporous metal-organic framework functionalized with in situ generated NHC-CuCl units (1) has been successfully synthesized from a novel imidazolium-containing ligand. In particular, the MOF 1 can catalyze β-boration of α,β-unsaturated carbonyl compounds, while the isoreticular version of 1 (1-im) modified with only imidazolium moiety cannot. This work demonstrates for the first time the heterogeneous catalysis of NHC-Cu(i)Cl within a MOF solid.