[Pd(4-R3Si-IPr)(allyl)Cl], a Family of Silyl-Substituted Pd–NHC Complexes: Catalytic Systems for the Buchwald–Hartwig Amination

Silylimidazolium Hexafluorophosphate Salts as Synthetic Precursors to N-Heterocyclic Carbene Pentafluorophosphorus Adducts

N-Heterocyclic carbene pentafluorophosphorus (NHC-PF5) adducts are six-coordinate phosphorus(v) compounds with emerging applications but poor synthetic accessibility. We have developed a simple and high yielding protocol for synthesizing imidazolylidene NHC-PF5 adducts from silylimidazolium hexafluorophosphate salts. Using this methodology, we have prepared a series of NHC-PF5 adducts in high yields, including new NHC-PF5 building blocks amenable to subsequent synthetic diversification. We also demonstrate that a similar approach enables access to analogous, synthetically elusive NHC-BF3 and NHC-SbF5 adducts.

Quantitative Determination of Active Species Transforming the R-NHC Coupling Process under Catalytic Conditions

Palladium complexes with N-heterocyclic carbenes (Pd/NHC) serve as prominent precatalysts in numerous Pd-catalyzed organic reactions. While the evolution of Pd/NHC complexes, which involves the cleavage of the Pd-C(NHC) bond via reductive elimination and dissociation, is acknowledged to influence the catalysis mechanism and the performance of the catalytic systems, conventional analytic techniques [such as NMR, IR, UV-vis, gas chromatography-mass spectrometry (GC-MS), and high-performance liquid chromatography (HPLC)] frequently fail to quantitatively monitor the transformations of Pd/NHC complexes at catalyst concentrations typical of real-world conditions (below approximately 1 mol %). In this study, for the first time, we show the viability of using electrospray ionization mass spectrometry (ESI-MS). This approach was combined with the use of selectively deuterated H-NHC, Ph-NHC, and O-NHC coupling products as internal standards, allowing for an in-depth quantitative analysis of the evolution of Pd/NHC catalysts within actual catalytic systems. The reliability of this approach was affirmed by aligning the ESI-MS results with the NMR spectroscopy data obtained at greater Pd/NHC precatalyst concentrations (2-5 mol %) in the Mizoroki-Heck, Sonogashira, and alkyne transfer hydrogenation reactions. The efficacy of the ESI-MS methodology was further demonstrated through its application in the Mizoroki-Heck reaction at Pd/NHC loadings of 5, 0.5, 0.05, and 0.005 mol %. In this work, for the first time, we present a methodology for the quantitative characterization of pivotal catalyst transformation processes commonly observed in M/NHC systems.

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.

New class of RSO2-NHC ligands and Pd/RSO2-NHC complexes with tailored electronic properties and high performance in catalytic C-C and C-N bonds formation

Imidazolium salts have found ubiquitous applications as N-heterocyclic carbene precursors and metal nanoparticle stabilizers in catalysis and metallodrug research. Substituents directly attached to the imidazole ring can have a significant influence on the electronic, steric, and other properties of NHC-proligands as well as their metal complexes. In the present study, for the first time, a new type of Pd/NHC complex with the RSO2 group directly attached to the imidazol-2-ylidene ligand core was designed and synthesized. The electronic properties as well as structural features of the new ligands were evaluated by means of experimental and computational methods. Interestingly, the introduction of a 4-aryl(alkyl)sulfonyl group only slightly decreased the electron donation, but it significantly increased the π-acceptance and slightly enhanced the buried volume (%Vbur) of new imidazol-2-ylidenes. New Pd/NHC complexes were obtained through selective C(2)H-palladation of some of the synthesized 4-RSO2-functionalized imidazolium salts under mild conditions. Several complexes demonstrated good activity in the catalysis of model cross-coupling reactions, outperforming the activity of similar complexes with non-substituted NHC ligands.

Evidence for “cocktail”-type catalysis in Buchwald–Hartwig reaction. A mechanistic study

The mechanism of the C–N cross-coupling reaction, catalyzed by Pd/NHC, was evaluated at the molecular and nanoscale levels. The first evidence for the involvement of a “cocktail”-type system in the Buchwald–Hartwig reaction is provided.

Benzyloxycalix[8]arene supported Pd–NHC cinnamyl complexes for Buchwald–Hartwig C–N cross-couplings

The synthesis of a Pd–NHC cinnamyl-complex supported on a calix[8]arene and its use in Buchwald–Hartwig amination is reported. Thanks to the support, the products were isolated with low levels of residual palladium, in some cases below standards.

Sterically enriched bulky 1,3-bis(N,N′-aralkyl)benzimidazolium based Pd-PEPPSI complexes for Buchwald–Hartwig amination reactions

A simple and efficient synthesis of a series of unexisting Pd-PEPPSI complexes is summarized. These complexes are exploited for their high catalytic activity towards Buchwald–Hartwig amination.

Sterically hindered N-heterocyclic carbene/palladium(ii) catalyzed Suzuki–Miyaura coupling of nitrobenzenes

A palladium-catalyzed Suzuki coupling reaction of nitroarenes has been developed using 5-(2,4,6-triisopropylphenyl)-2,3-imidazolylidene[1,5-a]pyridines as the ligands.

Rigid hindered N-heterocyclic carbene palladium precatalysts: synthesis, characterization and catalytic amination

Rigid hindered N-heterocyclic carbene palladium complexes have been developed and exhibited high activities for a variety of (hetero)aryl chlorides with (hetero)anilines and amines under aerobic conditions.