Room-Temperature Synthesis of Isoindolone Spirosuccinimides: Merger of Visible-Light Photocatalysis and Cobalt-Catalyzed C-H Activation

Recent advances in spirocyclization of maleimides via transition-metal catalyzed C-H activation

In recent years, the maleimide scaffold has received a great deal of attention in C-H activation. Several types of products can be constructed using maleimides as a coupling partner. Alkylation, alkenylation, annulation, dehydrogenative annulation and spirocyclization are various reactions shown by maleimides in C-H activation. Thus, the maleimide scaffold has been extensively studied in the last few years in C-H activation owing to its unique reactivity. Among the diverse class of reactions of maleimides, spirocyclization is a less explored reaction. The spirocycles, in particular the spirosuccinimides are interesting candidates in drug discovery and materials chemistry. Therefore the method of spirocyclization of maleimides via C-H activation becomes an important strategy for the synthesis of a diverse array of spirosuccinimides. This review summarizes the reports available in this field from 2015-2023 and also highlights the scopes and prospects of this method.

Palladium-Catalyzed Chelation-Assisted Aldehyde C-H Bond Activation of Quinoline-8-carbaldehydes: Synthesis of Amides from Aldehydes with Anilines and Other Amines

A palladium-catalyzed chelation-assisted direct aldehyde C-H bond amidation of quinoline-8-carbaldehydes with an amine was developed under mild reaction conditions. A wide range of amides were obtained in good to excellent yields from aldehyde with a variety of aniline derivatives and aliphatic amines. Our methodology was successfully applied to synthesize known DNA intercalating agents and can be easily scaled up to a gram scale.

Glyoxylic Acid Monohydrate-Promoted Formation of the C-SO2 Bond Starting from Maleimides/Quinones and Sodium Sulfinates

By using glyoxylic acid monohydrate as a promoter, a wide range of substances containing a C-SO2 bond could be obtained from N-substituted maleimides or quinones and sodium sulfinates. The protocol features mild reaction conditions, short reaction time, and good atomic economics, which provides an alternative protocol for the α-sulfonylation of α,β-unsaturated ketones.

The recent advances in cobalt-catalyzed C(sp3)-H functionalization reactions

Over the past decades, reactions involving C-H functionalization have become a hot theme in organic transformations because they have a lot of potential for the streamlined synthesis of complex molecules. C(sp³)-H bonds are present in most organic species. Since organic molecules have massive significance in various aspects of life, the exploitation and functionalization of C(sp³)-H bonds hold enormous importance. In recent years, the first-row transition metal-catalyzed direct and selective functionalization of C-H bonds has emerged as a simple and environmentally friendly synthetic method due to its low cost, unique reactivity profiles and easy availability. Therefore, research advancements are being made to conceive catalytic systems that foster direct C(sp³)-H functionalization under benign reaction conditions. Cobalt-based catalysts offer mild and convenient reaction conditions at a reasonable expense compared to conventional 2^nd and 3^rd-row transition metal catalysts. Consequently, the probing of Co-based catalysts for C(sp³)-H functionalization is one of the hot topics from the outlook of an organic chemist. This review primarily focuses on the literature from 2018 to 2022 and sheds light on the substrate scope, selectivity, benefits and limitations of cobalt catalysts for organic transformations.

Synergistic Visible Light and Pd-Catalyzed C-H Alkylation of 1-Naphthylamines with α-Diazoesters

The combination of visible light irradiation and Pd-catalysis has been practically employed for the C-H alkylation reactions of naphthylamines and α-diazo esters, leading to the synthesis of α-naphthyl functionalized acetates via C-C bond construction under mild reaction conditions and under solvent-free conditions. The light irradiation has been proven to play a pivotal role in the reactions, probably by promoting the generation of active carbene species from α-diazo esters.

Palladium-Catalyzed Directed Aldehyde C-H Arylation of Quinoline-8-carbaldehydes: Exploring the Reactivity Differences between Aryl (Pseudo) Halides

We have developed a method for Pd-catalyzed direct C-H arylation of quinoline-8-carbaldehydes with either aryl iodides or aryl diazonium salts for the synthesis of aryl quinolinyl ketones. Aryl iodide substituted with an electron-donating group favors the reaction, whereas aryl diazonium salt substituted with an electron-withdrawing group showed excellent reactivity. A range of aryl quinolinyl ketones were synthesized in good-to-excellent yields, with very good functional group tolerance. Our methodology was successfully applied to synthesize highly potent tubulin polymerization inhibitors and can be easily scaled up to a gram scale.

Dual Photoredox Cobalt Catalyzed [4+1] Annulation and C-H Alkoxylation

Herein, we developed two distinct pyridine N-oxide directed C-H activation protocols to achieve [4+1] annulation and alkoxylation of benzamide derivatives by merging Co-catalysis with visible light photoredox catalysis. The protocols deliver the respective products in good yields under facile conditions at room temperature. The use of cheap photocatalyst coupled with molecular oxygen bypassing the need of stoichiometric oxidants forms the chief highlight of the work. The protocols are scalable and the products could be further modified. Additionally, preliminary studies were carried out to probe the reaction mechanism.

Electrochemical 5-exo-dig aza-cyclization of 2-alkynylbenzamides toward 3-hydroxyisoindolinone derivatives

Preparation of biologically relevant 3-hydroxyisoindolinones from readily available 2-alkynylbenzamides is an appealing synthetic approach. However, such kinds of compounds preferably undergo O-attacked 5-exo-dig/6-endo-dig cyclizations. Herein, we report an electrochemically generated amidyl radical proceeding via a highly selective N-attacked 5-exo-dig radical cyclization to form 3-hydroxyisoindolinone derivatives. This reaction features simple operation, good selectivity, and broad substrate scope. Moreover, gram-scale preparation and synthetic elaborations imply the potential applicability of this protocol for the synthesis of diverse isoindolinone derivatives.

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.

Reaction Path Determination of Rhodium(I)-Catalyzed C-H Alkylation of N-8-Aminoquinolinyl Aromatic Amides with Maleimides

The rhodium(I)-catalyzed reaction of N-8-aminoquinolinyl aromatic amides with maleimides results in C-H alkylation at the ortho position of the amide. The reaction path and formation of the alkylation product with density functional theory (DFT) calculations were done. The detailed computational study showed that the reaction proceeds in the following steps: (I) deprotonation of the NH amide proton, (II) oxidative addition of the ortho C-H bond, (III) migratory insertion of the maleimide, (IV) reductive elimination with the C-C bond formation, and (V) protonation. The energetic span model showed that the turnover frequency (TOF)-determining transition state (TDTS) is the oxidative addition, while the TOF-determining intermediate (TDI) is the formation of an Rh(I)-complex after N-H deprotonation. It was also found that the change in the oxidation number of the Rh catalyst is a key determinant of the reaction path.

C-H bond functionalization by dual catalysis: merging of high-valent cobalt and photoredox catalysis

The merger of transition metal catalysis and photocatalysis has emerged as a versatile platform that opened the gateway to diverse low-energy pathways for several synthetic transformations. However, amidst the first-row transition metals, directed C-H bond functionalization mediated by high-valent cobalt catalysis has advanced with rising momentum owing to its unique reactivity and the ability to participate in both one- and two-electron transfer reactions. However, the use of expensive, privileged Cp* ligands or use of stoichiometric silver(I) or manganese(III) is unavoidable. Despite significant advances in their respective fields, the combination of these two "green" approaches to further the vested interest of the scientific research community towards the development of ecofriendly and sustainable protocols is noticeably limited. Thus, the methodology based on high-cobalt-photoredox dual-catalytic strategy has high dormant potential and is worthy to explore. Herein, we highlight the recent advances in the high-valent cobalt-catalyzed sustainable catalytic approach by harnessing light energy for oxidative C-H bond functionalization. With this, we hope to inspire the development of unexplored cobalt-photoredox-catalyzed reactions with improved efficiency and selectivity.

Room-temperature C-H bond alkynylation by merging cobalt and photocatalysts

A new protocol is developed for the mono- and bis-ortho-C-H alkynylation of easily accessible benzamide derivatives using alkynyl bromides at room temperature by merging cobalt and photocatalysts. The diverse reactivity of various alkynyl bromides towards the C-H alkynylation and competing C-H/N-H bond annulation reactions has been demonstrated to give the corresponding products in good yields with excellent functional group tolerance.

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.

Synthesis of Chiral Spirolactams via Sequential C-H Olefination/Asymmetric [4+1] Spirocyclization under a Simple CoII /Chiral Spiro Phosphoric Acid Binary System

An unprecedented enantioselective synthesis of spiro-γ-lactams via a sequential C-H olefination/asymmetric [4+1] spirocyclization under a simple Co^II /chiral spiro phosphoric acid (SPA) binary system is reported. A range of biologically important spiro-γ-lactams are obtained with high levels of enantioselectivity (up to 98 % ee). The concise, asymmetric synthesis of an aldose reductase inhibitor was successfully achieved. Notably, contrast to previous reports that relied on the use of cyclopentadienyl or its derivatives (achiral Cp*, Cp^tBu , or chiral Cp^x ) ligated Co^III complexes requiring tedious steps to prepare, cheap and commercially available cobalt(II) acetate tetrahydrate was used as an efficient precatalyst.

On the application of 3d metals for C-H activation toward bioactive compounds: The key step for the synthesis of silver bullets

Several valuable biologically active molecules can be obtained through C-H activation processes. However, the use of expensive and not readily accessible catalysts complicates the process of pharmacological application of these compounds. A plausible way to overcome this issue is developing and using cheaper, more accessible, and equally effective catalysts. First-row transition (3d) metals have shown to be important catalysts in this matter. This review summarizes the use of 3d metal catalysts in C-H activation processes to obtain potentially (or proved) biologically active compounds.