The Buchwald-Hartwig Amination After 25 Years

Synthesis of Aza[n]helicenes (n = 4-7) via Photocyclodehydrochlorination of 1-Chloro-N-aryl-2-naphthamides

A series of aza[n]helicenes (n = 4-7) was synthesized using a photocyclodehydrochlorination of 1-chloro-N-aryl-2-naphthamides as a general synthetic procedure introducing the nitrogen atom to the third ring of the helicene framework. The effect of the nitrogen presence in the helicene skeleton on the physicochemical properties of the prepared aza[n]helicenes was studied and compared to those of the parent carbo-analogues. The insertion of a nitrogen atom into the outer edge of the helicene molecule has a severe impact on certain physicochemical properties such as optical rotation, electrostatic potentials, and intermolecular interactions. On the other hand, some other properties such as UV/vis, fluorescence, and phosphorescence spectra remained almost unaffected when compared to the parent carbohelicenes. A nitrogen atom can be also used for further derivatization, which can lead to further modification of helicene properties, as manifested here in the fluorescence changes induced by protonation.

Metal-Organic Framework Surface Functionalization Enhancing the Activity and Stability of Palladium Nanoparticles for Carbon-Halogen Bond Activation

Supported metal nanocatalyst is one of the efficient tools for organic transformations. However, catalyst deactivation caused by the migration, aggregation, and leaching of active metal species in the reaction process remains challenging. Herein, a metal-organic framework (MOF), MIL-101, was employed to covalently graft the PPh₃ ligand on its surface and then supported palladium nanoparticles (Pd NPs), affording Pd/MIL-101-PPh₃. A variety of spectral characterizations and DFT calculation reveal that there is an electron-donating effect of the MOF surface PPh₃ toward Pd NPs, which markedly boosts the activation of the carbon-halogen bond in aryl halides. Consequently, Pd/MIL-101-PPh₃ exhibits excellent activity for the three-component reaction of 2-iodoaniline, CO₂, and isocyanide, as well as Suzuki-Miyaura and Heck coupling reactions, far exceeding amino-functionalized Pd/MIL-101-NH₂, naked Pd/MIL-101, and other commercial-supported Pd catalysts. Furthermore, Pd/MIL-101-PPh₃ can also frustrate the migration, aggregation, and leaching of reactive Pd species in the reaction process due to the molecular fence effect generated by MOF surface functionalization.

Iron-Catalyzed Oxidative C-O and C-N Coupling Reactions Using Air as Sole Oxidant

We describe the oxygenation of tertiary arylamines, and the amination of tertiary arylamines and phenols. The key step of these coupling reactions is an iron-catalyzed oxidative C-O or C-N bond formation which generally provides the corresponding products in high yields and with excellent regioselectivity. The transformations are accomplished using hexadecafluorophthalocyanine-iron(II) (FePcF16 ) as catalyst in the presence of an acid or a base additive and require only ambient air as sole oxidant.

The Potential of Micellar Media in the Synthesis of DNA-Encoded Libraries

DNA‐encoded library (DEL) technology has become widely used in drug discovery research. The construction of DELs requires robust organic transformations that proceed in aqueous media under mild conditions. Unfortunately, the application of water as reaction medium for organic synthesis is not evident due to the generally limited solubility of organic reagents. However, the use of surfactants can offer a solution to this issue. Oil‐in‐water microemulsions formed by surfactant micelles are able to localize hydrophobic reagents inside them, resulting in high local concentrations of the organic substances in an otherwise poorly solvated environment. This review provides a conceptual and critical summary of micellar synthesis possibilities that are well suited to DEL synthesis. Existing examples of micellar DEL approaches, together with a selection of micellar organic transformations fundamentally suitable for DEL are discussed.

Mapping Dual-Base-Enabled Nickel-Catalyzed Aryl Amidations: Application in the Synthesis of 4-Quinolones

The C-N cross-coupling of (hetero)aryl (pseudo)halides with NH substrates employing nickel catalysts and organic amine bases represents an emergent strategy for the sustainable synthesis of (hetero)anilines. However, unlike protocols that rely on photoredox/electrochemical/reductant methods within Ni^I/III cycles, the reaction steps that comprise a putative Ni^0/II C-N cross-coupling cycle for a thermally promoted catalyst system using organic amine base have not been elucidated. Here we disclose an efficient new nickel-catalyzed protocol for the C-N cross-coupling of amides and 2'-(pseudo)halide-substituted acetophenones, for the first time where the (pseudo)halide is chloride or sulfonate, which makes use of the commercial bisphosphine ligand PAd2-DalPhos (L4) in combination with an organic amine base/halide scavenger, leading to 4-quinolones. Room-temperature stoichiometric experiments involving isolated Ni^{0, I, and II} species support a Ni^0/II pathway, where the combined action of DBU/NaTFA allows for room-temperature amide cross-couplings.

A "Pool and Split" Approach to the Optimization of Challenging Pd-Catalyzed C-N Cross-Coupling Reactions

A screening method for the rapid identification of catalytic conditions for Pd-catalyzed C-N cross-coupling reactions is reported. The strategy evaluates mixtures of precatalysts, ligands, and bases to identify productive conditions that are subsequently optimized through two deconvolution steps, which uncover the active catalyst and identify the optimal solvent and base for the catalytic system. The efficacy of this approach was demonstrated through application to a previously reported reaction, whereby both the literature conditions and additional solutions were retrieved. The same approach to Ni-catalyzed C-N cross-coupling was investigated in parallel but was found to be less successful due to limited activity of the evaluated reagent combinations. Finally, the utility of this method was showcased by identifying effective conditions for the Pd-catalyzed cross-coupling of complex molecules, which not only revealed nonobvious solutions for the processes under evaluation but also resulted in the discovery of new chemical reactions.

Structural and Thermal Characterization of Halogenated Azidopyridines: Under-Reported Synthons for Medicinal Chemistry

Owing to their participation in Click reactions, bifunctional azides are valuable intermediates in the preparation of medicines and biochemical tool compounds. Despite the privileged nature of pyridines among pharmaceutical scaffolds, reports of the synthesis and characterization of azidopyridines bearing a halogen substituent for further elaboration are almost completely unknown in the literature. As azidopyridines carry nearly equal numbers of nitrogen and carbon atoms, we hypothesized that safety concerns limited the application of these useful bifunctional building blocks in medicinal and biological chemistry. To address this concern, we prepared and characterized nine azidopyridines bearing a single fluorine, chlorine, or bromine atom. All were examined by differential scanning calorimetry (DSC), in which they demonstrated exotherms of 228-326 kJ/mol and onset temperatures between 119 and 135 °C. Selected azidopyridines were advanced to mechanical stress testing, in which impact sensitivity was noted for one regioisomer of C5H3FN4. The utility of these versatile intermediates was demonstrated through their use in a variety of Click reactions and the diversification of the halogen handles.

Catalytic Amination of Phenols with Amines

Given the wide prevalence and ready availability of both phenols and amines, aniline synthesis through direct coupling between these starting materials would be extremely attractive. Herein, we describe a rhodium-catalyzed amination of phenols, which provides concise access to diverse anilines, with water as the sole byproduct. The arenophilic rhodium catalyst facilitates the inherently difficult keto-enol tautomerization of phenols by means of π-coordination, allowing for the subsequent dehydrative condensation with amines. We demonstrate the generality of this redox-neutral catalysis by carrying out reactions of a large array of phenols with various electronic properties and a wide variety of primary and secondary amines. Several examples of late-stage functionalization of structurally complex bioactive molecules, including pharmaceuticals, further illustrate the potential broad utility of the method.

Solid-State C-N Cross-Coupling Reactions with Carbazoles as Nitrogen Nucleophiles Using Mechanochemistry

The palladium-catalyzed solid-state C-N cross-coupling of carbazoles with aryl halides via a high-temperature ball-milling technique has been reported. This reaction allowed simple, fast, and efficient synthesis of N-arylcarbazole derivatives in good to excellent yields without the use of large amounts of organic solvents in air. Importantly, the developed solid-state coupling approach enabled the cross-coupling of poorly soluble aryl halides with large polyaromatic structures that are barely reactive under conventional solution-based conditions.

Highly Selective Hydrogenative Conversion of Nitriles into Tertiary, Secondary, and Primary Amines under Flow Reaction Conditions

Flow reaction methods have been developed to selectively synthesize tertiary, secondary, and primary amines depending on heterogeneous platinum-group metal species under catalytic hydrogenation conditions using nitriles as starting materials. A 10 % Pd/C-packed catalyst cartridge affords symmetrically substituted tertiary amines in good to excellent yields. A 10 % Rh/C-packed catalyst cartridge enables the divergent synthesis of secondary and primary amines, with either cyclohexane or acetic acid as a solvent, respectively. Reaction parameters, such as the metal catalyst, solvent, and reaction temperature, and continuous-flow conditions, such as flow direction and second support of the catalyst in a catalyst cartridge, are quite important for controlling the reaction between the hydrogenation of nitriles and nucleophilic attack of in situ-generated amines to imine intermediates. A wide variety of aliphatic and aromatic nitriles could be highly selectively transformed into the corresponding tertiary, secondary, and primary amines by simply changing the metal species of the catalyst or flow parameters. Furthermore, the selective continuous-flow methodologies are applied over at least 72 h to afford three different types of amines in 80-99 % yield without decrease in catalytic activities.

Recent advances of aminoazanium salts as amination reagents in organic synthesis

This review summarizes the utilization of aminoazaniums as amination reagents in organic synthesis, including the amination of aldehydes, boronic esters, olefins, etc.

Visible light-induced Ni-catalyzed C–heteroatom cross-coupling of aryl halides via LMCT with DBU to access a Ni(I)/Ni(III) cycle

Cross-coupling of aryl halides with nucleophiles is a synthetically attractive strategy to construct C–heteroatom bonds. Here we report a highly efficient photoinduced Ni-catalyzed method for the C–heteroatom cross-coupling of aryl...

Iodonitrene: a direct metal-free electrophilic aminating reagent

Iodonitrene is a new type of reactive electrophilic aminating reagent that opens up opportunities for new discoveries.

Buchwald-Hartwig Amination of Aryl Esters and Chlorides catalyzed by Dianisole-decorated Pd-NHC complex

A modular and generic method for the Buchwald-Hartwig amination reactions of relatively unreactive aryl esters as acyl electrophiles and aryl chlorides as aryl electrophiles is developed, delivering efficient synthesis of...

Directing Group Strategies in Rhodium-Catalyzed C-H Amination

Construction of a carbon-nitrogen bond is one of the most prevalent operations in nature and organic synthesis. The resulting amino compounds are privileged structural fragments in natural products, pharmaceutical drugs,...

Decarboxylative Amination of Benzoic Acids Bearing Electron-Donating Substituents and Non-Activated Amines

Efficient methods for decarboxylative activation of benzoic acids into great valuable products are highly sought after. Here we report a highly desirable and straightforward decarboxylative amination of readily available benzoic...

Air-Stable, Well-Defined Palladium–BIAN–NHC Chloro Dimer: Fast-Activating, Highly Efficient Catalyst for Cross-Coupling

We report the synthesis, characterization and reactivity of an air-stable, well-defined acenaphthoimidazolylidene palladium–BIAN–NHC chloro dimer complex, [Pd(BIAN–IPr)(μ-Cl)Cl]2. This rapidly activating catalyst merges the reactive properties of palladium chloro dimers, [Pd(NHC)(μ-Cl)Cl]2,...

Synthetic route towards 1,2,3,4-tetrahydroquinoxaline/piperidine combined tricyclic ring system

The synthetic route toward novel tricyclic, nitrogen-containing system is disclosed. Three novel compounds possessing structural features of 1,2,3,4-tetrahydroquinoxaline and decahydropyrido[3,4-b]pyrazine are synthesized starting from readily available precursors in six or seven steps, of which the last three or four steps respectively are diastereoselective. Key reaction steps include N-acylation, Hofmann rearrangement and ring-closing Buchwald? Hartwig reaction. Compounds trans-8, cis-12 and trans-12 are synthesized in order to prove that this novel, tricyclic system can be functionalized with various groups. Synthetic significance of this heterocyclic system lies in the possibility for the orthogonal functionalization of three different amino groups, allowing fine structural tuning.

Selective Buchwald–Hartwig arylation of C-amino-1,2,4-triazoles and other coordinating aminoheterocycles enabled by bulky NHC ligands and TPEDO activator

A facile method for selective N-(hetero)arylation of coordinating 3(5)-amino-1,2,4-triazoles under Pd/NHC catalysis using TPEDO as a new efficient Pd(ii) to Pd(0) reductant has been developed.

Bis-isonicotinoyl linkers containing polyaromatic scaffolds: synthesis, structure and spectroscopic properties

In this study, a new series of extended linkers containing different polyaromatic chromophores (biphenyl, naphthalene, anthracene, fluorene, 9,9-dimethylfluorene and fluorenone) functionalized with isonicotinoyl moieties have been synthesized by Pd-catalyzed cross-coupling reactions involving isonicotinamide and the appropriate aromatic dibromide. The optimized protocol led to the isolation of the target molecules in good yield and with high purity. These were characterized by ¹H NMR, FTIR, MS, and elemental analysis and their solid state structures were solved by single-crystal X-ray diffraction analysis. Electronic absorption and emission spectra were collected both in solution (DMF) and in the solid state. TDDFT calculations were carried out to investigate the effect of the isonicotinoyl moieties on the spectral features of the central chromophores. Although in solution only the linker containing a fluorenone scaffold shows a weak fluorescence, all the isolated linkers turned out to be fluorescent in the solid state, thus paving the way for their use for the fabrication of fluorescent MOFs.

Synthesis of Azacarbolines via PhIO2-Promoted Intramolecular Oxidative Cyclization of α-Indolylhydrazones

An unprecedented synthesis of polysubstituted indole-fused pyridazines (azacarbolines) from α-indolylhydrazones under oxidative conditions using a combination of iodylbenzene (PhIO₂) and trifluoroacetic acid (TFA) has been developed. This transformation is conducted without the need for transition metals, harsh conditions, or an inert atmosphere.

Buchwald-Hartwig Amination of Aryl Halides with Heterocyclic Amines in the Synthesis of Highly Fluorescent Benzodifuran-Based Star-Shaped Organic Semiconductors

The study of palladium-catalyzed amination of bromobenzene with aromatic and heterocyclic amines, widely used in the synthesis of organic semiconductors, was performed. The best conditions for the coupling of aryl bromides with carbazole, diphenylamine, phenoxazine, phenothiazine, 9,9-dimethyl-9,10-dihydroacridine, and their derivatives have been developed. Based on the results, nine new star-shaped organic semiconductors, exhibiting up to 100% fluorescent quantum yield in the 400-550 nm range, have been synthesized in good yields. The TDDFT calculations of the absorption spectra revealed a good correlation with experimental results and slight solvatochromic effects with a change in the polarity of the solvent.

Expanding the Diversity at the C-4 Position of Pyrido[2,3-d]pyrimidin-7(8H)-ones to Achieve Biological Activity against ZAP-70

Pyrido[2,3-d]pyrimidin-7(8H)-ones have attracted widespread interest due to their similarity with nitrogenous bases found in DNA and RNA and their potential applicability as tyrosine kinase inhibitors. Such structures, presenting up to five diversity centers, have allowed the synthesis of a wide range of differently substituted compounds; however, the diversity at the C4 position has mostly been limited to a few substituents. In this paper, a general synthetic methodology for the synthesis of 4-substituted-2-(phenylamino)-5,6-dihydropyrido[2,3-d]pyrimidin-7(8H)-ones is described. By using cross-coupling reactions, such as Ullmann, Buchwald–Hartwig, Suzuki–Miyaura, or Sonogashira reactions, catalyzed by Cu or Pd, we were able to describe new potential biologically active compounds. The resulting pyrido[2,3-d]pyrimidin-7(8H)-ones include N-alkyl, N-aryl, O-aryl, S-aryl, aryl, and arylethynyl substituents at C4, which have never been explored in connection with the biological activity of such heterocycles as tyrosine kinase inhibitors, in particular as ZAP-70 inhibitors.

O-Protected NH-free hydroxylamines: emerging electrophilic aminating reagents for organic synthesis

In this highlight, O-protected NH-free hydroxylamine derivatives have been evaluated in the construction of nitrogen-enriched compounds, such as primary amines, amides, and N-heterocycles, with high regio-, chemo- and stereoselectivity in the unprotected form, showcasing the late-stage functionalization of natural products, drugs and functional molecules by biocatalysis, organocatalysis, and transition metal catalysis. The reactivity dichotomy among these N-O reagents has been explored based on SET and metal-nitrenoids.

Micellar Buchwald-Hartwig Coupling of Aryl and Heteroarylamines for the Synthesis of DNA-Encoded Libraries

DNA-encoded libraries are a very efficient means of identifying ligands for protein targets in high throughput. To fully maximize their use, it is essential to be able to carry out efficient reactions on DNA-conjugated substrates. Arylamines are privileged motifs in druglike molecules, and methods for their incorporation into DNA-encoded libraries are highly desirable. One of the preferred methods for their preparation, the Buchwald-Hartwig coupling, does not perform well on DNA conjugates using current approaches. We report the application of our recently developed micellar technology for on-DNA chemistry to the Buchwald-Hartwig reaction. Optimization of conditions led to a robust, high-yielding method for the synthesis of DNA-conjugated aryl and heteroarylamines, which is broad in substrate scope for both the arylamine and the DNA-conjugated aryl halide and is fully compatible with DNA-encoding and decoding procedures. This method will enable the preparation of diverse, high-fidelity libraries of biarylamines.

Metal-catalyzed reactions for the C(sp2)–N bond formation: achievements of recent years

The review deals with the main catalytic methods for the C(sp2)–N bond formation, including Buchwald–Hartwig palladium-catalyzed amination of aryl and heteroaryl halides, renaissance of the Ullmann chemistry, i.e., the application of catalysis by copper complexes to form the carbon–nitrogen bond, and Chan–Lam reactions of (hetero)arylboronic acids with amines. Also, oxidative amination with C–H activation, which has been booming during the last decade, is addressed. Particular attention is paid to achievements in the application of heterogenized catalysts.

The bibliography includes 350 references.

Acetonitrile and benzonitrile as versatile amino sources in copper-catalyzed mild electrochemical C-H amidation reactions

A mild, efficient electrochemical approach to the site-selective direct C–H amidation of benzene and its derivatives with acetonitrile and benzonitrile has been developed. It has been shown that joint electrochemical oxidation of various arenes in the presence of a copper salt as a catalyst and nitriles leads to the formation of N-phenylacetamide from benzene and N-benzylacetamides from benzyl derivatives (up to 78% yield). A favorable feature of the process is mild conditions (room temperature, ambient pressure, no strong oxidants) that meet the criteria of green chemistry.

Different pathways of C–H transformation depending on the substrate nature and oxidation potential.

Halogen-Bridged Methylnaphthyl Palladium Dimers as Versatile Catalyst Precursors in Coupling Reactions

Halogen‐bridged methylnaphthyl (MeNAP) palladium dimers are presented as multipurpose Pd‐precursors, ideally suited for catalytic method development and preparative organic synthesis. By simply mixing with phosphine or carbene ligands, they are in situ converted into well‐defined monoligated complexes. Their catalytic performance was benchmarked against state‐of‐the‐art systems in challenging Buchwald–Hartwig, Heck, Suzuki and Negishi couplings, and ketone arylations. Their use enabled record‐setting activities, beyond those achievable by optimization of the ligand alone. The MeNAP catalysts permit syntheses of tetra‐ortho‐substituted arenes and bulky anilines in near‐quantitative yields at room temperature, allow mono‐arylations of small ketones, and enable so far elusive cross‐couplings of secondary alkyl boronic acids with aryl chlorides.