Multi-component reactions (MCRs) are considered to be an important methodological arsenal in synthetic and medicinal chemistry. These reactions have been strategically employed in various synthetic transformations where classical methods usually involve many steps with tedious procedures. The MCR approach provides high yields, atom-/step economy, reduced reaction time, is eco-friendly, and acts as an amenable tool for the generation of a library of new chemical entities (NCEs), especially in the drug discovery process. Extensive research has led to copious developments in the field of MCRs. The developments have emerged with different synthetic approaches, including C–H activation, coupling, and cycloaddition, and eventually, such an amalgamation has enabled access to a broad spectrum of organic frameworks. The present review summarizes recent advancements in MCRs and their mechanistic insights from the last triennium (2017–2020).

Recent advances in multi-component reactions and their mechanistic insights: a triennium review

Transition-metal-catalyzed C–H bond functionalizations have had an enormous influence on organic synthesis in recent times. However, the use of low-abundance 4d and 5d metals is almost inevitable, and they are in high demand. This will be a cause of concern, and hence, it is important to develop methods based on 3d metals, which are widely present in the Earth’s crust. In this regard, the use of 3d metal catalysts or their precursors for catalysis, in general, and C–H bond functionalizations, in particular, has gained significant momentum in the recent times. The major development in catalytic C–H bond functionalizations with 3d metals has been achieved predominantly with strongly coordinating directing groups such as pyridyl, pyrimidinyl, pyrazolyl, and 8-amino-quinolinyl groups. Thus, prefunctionalization of substrates with these directing groups is necessary, which contradicts the step- and atom-economy of C–H bond activation. However, commonly available functional groups such as aldehyde, ketone, carboxylic acid, amide, hydroxy, and N-oxides loosely bind to metals through weak-coordination. These weakly coordinating directing groups orient the metal to activate C–H bond regioselectively without the need for preinstalled strongly coordinating directing groups. Although it is challenging, this contemporary topic has been actively pursued by many researchers in recent times. Through this article, we provide a comprehensive overview of 3d metal-catalyzed, weakly coordinating, directing-group-enabled C–H bond functionalizations reported until March 2021. 

Weak-Coordination in C–H Bond Functionalizations Catalyzed by 3d Metals

Ionic liquids (ILs) and deep eutectic solvents (DESs) debuted with a promise of a superior sustainability footprint due to their low vapor pressure. However, their toxicity and high cost compromise this footprint, impeding their real-world applications. Fortunately, their property tunability through a rational selection of precursors, including bioderived ones, provides a strategy to ameliorate toxicity, lower cost and endow new functions. This review addresses whether ILs and DESs are sustainable solvents or contribute to sustainable chemical processes.

Enabling Sustainable Chemistry with Ionic Liquids and Deep Eutectic Solvents: a Fad or the Future?

In the past two decades, transition metal-catalyzed C–H functionalization followed by annulations with various coupling partners has received much attention in organic synthesis. In particular, Rh(III) or Ir(III) catalyzed regioselective C–H functionalization followed by the cyclization of diverse arenes with metal carbene precursors has become a highly investigated research field in recent years. The purpose of this review is to provide a comprehensive essay on the Rh(III)/Ir(III) catalyzed C–H functionalizations/annulations via carbene migratory insertion, focusing on the diverse metal carbene precursors with arenes/alkenes reported so far. The highlighted examples are categorized by nitrogen, oxygen, sulfur-containing heterocycles and carbocyclizations. Further, key mechanistic approaches are also briefly described.

Recent advances in Rh(III)/Ir(III)-catalyzed C–H functionalization/annulation via carbene migratory insertion

We describe a new functional group metathesis between aryl nitriles and aryl thioethers. The catalytic system nickel/dcype is essential to achieve this fully reversible transformation in good to excellent yields. Furthermore, the cyanide- and thiol-free reaction shows high functional group tolerance and great efficiency for the late-stage derivatization of commercial molecules. Finally, synthetic applications demonstrate its versatility and utility in multistep synthesis.

Nickel-Catalyzed Reversible Functional Group Metathesis between Aryl Nitriles and Aryl Thioethers.

Aryl nitriles are compounds with wide significance. They have made their own space in various sectors including pharmaceuticals, industries, natural product chemistry, and so on. Furthermore, they are also key intermediates in various transformations in organic chemistry. Transition metal-catalyzed cyanation reactions have proved to be a better replacement for the existing traditional synthetic strategies for aryl nitriles. Palladium is one of the most studied transition metals other than copper and nickel owing to its wide functional group compatibility and catalytic efficacy. There have been drastic developments in the field of palladium-catalyzed cyanation since its discovery in the 1973. This review summarizes the recent developments in the palladium-catalyzed cyanation of aryl halides and covers literature from 2012–2020.

/pdf/recent-advances-and-prospects-in-the-palladium-catalyzed-2a6atqe3le.pdf

Recent advances and prospects in the palladium-catalyzed cyanation of aryl halides

Spirooxindole is a principal bioactive agent and is observed in several natural products including alkaloids. They are broadly studied in the pharmaceutical field and have a significant role in the evolution of drugs such as anti-viral, anti-cancer, anti-microbial etc. In organic chemistry, an indispensable role is presented by transition metal catalysts. An effective synthetic perspective to spirooxindoles is the use of transition metals as the catalyst. This review discusses the synthesis of spirooxindoles catalyzed by transition metals and covers literature up to 2020.

/pdf/transition-metal-catalyzed-synthesis-of-spirooxindoles-3ryn6dur8h.pdf

Transition metal-catalyzed synthesis of spirooxindoles

Copper-catalyzed C–H functionalization is becoming a significant area in organic chemistry. Copper is now widely used as a catalyst in organic synthesis as it is inexpensive and not very toxic. Functionalization of C–H bonds to construct wide varieties of organic compounds has received much attention in recent times. This review focuses on the recent advances in Cu-catalyzed C–H functionalization and covers literature from 2018–2020.

/pdf/progress-and-prospects-in-copper-catalyzed-c-h-wzrhq3pawi.pdf

Progress and prospects in copper-catalyzed C–H functionalization

Heterocyclic compounds have become an inevitable part of organic chemistry due to their ubiquitous presence in bioactive compounds. Copper-catalyzed multicomponent synthesis of heterocycles has developed as the most convenient and facile synthetic route towards complex heterocyclic motifs. In this review, we discuss the advancements in the field of copper-catalyzed multicomponent reactions for the preparation of heterocycles since 2018.

/pdf/recent-developments-and-perspectives-in-the-copper-catalyzed-23mtzuoxjz.pdf

Recent developments and perspectives in the copper-catalyzed multicomponent synthesis of heterocycles

Recent advances and perspectives in manganese-catalyzed C–H activation

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