Robust Acenaphthoimidazolylidene Palladacycles: Highly Efficient Catalysts for the Amination of N-Heteroaryl Chlorides

Highly efficient α-arylation of aryl ketones with aryl chlorides by using bulky imidazolylidene-ligated oxazoline palladacycles

α-Aryl derivatives of carbonyl compounds are important building blocks. Herein, we presented an efficient catalytic system for the α-arylation of aryl ketones with inactive aryl chlorides by firstly using N,N'-bis(2,6-diisopropylphenyl)-imidazol-2-ylidene (IPr)-ligated chiral oxazoline palladacycles, and tolerated a wide range of substrates at low catalyst loadings, leading to the desired products in good to excellent yields.

A Second-Generation Palladacycle Architecture Bearing a N-Heterocyclic Carbene and Its Catalytic Behavior in Buchwald–Hartwig Amination Catalysis

Palladacyclic architectures have been shown as versatile motifs in cross-coupling reactions. NHC-ligated palladacycles possessing unique electronic and steric properties have helped to stabilize the catalytically active species and provide additional control over reaction selectivity. Here, we report on a synthetic protocol leading to palladacycle complexes using a mild base and an environmentally desirable solvent, with a focus on complexes bearing backbone-substituted N-heterocyclic carbene ligands. The readily accessible complexes exhibit high catalytic activity in the Buchwald–Hartwig amination. This is achieved using low catalyst loading and mild reaction conditions in a green solvent.

Practical chemoselective aromatic substitution: the synthesis of N-(4-halo-2-nitrophenyl)benzenesulfonamide through the efficient nitration and halogenation of N-phenylbenzenesulfonamide

A novel route involving the metal-promoted tandem nitration and halogenation of N-phenylbenzenesulfonamide to synthesize N-(4-halo-2-nitrophenyl)benzenesulfonamide derivatives has been developed. The method shows highly practical chemoselective and functional group compatibility. In addition, it employs insensitive and inexpensive Cu(NO3)2·3H2O, Fe(NO3)3·9H2O, and NH4NO3 as the nitration reagents, and it provides a direct approach for the preparation of 4-halo-2-nitroaniline, which is a crucial intermediate for the synthesis of benzimidazoles and quinoxaline derivatives.

General Method of Synthesis of 5-(Het)arylamino-1,2,3-triazoles via Buchwald–Hartwig Reaction of 5-Amino- or 5-Halo-1,2,3-triazoles

An efficient access to the novel 5-(het)arylamino-1,2,3-triazole derivatives has been developed. The method is based on Buchwald–Hartwig cross-coupling reaction of 5-Amino or 5-Halo-1,2,3-triazoles with (het)aryl halides and amines, respectively. As result, it was found that palladium complex [(THP-Dipp)Pd(cinn)Cl] bearing expanded-ring N-heterocyclic carbene ligand is the most active catalyst for the process to afford the target molecules in high yields.

Palladacycle-Catalyzed Regioselective Heck Reaction Using Diaryliodonium Triflates and Aryl Iodides

We describe a P-containing palladacycle-catalyzed regioselective Heck reaction of 2,3-dihydrofuran with diaryliodonium salts and aryl iodides to afford 2-aryl-2,5-dihydrofurans and 2-aryl-2,3-dihydrofurans, respectively, in good yields. Mechanistic studies revealed that the oxidative addition of diaryliodonium salts to palladacycles to form Pd(IV) species showed high chemoselectivity and that electron-rich aryl moieties were preferentially transferred to the Heck product. DFT calculations indicated that the regioselectivity-determining step is the reductive elimination reaction rather than the isomerization and reinsertion of Pd(IV)-hydride intermediates into the double bond.

Reductive Aldol-type Reactions in the Synthesis of Pharmaceuticals

The efficient chemo-, regio- and stereoselective formation of saturated carbon-carbon fragment is the critical challenge of organic synthesis; therefore, developing new methods for formation of these bonds is paramount. The rising interest for reductive aldol-type reactions is conditioned by its versatile applications, allowing the efficient formation of carbon-carbon bonds. The review aims to highlight the advantages and disadvantage of reductive aldol-type reactions to total synthesis of pharmaceutical substances in order to summarize knowledge and encourage further investigation of the field.

Acenaphthene-Based N-Heterocyclic Carbene Metal Complexes: Synthesis and Application in Catalysis

N-Heterocyclic carbene (NHC) ligands have become a privileged structural motif in modern homogenous and heterogeneous catalysis. The last two decades have brought a plethora of structurally and electronically diversified carbene ligands, enabling the development of cutting-edge transformations, especially in the area of carbon-carbon bond formation. Although most of these were accomplished with common imidazolylidene and imidazolinylidene ligands, the most challenging ones were only accessible with the acenaphthylene-derived N-heterocyclic carbene ligands bearing a π-extended system. Their superior σ-donor capabilities with simultaneous ease of modification of the rigid backbone enhance the catalytic activity and stability of their transition metal complexes, which makes BIAN-NHC (BIAN—bis(imino)acenaphthene) ligands an attractive tool for the development of challenging reactions. The present review summarizes synthetic efforts towards BIAN-NHC metal complexes bearing acenaphthylene subunits and their applications in modern catalysis, with special emphasis put on recently developed enantioselective processes.

BIAN-NHC Ligands in Transition-Metal-Catalysis: A Perfect Union of Sterically Encumbered, Electronically Tunable N-Heterocyclic Carbenes?

The discovery of NHCs (NHC = N-heterocyclic carbenes) as ancillary ligands in transition-metal-catalysis ranks as one of the most important developments in synthesis and catalysis. It is now well-recognized that the strong σ-donating properties of NHCs along with the ease of scaffold modification and a steric shielding of the N-wingtip substituents around the metal center enable dramatic improvements in catalytic processes, including the discovery of reactions that are not possible using other ancillary ligands. In this context, although the classical NHCs based on imidazolylidene and imidazolinylidene ring systems are now well-established, recently tremendous progress has been made in the development and catalytic applications of BIAN-NHC (BIAN = bis(imino)acenaphthene) class of ligands. The enhanced reactivity of BIAN-NHCs is a direct result of the combination of electronic and steric properties that collectively allow for a major expansion of the scope of catalytic processes that can be accomplished using NHCs. BIAN-NHC ligands take advantage of (1) the stronger σ-donation, (2) lower lying LUMO orbitals, (3) the presence of an extended π-system, (4) the rigid backbone that pushes the N-wingtip substituents closer to the metal center by buttressing effect, thus resulting in a significantly improved control of the catalytic center and enhanced air-stability of BIAN-NHC-metal complexes at low oxidation state. Acenaphthoquinone as a precursor enables facile scaffold modification, including for the first time the high yielding synthesis of unsymmetrical NHCs with unique catalytic properties. Overall, this results in a highly attractive, easily accessible class of ligands that bring major advances and emerge as a leading practical alternative to classical NHCs in various aspects of catalysis, cross-coupling and C-H activation endeavors.

Nickel-Catalyzed Amination of Aryl Chlorides with Amides

A nickel-catalyzed amination of aryl chlorides with diverse amides via C-N bond cleavage has been realized under mild conditions. A broad substrate scope with excellent functional group tolerance at a low catalyst loading makes the protocol powerful for synthesizing various aromatic amines. The aryl chlorides could selectively couple to the amino fragments rather than the carbonyl moieties of amides. Our protocol complements the conventional amination of aryl chlorides and expands the usage of inactive amides.

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.