Visible-Light-Mediated Nickel(II)-Catalyzed C–N Cross-Coupling in Water: Green and Regioselective Access for the Synthesis of Pyrazole-Containing Compounds

Oxidative Nickel-Catalyzed ortho-C-H Amination of (Iso)quinolines with Alicyclic Amines Directed by a Sacrificial N-Oxide Group

Transition metal (TM)-catalyzed direct amination of C-H bonds on free or fused pyridine (Py) rings with free amines still remains scarce because amines and the Py ring tend to adopt a nonproductive N-bound coordination with many TMs, leading to a significant decrease of catalytic reactivity. We herein disclose a nickel-catalyzed and a sacrificial N-oxide group directed oxidative coupling of (iso)quinolyl C-H bonds and alicyclic amines, which furnishes bioimportant amino(iso)quinolines efficiently and selectively in a single step. Noteworthy, this protocol avoids the use of aggressive reactants and very strong bases usually required when aminating on nonoxidized Py rings.

Transition-Metal-Catalyzed Directed C-H Functionalization in/on Water

Directing group assisted C-H bond functionalization using transition-metal-catalysis has emerged as a reliable synthetic tool for the construction of regioselective carbon-carbon/heteroatom bonds. Off late, "in/on water directed transition-metal-catalysis", though still underdeveloped, has appeared as one of the prominent themes in sustainable organic chemistry. This article covers the advancements, mechanistic insights and application of the sustainable directed C-H bond functionalization of (hetero)arenes in/on water in the presence of transition-metal-catalysis.

A Traceless Heterocyclic Amine Mediator in Regioselectivity-Switchable Formal [1 + 2 + 2] Cycloaddition Reaction to 1,3,4- and 1,4,5-Trisubstituted Pyrazoles

Switchable multicomponent reactions have been attractive tools for the construction of compound libraries with skeleton diversity and complexity by slightly changing the reaction conditions. Described herein is a regioselectivity-switchable formal [1 + 2 + 2] cycloaddition reaction from difluoroalkyl compounds, enaminones, and RNHNH₂, ultimately using 1-methylindazol-3-amine as a traceless mediator to switch the inherent regioselectivity of 1,3,4-trisubstituted pyrazole formation to 1,4,5-trisubstituted pyrazoles. Remarkable features of this work include mild conditions, simple operation, and broad scopes.

Silver(I)-Catalyzed C4-H Amination of 1-Naphthylamine Derivatives with Azodicarboxylates at Room Temperature

A highly facile and efficient protocol for silver(I)-catalyzed C4–H amination of 1-naphthylamine derivatives with readily available azodicarboxylates utilizing picolinamide as a bidentate directing group is reported, providing an alternative strategy for the synthesis of 4-aminated 1-naphthylamine derivatives. The reaction proceeded smoothly in acetone at room temperature undergoing a self-redox process under the base and external oxidant-free conditions, affording the desired products with good to excellent yields.

Photoredox-Catalyzed C-H Functionalization Reactions

The fields of C-H functionalization and photoredox catalysis have garnered enormous interest and utility in the past several decades. Many different scientific disciplines have relied on C-H functionalization and photoredox strategies including natural product synthesis, drug discovery, radiolabeling, bioconjugation, materials, and fine chemical synthesis. In this Review, we highlight the use of photoredox catalysis in C-H functionalization reactions. We separate the review into inorganic/organometallic photoredox catalysts and organic-based photoredox catalytic systems. Further subdivision by reaction class─either sp2 or sp3 C-H functionalization─lends perspective and tactical strategies for use of these methods in synthetic applications.

The cobalt(ii)-catalyzed acyloxylation of picolinamides with bifunctional silver carboxylate via C–H bond activation

The cobalt(ii)-catalyzed C–H bond acyloxylation of picolinamides with bifunctional silver carboxylate has been developed. The mild and practical esterification provides an atom-economic route to access to polysubstituted naphthalene compounds.

Visible light photocatalysis in the synthesis of pharmaceutically relevant heterocyclic scaffolds

Visible light and photoredox catalysis have emerged as a powerful and long-lasting tool for organic synthesis, demonstrating the importance of a variety of chemical bond formation methods.

Merging cobalt and photoredox catalysis for the C8-H alkoxylation of 1-naphthylamine derivatives with alcohols

A combined cobalt and photoredox catalysis system to realize the C8-H alkoxylation of 1-naphthylamine derivatives with alcohols was developed. Using commercially available alkyl alcohols as raw materials and Co(OAc)₂ and rose bengal as catalysts, 1-naphthylamine derivatives reacted with alcohols to generate the corresponding C8-H alkoxylation products in good yields.

Applications of Nickel(II) Compounds in Organic Synthesis

This review article summarizes the applications of nickel(II) compounds in organic synthesis since 2016. In recent years, the field of nickel(II) catalysis is gaining considerable interest due to readily available, low-cost nickel(II)-compounds and several key properties of nickel. This review article is organized by the reaction type, although some reactions can be placed in multiple sections.

Merging Photoredox Catalysis with Transition Metal Catalysis: Direct C4-H Sulfamidation of 1-Naphthylamine Derivatives

A mild and efficient protocol for the copper(I)-catalyzed C4-H sulfamidation of 1-naphthylamine derivatives with diphenylsulfonimide (NHSI) was explored at room temperature, affording the desire produces in moderate to good yields. The control experiments indicated that this visible-light-promoted reaction might proceed via a single-electron-transfer process. In addition, preliminary DFT studies for the intermediates in the catalytic cycle were also explored, indicating that the C4 site in the naphthyl ring is the most likely electrophilic reactive site and providing some exact basis for the plausible mechanism.

Silver(I) Promoted the C4-H Bond Phosphonation of 1-Naphthylamine Derivatives with H-Phosphonates

A simple and efficient protocol for silver-promoted direct C-H phosphonation of 1-naphthylamine derivatives with H-phosphonates was described. This reaction proceeded smoothly for 1-naphthylamine derivatives at the C4 site, providing a facile and efficient route to 4-phosphonated 1-naphthylamine derivatives. This phosphonation could tolerate a diverse type of functional groups at the pyridinyl and naphthyl moieties. Further functionalization of the phosphonated product was also explored at the C2 and C8 sites, such as fluoridation, methylation, methoxylation, and amination. In addition, DFT studies of the reaction intermediate showed that the most electrophilic reactive site is at the C4 site in the naphthyl ring.

An organophotoredox-catalyzed C(sp2)-N cross coupling reaction of cyclic aldimines with cyclic aliphatic amines

An organophotocatalyzed C(sp2)-H/N-H cross-dehydrogenative coupling of cyclic aldimines with aliphatic amines has been developed, which represents the first example of visible-light-induced C-H amination of N-sulfonylated imines. This methodology enables the streamline assembly of amine derivatives via radical mediated C-N bond formation. The current protocol features transition-metal-free, mild conditions, good functional group tolerance and good yields.

Metal-free regioselective C-H amination for the synthesis of pyrazole-containing 2H-indazoles

A general and practical regioselective approach for the C-H amination of 2H-indazoles under transition-metal-free conditions was developed. A series of substrates were tested showing eminent functional group tolerance and affording the C-N functionalization products in good to excellent yields. Mechanism studies revealed that a radical process was involved in this transformation.

Visible-light-induced ligand-free RuCl3 catalyzed C-H phosphorylation in water

Visible-light-induced C-H phosphorylation of para-C_Ar-H and heteroarenes was realized using cost-effective RuCl₃ as a catalyst. The reaction conditions are green and environmentally friendly, using water as a solvent at room temperature and without ligands. A broad range of highly functional organophosphorus compounds were obtained via a cross-dehydrogenation-coupling (CDC) reaction. In addition, we also proved that RuCl₃ is a photocatalyst via its absorption spectrum and on/off light experiments.

Cobalt(II)-Catalyzed [4+2] Annulation of Picolinamides with Alkynes via C-H Bond Activation

A cobalt(II)-catalyzed [4+2] annulation of picolinamides with alkynes via C-H bond activation has been developed. The operationally simple annulation reaction allows for the synthesis of acyl-substituted 1H-benzoquinoline bearing multiple aromatic rings (up to 96 % yield) without co-oxidant or other oxidation factors under mild conditions. Several control experiments were carried out. This practical [4+2] annulation provides an efficient route to access highly functionalized compounds.

Visible-Light-Induced Metal-Free Synthesis of 2-Phosphorylated Thioflavones in Water

The introduction of phosphorus functional groups into the skeleton of thioflavones is an attractive task and of great significance. Herein, a metal-free visible-light-induced radical cascade cyclization was developed for the preparation of 2-phosphorylated thioflavones from methylthiolated alkynones and phosphine oxides. In water as a green reaction medium, a large number of such 2-phosphorylated thioflavones were prepared, catalyzed by 4CzIPN [1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene] under visible-light irradiation. These reactions could be performed at ambient temperature and feature simple operation, wide reaction scope, and recyclability of aqueous media.

Unravelling the mechanism of cobalt-catalysed remote C-H nitration of 8-aminoquinolinamides and expansion of substrate scope towards 1-naphthylpicolinamide

Previously, an unexpected Co-catalysed remote C-H nitration of 8-aminoquinolinamide compounds was developed. This report provided a novel reactivity for Co which was assumed to proceed through the mechanistic pathway already known for analogous Cu-catalysed remote couplings of the same substrates. In order to shed light into this intriguing, and previously unobserved reactivity for Co, a thorough computational study has now been performed, which has allowed for a full understanding of the operative mechanism. This study demonstrates that the Co-catalysed remote coupling does not occur through the previously proposed Single Electron Transfer (SET) mechanism, but actually operates through a high-spin induced remote radical coupling mechanism, through a key intermediate with significant proportion of spin density at the 5- and 7-positions of the aminoquinoline ring. Additionally, new experimental data provides expansion of the synthetic utility of the original nitration procedure towards 1-naphthylpicolinamide which unexpectedly appears to operate via a subtly different mechanism despite having a similar chelate environment.

Visible-light-induced aerobic C3-H fluoroalkoxylation of quinoxalin-2(1H)-ones with fluoroalkyl alcohols

A novel and efficient method of visible-light-induced C3-H fluoroalkoxylation of quinoxalin-2(1H)-ones with fluoroalkyl alcohols is developed. This approach uses readily available fluoroalkyl alcohols as fluoroalkoxylation reagents and displays a wide substrate scope, providing the fluoroalkoxylated products in moderate to good yields. Compared with the previous method, such a transformation uses oxygen as an oxidant, which avoids the utilization of plenty of PhI(TFA)2. In addition, this strategy also gives a practical tool for the rapid synthesis of histamine-4 receptor antagonist and new N-containing bidentate ligands. A radical mechanism was suggested according to the results of control experiments.

A metal-free visible-light-promoted phosphorylation/cyclization reaction in water towards 3-phosphorylated benzothiophenes

A metal-free visible-light-induced phosphorylation/cyclization reaction was developed in water at room temperature for the synthesis of 3-phosphorylated benzothiophenes.

Transition-metal- and oxidant-free three-component reaction of quinoline N-oxides, sodium metabisulfite and aryldiazonium tetrafluoroborates via a dual radical coupling process

A convenient and straightforward three-component transformation of quinoline N-oxides, sodium metabisulfite and aryldiazonium tetrafluoroborates has been developed, providing the target products in moderate to good yields. Compared with previous studies, the present methodology avoids the use of transition-metal catalysts and excess oxidants, providing a simple and practical alternative approach for the construction of 2-sulfonylquinolines. Control experiments indicate that a dual radical coupling process is responsible for this reaction.

Nickel(II)-Catalyzed [5 + 1] Annulation of 2-Carbonyl-1-propargylindoles with Hydroxylamine To Synthesize Pyrazino[1,2-a]indole-2-oxides in Water

An atom-economical and practical method for the efficient synthesis of various pyrazino[1,2-a]indole-2-oxides was developed through a nickel(II)-catalyzed [5 + 1] annulation of 2-carbonyl-1-propargylindoles with hydroxylamine in water without using an organic solvent. The reaction involved an initial condensation of 2-carbonyl-1-propargylindoles with hydroxylamine to afford oxime intermediates, which then underwent a nickel(II)-catalyzed 6-exo-dig cyclization. Preliminary studies showed that (n-Bu)₄NI served as a phase transfer catalyst and promoted the formation of active nickel(II) species. More importantly, the nickel(II) salt and phase transfer catalyst-in-water could be recycled seven times, and a gram scalable product was easily obtained in good yields through a filtration and washing protocol.

Iodine-Catalyzed Regioselective Oxidative Cyclization of Aldehyde Hydrazones with Electron-Deficient Olefins for the Synthesis of Mefenpyr-Diethyl

A regioselective synthesis of polysubstituted dihydropyrazoles and pyrazoles through an iodine-catalyzed oxidative cyclization strategy of aldehyde hydrazones with electron-deficient olefins is described. The protocol adopts very mild reaction conditions and provides desirable yields. The reaction is supposed to proceed via a cascade C-H functionalization, C-N bond formation, and oxidation sequential processes. The overall simplicity and regioselectivity of the catalytic system make this approach a valuable and step-economical tool to construct a C-C bond for the synthesis of Mefenpyr-Diethyl.

Transition-metal free oxidative C-H etherification of acylanilines with alcohols through a radical pathway

A transition metal free approach for the synthesis of methyl/ethyl aryl ether via oxidative C-H etherification of acylanilines with alcohols has been developed. Various acylanilines are compatible under standard conditions, giving the corresponding products in moderate to good yields. This strategy avoids transition-metal catalyst and excessive alcohol, providing a simple and reliable alternative method for the synthesis of methyl/ethyl aryl ether. Control experiments reveal that a radical mechanism is involved in this transformation.

Synthesis of N-Arylpyrazoles by Palladium-Catalyzed Coupling of Aryl Triflates with Pyrazole Derivatives

A method for the synthesis of N-arylpyrazoles by palladium-catalyzed coupling of aryl triflates with pyrazole derivatives is described. Using tBuBrettPhos as a ligand, the palladium-catalyzed C-N coupling of a variety of aryl triflates including ortho-substituted ones with pyrazole derivatives proceeded efficiently to give N-arylpyrazole products in high yields. 3-Trimethylsilylpyrazole was found to be an excellent pyrazole substrate for the coupling, and the corresponding product, 1-aryl-3-trimethylsilylpyrazole, also served as a great template for the syntheses of N-arylpyrazole derivatives, as demonstrated by regioselective halogenation at the 3-, 4-, and 5-positions of the pyrazole ring.

Palladium-catalyzed direct C–H nitration and intramolecular C–H functionalization for the synthesis of 3-nitro-1-(phenylsulfonyl)-1H-indazole derivatives

Various 3-nitro-1-(phenylsulfonyl)-1H-indazole derivatives have been obtained in moderate to high yields using palladium catalysis.