Palladium(II) Complexes Bearing an Indazole-Derived N-Heterocyclic Carbene and Phosphine Coligands as Catalysts for the Sonogashira Coupling and the Hydroamination of Alkynes

Indazolin-3-ylidenes (Indy*): easily accessible, sterically-hindered indazole-derived N-heterocyclic carbenes and their application in gold catalysis

Sterically-hindered N-heterocyclic carbenes (NHCs) with functionalized N-wingtips are a pivotal class of ligands in organic synthesis. Herein, we report the first class of sterically-hindered N-heterocyclic carbenes based on the indazole framework. These ligands combine the strong σ-donation of the carbene center due to the carbene placement at the C3-indazole position with the sterically-hindered and flexible N-substitution with the versatile 2,6-bis(diphenylmethyl)aryl moiety that extends beyond the metal centre for the first time in non-classical N-heterocyclic carbenes. The ligands are readily accessible by the rare Cadogan indazole synthesis of sterically-hindered N-aryl-1-(2-nitrophenyl)methanimines. Steric and electronic characterization as well as catalytic studies in the synthesis of oxazolines are described. Considering the unique properties of indazole-derived carbenes, we anticipate that this class of compounds will find broad application in organic synthesis and catalysis.

A novel approach to study catalytic reactions via electrophoretic NMR on the example of Pd/NHC-catalyzed Mizoroki-Heck cross-coupling reaction

Investigation of catalytic reactions using nuclear magnetic resonance (NMR) is a crucial task, which is often challenging to perform due to rather complex transformations at the metal center. In this work, it was shown that electrophoretic NMR can be a suitable method for studying catalytic reactions and for observing the changes in the catalyst nature. As an important example involving palladium catalysts with N-heterocyclic carbine ligands (NHCs), the breakage of the Pd-NHC bond can occur during the catalytic process. Electrophoretic NMR allows the distinction of compounds in the spectra depending on the charge, thus bringing new opportunities to mechanistic studies. Here, we present independent evidence of R-NHC product formation in the Pd-catalyzed Mizoroki-Heck reaction-the key process for catalyst change from the molecular to nano-scale type.

A Bis (BICAAC) Palladium(II) Complex: Synthesis and Implementation as Catalyst in Heck-Mizoroki and Suzuki-Miyaura Cross Coupling Reactions

Carbenes are one of the most appealing, well-explored, and exciting ligands in modern chemistry due to their tunable stereoelectronic properties and a wide area of applications. A palladium complex (BICAAC)₂PdCl₂ with a recently discovered cyclic (alkyl)(amino)carbene having bicyclo[2.2.2] octane skeleton (BICAAC) was synthesized and characterized. The enhanced σ-donating and π-accepting ability of this carbene lend a hand to form a robust Pd-carbene bond, which allowed us to probe its reactivity as a precatalyst in Heck-Mizoroki and Suzuki-Miyaura cross-coupling reactions with low catalyst loading in open-air conditions. The diverse range of substrates was explored for both the cross-coupling reactions. To get a better understanding of the catalytic reactions, several analytical techniques such as field-emission scanning electron microscopy, high-resolution transmission electron microscopy, and powder X-ray diffraction were employed in a conclusive manner.

Preparation of Complexes Bearing N-Alkylated, Anionic or Protic CAACs Through Oxidative Addition of 2-Halogenoindole Derivatives

CAAC precursors 2-chloro-3,3-dimethylindole 1 and 2-chloro-1-ethyl-3,3-dimethylindolium tetrafluoroborate 2BF₄ have been prepared and oxidatively added to [M(PPh₃ )₄ ] (M=Pd, Pt). Salt 2BF₄ reacts with [Pd(PPh₃ )₄ ] in toluene at 25 °C over 4 days to yield complex cis-[3]BF₄ featuring an N-ethyl substituted CAAC, two cis-arranged phosphines and a chloro ligand. Compound trans-[3]BF₄ was obtained from the same reaction at 80 °C over 1 day. Salt 2BF₄ reacts with [Pt(PPh₃ )₄ ] to give cis-[4]BF₄ . The neutral indole derivative 1 adds oxidatively to [Pt(PPh₃ )₄ ] to give trans-[5] featuring a CAAC ligand with an unsubstituted ring-nitrogen atom. This nitrogen atom has been protonated with py⋅HBF₄ to give trans-[6]BF₄ bearing a protic CAAC ligand. The Pd^II complex trans-[7]BF₄ bearing a protic CAAC ligand was obtained in a one-pot reaction from 1 and [Pd(PPh₃ )₄ ] in the presence of py⋅HBF₄ .

Nonmetal-catalyzed hydroamination of ynamides with amines

A TfOH-catalyzed hydroamination of ynamides with primary and secondary amines under mild reaction conditions is described for the efficient synthesis of N-arylimines and ethene-1,1-diamines.

Pyridine Wingtip in [Pd(Py-tzNHC)2]2+ Complex Is a Proton Shuttle in the Catalytic Hydroamination of Alkynes

The cationic palladium(II) complex 1 of pyridyl-mesoionic carbene ligand catalyzes Markovnikov-selective intermolecular hydroamination between anilines and terminal alkynes into the corresponding imines. The reaction proceeds at room temperature, in the absence of additives, with exquisite selectivity and diverse functional group tolerance. The key intrinsic feature of the catalyst is the pyridine wingtip confined to the proximity of the alkynophilic metal active site, which mimics the function of enzyme-like architectures by assisting entropically favored proton transfers.

Synthesis of 1,2-Dihydroquinazolines via Rearrangement of Indazolium Salts

A convenient synthesis of 1,2-dihydroquinazolines via rearrangement of indazolium salts was described. A mechanistic study using isotope labeling experiment revealed that the rearrangement passes through cleavage of N-N bond/ring opening after basic deprotonation of 2-benzyl in indazolium salts to yield intermediate E, which proceeds in an intramolecular N-nucleophilic addition to form the observed product. Computational analyses imply that the pathway of the rearrangement is determined by the energy barriers of the ring-closing process and the stability of the product.

A macrocyclic silver polycarbene complex based on 1,2,4-triazole units: synthesis and postsynthetic modification

A method for the synthesis of a new rectangular 1,2,4-triazole-containing macrocycle via photochemical [2 + 2] cycloaddition with a metal–carbene template was developed.

Cyclometalated Palladium(II) N-Heterocyclic Carbene Complexes: Anticancer Agents for Potent In Vitro Cytotoxicity and In Vivo Tumor Growth Suppression

Palladium(II) complexes are generally reactive toward substitution/reduction, and their biological applications are seldom explored. A new series of palladium(II) N-heterocyclic carbene (NHC) complexes that are stable in the presence of biological thiols are reported. A representative complex, [Pd(C^N^N)(N,N'-nBu2 NHC)](CF3 SO3 ) (Pd1 d, HC^N^N=6-phenyl-2,2'-bipyridine, N,N'-nBu2 NHC=N,N'-di-n-butylimidazolylidene), displays potent killing activity toward cancer cell lines (IC50 =0.09-0.5 μm) but is less cytotoxic toward a normal human fibroblast cell line (CCD-19Lu, IC50 =11.8 μm). In vivo anticancer studies revealed that Pd1 d significantly inhibited tumor growth in a nude mice model. Proteomics data and in vitro biochemical assays reveal that Pd1 d exerts anticancer effects, including inhibition of an epidermal growth factor receptor pathway, induction of mitochondrial dysfunction, and antiangiogenic activity to endothelial cells.

Synthesis, structure and catalysis of a NHC–Pd(ii) complex based on a tetradentate mixed ligand

NHC–Pd(ii) complex 1 has been prepared. The catalytic activity of 1 in C–C coupling reactions was studied.

Titanium complexes supported by imidazo[1,5-a]pyridine-containing pyrrolyl ligand as catalysts for hydroamination and polymerization reactions, and as an antitumor reagent

The syntheses, structures, catalytic properties and antitumor activities of three titanium complexes supported by an imidazo[1,5-a]pyridine-containing pyrrolyl ligand are reported.

Sydnone anions and abnormal N-heterocyclic carbenes of O-ethylsydnones. Characterizations, calculations and catalyses

Sydnone anions and their deprotonated O-ethyl derivatives (aNHCs) form catalytically active Pd complexes.