Cationic Cobalt(III) Catalyzed Indole Synthesis: The Regioselective Intermolecular Cyclization of N-Nitrosoanilines and Alkynes

Harnessing sulfilimine as an oxidizing directing group in Cp*Co(III)-catalyzed [4+2] annulation with alkynes and 1,3-diynes

We have developed the first Cp*Co(III)-catalyzed [4+2] annulation utilizing sulfilimine as an oxidizing directing group in a redox-neutral fashion. The N-S bond of the sulfilimine serves as an internal oxidant, thereby eliminating the need for any internal oxidant. The reaction worked with various alkynes and also exhibited an excellent regioselectivity with 1,3-diynes furnishing the 3-alkynylated isoquinolones.

Tert-Butyl Nitrite-initiated C-N Bond Cleavage of 1-Nitromethyl-N-aryltetrahydroisoquinolines: Synthesis of Furoxans with N-NO Skeleton

A series of furoxan derivatives with N-nitroso groups were synthesized in good yields by TBN initiated radical sp³ C-N bond cleavage of 1-nitromethyl-N-aryltetrahydroisoquinolines. This reaction grafts the biologically important furoxan skeleton and N-nitroso group into on molecule, greatly improving the molecular complexity in one step transformation. The mechanistic study shows that this reaction is mediated by the in situ generated α-carbonyl nitrile oxide, which is afforded by TBN promoted C-N bond cleavage.

Metal-Promoted Heterocyclization: A Heterosynthetic Approach to Face a Pandemic Crisis

The outbreak of SARS-CoV-2 has drastically changed our everyday life and the life of scientists from all over the world. In the last year, the scientific community has faced this worldwide threat using any tool available in order to find an effective response. The recent formulation, production, and ongoing administration of vaccines represent a starting point in the battle against SARS-CoV-2, but they cannot be the only aid available. In this regard, the use of drugs capable to mitigate and fight the virus is a crucial aspect of the pharmacological strategy. Among the plethora of approved drugs, a consistent element is a heterocyclic framework inside its skeleton. Heterocycles have played a pivotal role for decades in the pharmaceutical industry due to their high bioactivity derived from anticancer, antiviral, and anti-inflammatory capabilities. In this context, the development of new performing and sustainable synthetic strategies to obtain heterocyclic molecules has become a key focus of scientists. In this review, we present the recent trends in metal-promoted heterocyclization, and we focus our attention on the construction of heterocycles associated with the skeleton of drugs targeting SARS-CoV-2 coronavirus.

Recent Advances and Strategies for the Transition‐Metal‐Catalyzed C−H Functionalization of N‐Nitrosoanilines

The challenges faced in the activation and functionalization of C−H bonds while producing high‐value aromatic intermediates are significant for synthetic organic chemistry. This review summarizes the advancements made toward the C−H functionalization of N‐nitrosoanilines in the past decade, in which the nitroso group coordinates to a transition metal, such as rhodium, palladium, cobalt, iridium, and ruthenium, acting as a directing group to allow the activation of the ortho C−H bond. The versatility of this synthetic approach has led to the regio‐ and chemoselective introduction of various functional groups giving rise to aromatic congeners with diverse functional and structural properties. C−H functionalized nitroso compounds can be further transformed into valuable synthetic intermediates and unique heterocyclic compounds. A description of the mechanism underlying each reaction is also included. This review includes the following sections:

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Recent advances in the synthesis of indoles from alkynes and nitrogen sources

This review highlights ten years of success in the synthesis of indoles using alkynes and nitrogen sources as substrates.

Indoles from Alkynes and Aryl Azides: Scope and Theoretical Assessment of Ruthenium Porphyrin-Catalyzed Reactions

A symbiotic experimental/computational study analyzed the Ru(TPP)(NAr)₂ -catalyzed one-pot formation of indoles from alkynes and aryl azides. Thirty different C₃ -substituted indoles were synthesized and the best performance, in term of yields and regioselectivities, was observed when reacting ArC≡CH alkynes with 3,5-(EWG)₂ C₆ H₃ N₃ azides, whereas the reaction was less efficient when using electron-rich aryl azides. A DFT analysis describes the reaction mechanism in terms of the energy costs and orbital/electronic evolutions; the limited reactivity of electron-rich azides was also justified. In summary, PhC≡CH alkyne interacts with one NAr imido ligand of Ru(TPP)(NAr)₂ to give a residually dangling C(Ph) group, which, by coupling with a C(H) unit of the N-aryl substituent, forms a 5+6 bicyclic molecule. In the process, two subsequent spin changes allow inverting the conformation of the sp² C(Ph) atom and its consequent electrophilic-like attack to the aromatic ring. The bicycle isomerizes to indole via a two-step outer sphere H-migration. Eventually, a 'Ru(TPP)(NAr)' mono-imido active catalyst is reformed after each azide/alkyne reaction.

Synthesis of Indoles through Domino Reactions of 2-Fluorotoluenes and Nitriles

Indoles are essential heterocycles in medicinal chemistry, and therefore, novel and efficient approaches to their synthesis are in high demand. Among indoles, 2-aryl indoles have been described as privileged scaffolds. Advanced herein is a straightforward, practical, and transition-metal-free assembly of 2-aryl indoles. Simply combining readily available 2-fluorotoluenes, nitriles, LiN(SiMe₃ )₂ , and CsF enables the generation of a diverse array of indoles (38 examples, 48-92 % yield). A range of substituents can be introduced into each position of the indole backbone (C4 to C7, and aryl groups at C2), providing handles for further elaboration.

Acetonitrile Activation: An Effective Two-Carbon Unit for Cyclization

A novel activation of acetonitrile for the construction of cyclobutenones by [2+2] cyclization was developed. Acetonitrile is utilized for the first time as two-carbon (C2) cyclization building block. The present protocol successfully inhibits the competitive cycloaddition with the C≡N bond of acetonitrile, but enables the in situ formation of an unsaturated carbon-carbon bond and the subsequent cycloaddition as a C2 unit. This chemistry features simple reaction conditions, high chemoselectivities, wide substrate scope, and offers a new and practical approach to cyclobutenones and cyclobuteneimines.

A machine-assisted approach for the preparation of follow-on pharmaceutical compound libraries

Coupling of a continuous flow process to a robotic microwave device has led to a simple platform for the preparation of compound libraries, specifically the anti-migraine compound Zolmitriptan and several unknown analogues.

Direct Functionalization of C-H Bonds by Iron, Nickel, and Cobalt Catalysis

Non-precious-metal-catalyzed reactions are of increasing importance in chemistry due to the outstanding ecological and economic properties of these metals. In the subfield of metal-catalyzed direct C-H functionalization reactions, recent years have shown an increasing number of publications dedicated to this topic. Nickel, cobalt, and last but not least iron, have started to enter a field which was long dominated by precious metals such as palladium, rhodium, ruthenium, and iridium. The present review article summarizes the development of iron-, nickel-, and cobalt-catalyzed C-H functionalization reactions until the end of 2016, and discusses the scope and limitations of these transformations.

Highly Stereoselective Cobalt(III)-Catalyzed Three-Component C-H Bond Addition Cascade

A highly stereoselective three-component C(sp(2) )-H bond addition across alkene and polarized π-bonds is reported for which Co(III) catalysis was shown to be much more effective than Rh(III) . The reaction proceeds at ambient temperature with both aryl and alkyl enones employed as efficient coupling partners. Moreover, the reaction exhibits extremely broad scope with respect to the aldehyde input; electron rich and poor aromatic, alkenyl, and branched and unbranched alkyl aldehydes all couple in good yield and with high diastereoselectivity. Multiple directing groups participate in this transformation, including pyrazole, pyridine, and imine functional groups. Both aromatic and alkenyl C(sp(2) )-H bonds undergo the three-component addition cascade, and the alkenyl addition product can readily be converted into diastereomerically pure five-membered lactones. Additionally, the first asymmetric reactions with Co(III) -catalyzed C-H functionalization are demonstrated with three-component C-H bond addition cascades employing N-tert-butanesulfinyl imines. These examples represent the first transition metal catalyzed C-H bond additions to N-tert-butanesulfinyl imines, which are versatile and extensively used intermediates for the asymmetric synthesis of amines.