Regioselective and guided C-H activation of 4-nitropyrazoles

Synthetic Chameleon Turns into Oximes, Nitrones, and Hydroxylamines when Exposed to Blue Light

A metal-free, user-friendly photochemical transformation of nitroalkanes to oximes, nitrones, and hydroxylamines has been developed. The visible-light-induced reactions are catalyzed by the readily available photoredox organocatalyst 4CzIPN and use inexpensive amines as reductants. Broad in scope and tolerant of multiple functional groups and heterocycles, the transformation proceeds under mild conditions. Its synthetic potential was demonstrated in the formal total synthesis of amathaspiramide F. A basic insight into the reaction mechanism was gained with the help of an NMR study.

Enantioselective C-H Arylation for Axially Chiral Heterobiaryls

An enantioselective palladium-catalyzed C-H arylation of functionalized pyrazoles/triazoles/imidazoles is developed, affording a variety of axially chiral ortho-nitro/formyl-substituted heterobiaryls with excellent enantioselectivities and good yields. The method features a deuterated P-chiral phosphorus ligand CD3-AntPhos, a broad substrate scope of functionalized heterobiaryls, mild reaction conditions, and low palladium loadings.

Redox-Neutral Vicinal Difunctionalization of Five-Membered Heteroarenes with Dual Electrophiles

A new reaction mode of palladium/norbornene (Pd/NBE) cooperative catalysis is reported involving the selective coupling of two different carbon-based electrophiles for vicinal double C-H functionalization of five-membered heteroarenes in a site-selective and redox-neutral manner. The key is to use alkynyl bromides as the second electrophile, which allows vicinal difunctionalization of a wide range of heteroarenes including pyrroles, thiophenes and furans at their C4 and C5 positions. One- or two-step tetrafunctionalizations of simple pyrrole and thiophene have also been realized. The C2-substituted NBEs prove most effective in these reactions, and the mechanistic exploration discloses the origin of the high selectivity of this transformation. Synthetic utility of this method has been exemplified in the concise preparations of thiophene-containing organic materials and a protein kinase inhibitor analogue. Preliminary success has also been achieved in a direct annulation event, using a tethered ketone as the second electrophile.

Rhodium-catalysed ortho-alkynylation of nitroarenes

The ortho-alkynylation of nitro-(hetero)arenes takes place in the presence of a Rh(iii) catalyst to deliver a wide variety of alkynylated nitroarenes regioselectively. These interesting products could be further derivatized by selective reduction of the nitro group or palladium-catalysed couplings. Experimental and computational mechanistic studies demonstrate that the reaction proceeds via a turnover-limiting electrophilic C-H metalation ortho to the strongly electron-withdrawing nitro group.

C-H activation reactions of nitroarenes: current status and outlook

Ring substitution reactions of nitroarenes remain an under-developed area of organic synthesis, confined to the narrow domains of S_NAr and S_NArH reactions. While searching for alternative methodologies, we took stock of the C-H activation reactions of nitroarenes which unearthed a variety of examples of nitro directed regioselective C-H functionalization reactions such as ortho-arylation, -benzylation/alkylation, and -allylation, oxidative Heck and C-H arylation reactions on (hetero)aromatic rings. A collective account of these reactions is presented in this review to showcase the existing landscape of C-H activation reactions of nitroarenes, to create interest in this field for further development and propagate this strategy as a superior alternative for ring substitution reactions of nitroarenes. The prospect of merging the C-H activation of nitroarenes with C-NO₂ activation, thereby harnessing NO₂ as a transformable multitasking directing group, is also illustrated.

Strategic Advances in Sequential C-Arylations of Heteroarenes

Sequence-specific C-arylation strategies have important applications in medicinal and material research. These strategies allow C-C bond formations in a regioselective manner to synthesize large molecular libraries for studying structure-activity profiles. The past decade has seen the development of single C-C bond forming reactions using various transition-metal catalysts, cryogenic metalation strategies, and metal-free methods. Sequential arylations of heterocycles allow for the formation of multiaryl derivatives and are a preferred choice over de novo synthetic routes. This perspective sheds light on recent strategic advances to develop various sequential synthetic routes for the multiarylation of heteroarenes. This perspective addresses many challenges in optimizing sequential routes with respect to catalysts, reaction parameters, and various strategies adopted to obtain diversely arylated products.

Transition-metal-catalyzed C–H functionalization of pyrazoles

This review describes recent advances in transition-metal-catalyzed C–H functionalization reactions of pyrazoles to form new C–C and C–heteroatom bonds on the pyrazole ring.

Polysubstituted 3-trifluoromethylpyrazoles: regioselective (3 + 2)-cycloaddition of trifluoroacetonitrile imines with enol ethers and functional group transformations

Non-catalysed addition of trifluoroacetonitrile imines to enol ethers provided fully regioselectively (3 + 2)-cycloadducts, which either spontaneously or via Brønsted acid-induced elimination of ROH molecules led to the formation of 3-trifluoromethylated pyrazoles. In the case of 2,3-dihydrofuran, the respective bicyclic intermediate was isolated and its structure was confirmed by X-ray analysis. Using the developed protocol the synthesis of a known antitumor compound SC-560 was performed in 45% yield. Subsequent functionalisations of selected 4-(ω-hydroxyalkyl)pyrazoles at C(5) through lithiation/addition, cross-coupling reactions or via intramolecular Pd-catalysed C-H arylations opened up an access to polysubstituted pyrazoles including unusual tricyclic systems comprising 7-membered rings (oxepane, thiepane and azepane) as the central unit.

Pd/NHC-catalyzed cross-coupling reactions of nitroarenes

N-Heterocyclic carbene (NHC) ligands effective for the cross-coupling of nitroarenes were identified.

Programmed synthesis of triarylnitroimidazoles via sequential cross-coupling reactions

Transition-metal-catalyzed programmed sequential arylation of 2-chloro-4-nitro-1H-imidazoles was achieved.

Copper-catalyzed direct C–H arylselenation of 4-nitro-pyrazoles and other heterocycles with selenium powder and aryl iodides. Access to unsymmetrical heteroaryl selenides

A one-pot, Cu-catalyzed direct C–H arylselenation protocol using elemental Se and aryl iodides was developed for nitro-substituted, N-alkylated pyrazoles, imidazoles and other heterocycles including 4H-chromen-4-one. This general and concise method allows one to obtain a large number of unsymmetrical heteroaryl selenides bearing a variety of substituents. The presence of the nitro group was confirmed to be essential for the C–H activation and can also be used for further functionalisation and manipulation. Several examples of heteroannulated benzoselenazines were also synthesized using the developed synthetic protocol.

In this work, we elaborated a general and straightforward method which permits the rapid assembly of unsymmetrical heteroaryl-aryl selenides containing 4-nitropyrazole, 4-nitroimidazole and a few other heterocyclic scaffolds.

Application of Silicon-Initiated Water Splitting for the Reduction of Organic Substrates

The use of water as a donor for hydrogen suitable for the reduction of several important classes of organic compounds is described. It is found that the reductive water splitting can be promoted by several metalloids among which silicon shows the best efficiency. The developed methodologies were applied for the reduction of nitro compounds, N-oxides, sulfoxides, alkenes, alkynes, hydrodehalogenation as well as for the gram-scale synthesis of several substrates of industrial importance.

Palladium Catalyzed Arylation and Benzylation of Nitroarenes Using Aryl Sulfonates and Benzyl Acetates

Pd-catalyzed arylation or benzylation of nitroazoles using aryl sulfonates or benzyl acetates is described. Electronically varied aryl tosylates and mesylates, as well as benzyl acetates, afford the arylated and benzylated products. Arylation of nitrobenzene is also reported. The relative rate for the arylation of halides is greater than that of tosylates using the reported reaction parameters. These studies enhance the scope of electrophiles for nitroarene arylations and benzylations, which was hitherto limited to the use of halide electrophiles.

Disilanes as oxygen scavengers and surrogates of hydrosilanes suitable for selective reduction of nitroarenes, phosphine oxides and other valuable substrates

In this report, we demonstrate that the reaction of nitroarenes with hexamethyldisilane under various conditions affords a different range of compounds with excellent selectivity.

Pd-Catalyzed and Copper Assisted Regioselective Sequential C2 and C7 Arylation of Thiazolo[5,4-f]quinazolin-9(8H)-one with Aryl Halides

A selective functionalization of thiazolo[5,4-f]quinazolin-9(8H)-one has been developed through sequential activation of C-H bonds to furnish diarylated compounds. This strategy allows the regioselective C2 and C7 arylation by a judicious choice of coupling partners and bases, requiring no additional ligands or directing groups. Differently substituted N(8)-benzylated-2,7-diaryl-thiazoloquinazolin-9(8H)-ones were thereby obtained in a facile manner. A one-pot procedure was also performed. These protocols provide a synthetically useful route for late-stage functionalization of this highly valuable scaffold, required in drug discovery.

Facile synthesis of hybrid Cu2O/Pd–Fe3O4 nanocatalysts for C–H arylation of 4-nitroimidazoles

A facile synthetic strategy for a hybrid Cu₂O/Pd–Fe₃O₄ nanocomposite via controlled thermal decomposition of Fe(CO)₅ and reduction of Pd(OAc)₂ and Cu(acac)₂ was developed.

Regioselectivity in palladium-catalysed direct arylation of 5-membered ring heteroaromatics

In the past few years, several results using modified catalysts and new reaction conditions have been reported permitting better control of the regioselectivity of Pd-catalysed arylations.

Conditions for palladium-catalyzed direct arylations of 4-bromo and 4-iodo N-substituted pyrazoles without C–Br or C–I bond cleavage

The Pd-catalyzed arylation at the C5 position ofN-protected pyrazole derivatives bearing bromo or iodo substituents at the C4 position is described.

Theoretical studies of palladium-catalyzed cycloaddition of alkynyl aryl ethers and alkynes

Density functional theory (DFT) was used to investigate palladium(0)-catalyzed cycloaddition of alkynyl aryl ethers and alkynes to generate 2-methylidene-2H-chromenes. Calculations indicated that the cycloaddition had five possible reaction pathways: I, II, III, IV, and V. In the palladium(0)-alkynyl aryl ether complex IM1, the oxidative addition of the Caryl-H bond occurred prior to the dissociation of a ligand PMe3. The dissociation of a ligand PMe3 from the five-coordinated complex IM2 was much easier to achieve than the hydrogen transfer reaction and the substitution reaction of alkynes. In the palladium(0)-hydride complex IM4, the hydrogen migration of H1 from palladium to carbon C1 was much easier to achieve than migration to carbon C2. In the four-coordinated aryl-palladium-alkyne complexes IM6a and IM6b, the alkyne insertion reaction into the Pd-Caryl bond occurred prior to that into the Pd-Calkenyl bond. The reaction channel IM1 → TS1 → IM2 → IM4 → TS3a → IM5a → IM6a → TS4a1 → IM7a1 → TS5a1 → IM8a was the most favorable among the catalytic reaction pathways of the cycloaddition of alkynyl aryl ethers and 2-butynes catalyzed by the palladium(0)/PMe3 complex. Moreover, hydrogen migration was the rate-determining step for this channel. The dominant product was 2-methylidene-2H-chromenes P1, which is in agreement with experimental studies.