Cobalt-Catalyzed C−H Nitration of Indoles by Employing a Removable Directing Group

Selective Arene Photonitration via Iron-Complex β-Homolysis

Nitroaromatics, as an important member and source of nitrogen-containing aromatics, is bringing enormous economic benefits in fields of pharmaceuticals, dyes, pesticides, functional materials, fertilizers, and explosives. Nonetheless, the notoriously polluting nitration industry, which suffers from excessive discharge of fumes and waste acids, poor functional group tolerance, and tremendous purification difficulty, renders mild, efficient, and environmentally friendly nitration a formidable challenge. Herein, we develop a visible-light-driven biocompatible arene C-H nitration strategy with good efficiency and regioselectivity, marvelous substrate applicability and functional group tolerance, and wide application in scale-up synthesis, total synthesis, and late-stage functionalization. A nitryl radical delivered through unusual β-homolysis of a photoexcited ferric-nitrate complex is proposed to be the key nitrification reagent in this system.

Nitration of Pyrrolo[2,1-a]isoquinolines

We have successfully modified a series of pyrrolo[2,1-a]isoquinolines via direct nitration under mild reaction conditions. Easily accessible nitrates including CAN, Cu(NO₃)₂·H₂O, and Fe(NO₃)₃·9H₂O all can serve as effective nitrating reagents for functionalizing pyrrolo[2,1-a]isoquinolines. Various nitro-bearing pyrrolo[2,1-a]isoquinolines have been efficiently prepared in acceptable to good yields.

Palladium-Catalysed Intermolecular Direct C–H Bond Arylation of Heteroarenes with Reagents Alternative to Aryl Halides: Current State of the Art

Abstract:

Abstract: This unprecedented review with 322 references provides a critical up-to-date picture of the Pd-catalysed intermolecular direct C–H bond arylation of heteroarenes with arylating reagents alternative to aryl halides that include aryl sulfonates (aryl triflates, tosylates, mesylates, and imidazole-1-sulfonates), diaryliodonium salts, [(diacetoxy)iodo]arenes, arenediazonium salts, 1-aryltriazenes, arylhydrazines and N’-arylhydrazides, arenesulfonyl chlorides, sodium arenesulfinates, arenesulfinic acids, and arenesulfonohydrazides. Particular attention has been paid to summarise the preparation of the various arylating reagents and to highlight the practicality, versatility, and limitations of the various developed arylation protocols, also comparing their results with those achieved in analogous Pd-catalysed arylation reactions involving the use of aryl halides as electrophiles. Mechanistic proposals have also been briefly summarised and discussed. However, data concerning Pd-catalysed direct C–H bond arylations involving the C–H bonds of aryl substituents of the examined heteroarene derivatives have not been taken into account.

Selective C3–H nitration of 2-sulfanilamidopyridines with tert-butyl nitrite

A selective C3–H nitration of bioactive 2-sulfanilamidopyridine derivatives, including corticosteroid 11-β-dehydrogenase isozyme, secretory phospholipase A2 inhibitor and human neutrophil elastase inhibitor, has been reported.

O-Directed C-H functionalization via cobaltacycles: a sustainable approach for C-C and C-heteroatom bond formations

This review focuses on providing comprehensive highlights of the recent advances in the field of cobalt-catalysed C-H functionalization and related synthetic concepts, relying on these through oxygen atom coordination. In recent years, 3d transition metal (Fe, Co, Cu & Ni) catalysed C-H functionalization reactions have received immense attention on account of its higher abundance and low cost, as compared to noble metals such as Ir, Rh, Ru and Pd. Among the first-row transition metals, cobalt is one of the extensively used metals for sustainable synthesis due to its unique reactivity towards the functionalization of inert C-H bonds. The functionalization of the inert C-H bond necessitates a proximal directing group. In this context, strongly coordinating nitrogen atom directed C-H functionalizations have been well explored. Nevertheless, strongly coordinating nitrogen-containing scaffolds, such as pyridine, pyrimidine, and 8-aminoquinoline, have to be installed and removed in a separate process. In contrast, C-H functionalizations through weakly coordinating atoms, such as oxygen, are largely underdeveloped. Since the oxygen atom is a part of many readily available functional groups, such as aldehydes, ketones, carboxylic acids, and esters, it could be used as directing groups for selective C-H functionalization reactions without any modification. Thus, the use of 3d transition metals, such as cobalt, along with weakly coordinating (oxygen) directing groups for C-H functionalization reactions are more sustainable, especially for the large-scale production of pharmaceuticals in industries. During the last decade, notable progress has been made using this concept. Nonetheless, almost all the reports are restricted to the formation of C-C and C-N bond. Therefore, there is a wide scope for developing this area for the formation of other bonds, such as C-X (halogens), C-B, C-S, and C-Se.

Transition metal-catalyzed C–H functionalizations of indoles

This review summarises a wide range of transformations on the indole skeleton, including arylation, alkenylation, alkynylation, acylation, nitration, borylation, and amidation, using transition-metal catalyzed C–H functionalization as the key step.

Mild Nitration of Pyrazolin-5-ones by a Combination of Fe(NO3 )3 and NaNO2 : Discovery of a New Readily Available Class of Fungicides, 4-Nitropyrazolin-5-ones

4-Nitropyrazolin-5-ones have been synthesized by the nitration of pyrazolin-5-ones at room temperature by employing the Fe(NO₃ )₃ /NaNO₂ system. The method demonstrated selectivity towards the 4-position of pyrazolin-5-ones even in the presence of NPh and allyl substituents, which are sensitive to nitration. It was shown that other systems containing Fe^III and nitrites, namely Fe(NO₃ )₃ /tBuONO, Fe(ClO₄ )₃ /NaNO₂ , and Fe(ClO₄ )₃ /tBuONO, were also effective. Presumably, Fe^III oxidizes the nitrite (NaNO₂ or tBuONO) to form the NO₂ free radical, which serves as the nitrating agent for pyrazolin-5-ones. The synthesized 4-nitropyrazolin-5-ones were discovered to be a new class of fungicides. Their in vitro activities against phytopathogenic fungi were found comparable or even superior to those of commercial fungicides (fluconazole, clotrimazole, triadimefon, and kresoxim-methyl). These results represent a promising starting point for the development of a new type of plant protection agents that can be easily synthesized from widely available reagents.

Recent advances in transition metal-catalyzed C(sp2)–H nitration

This review updates advances of direct C(sp²)–H nitration for the synthesis of nitroaromatic compounds and the mechanisms during the past decade.

3d Transition Metals for C-H Activation

C-H activation has surfaced as an increasingly powerful tool for molecular sciences, with notable applications to material sciences, crop protection, drug discovery, and pharmaceutical industries, among others. Despite major advances, the vast majority of these C-H functionalizations required precious 4d or 5d transition metal catalysts. Given the cost-effective and sustainable nature of earth-abundant first row transition metals, the development of less toxic, inexpensive 3d metal catalysts for C-H activation has gained considerable recent momentum as a significantly more environmentally-benign and economically-attractive alternative. Herein, we provide a comprehensive overview on first row transition metal catalysts for C-H activation until summer 2018.

Cobalt-catalyzed C–H activation: recent progress in heterocyclic chemistry

Cobalt-catalyzed C–H activation has gone through some major advancements in the past couple of decades.