Recent Advances in Four‐Coordinated Planar Cobalt Catalysis in Organic Synthesis

Halogen-Atom Transfer Enabled Z-Selective Styrene Synthesis via Dual Cobalt and Photocatalysis Through Coupling of Unactivated Alkyl Iodides With Terminal Arylalkynes

An efficient Z-selective cobalt-catalyzed reductive hydroalkylation of terminal aryl alkynes with unactivated alkyl iodides has been achieved, providing a straightforward and modular route to access 1,2-disubstituted Z-styrenes. This reaction operates under mild conditions without requiring over-stoichiometric amounts of metal terminal reductants. Excellent Z/E ratios and good to excellent yields can be achieved for diverse and complex scaffolds with remarkable functional-group compatibility. One potential utility of this reaction is demonstrated by the efficient synthesis of several syn homoallylic alcohols in a one-pot two-step sequence. Control experiments strongly support that the halogen-atom transfer (XAT) process is the key to generating carbon radicals. DFT studies suggest that the catalytic system involves the Co(II)/Co(III) cycle and the steric repulsion between the Co(II) catalyst, and the alkenyl radical in radical capture by Co(II) is the dominant factor controlling the Z/E selectivity. This approach represents the first example of merging photo-XAT with cobalt-catalyzed reductive coupling of terminal aryl alkynes with unactivated alkyl iodides.

Synthesis of lactones and lactams via C(sp3)-H bond functionalization

The field of directing group-assisted, transition-metal-catalyzed functionalization has undergone a significant transformation, evolving from the use of auxiliary group attachment for the exploitation of native functional groups in novel organic reactions. In particular, coordination-assisted C(sp3)-H bond functionalization has revolutionized synthetic planning to build molecular complexity. Recently, the use of native directing groups in transition-metal-catalyzed reactions has allowed a step-economic process for increased access to biologically important lactones and lactams. Accordingly, lactones and lactams are unavoidable structural motifs with widespread presence in many biological and pharmaceutical arenas, encouraging researchers to access and modify their structures for improved biological properties. In this review, we showcase the diverse aspects of transition metal catalysis, biocatalysis, and photocatalytic C(sp3)-H bond functionalization to access lactones and lactams assisted by carboxylic acid and amines/amides with auxiliary or transient directing groups or unique ligands. This article also emphasizes the role of specially designed complexes, artificial metalloenzymes, and biocatalysts in assembling lactones and lactams. Besides, three-component reactions involving CO as a C1 synthon play a vital role in developing these heterocycles. Importantly, the crucial role of ligands in determining regioselectivity and enhancing enantioselectivity is discussed thoroughly. For better clarity, this review is divided into twelve sections based on the catalysts involved, with subsections categorized by the type of bond activation or formation. Overall, this review aims to inspire the growth of C(sp3)-H bond functionalization, leading to the integration of lactams and lactones in organics.

Cobalt-Assisted α-Activation of Ketones, Their Arylation and Keto-Enol Equilibrium Confined into a Macrocycle

In this study, we explore the cobalt-mediated α-activation of ketones within modified porphyrin macrocycles, focusing on the arylation processes and the influence of macrocyclic confinement on keto-enol tautomerism. The activation of ketones such as acetone, butanone, 3-pentanone, and acetophenone and their possible transfer from a cobalt center on the carbaporphyrin fragment results in the formation of new C(sp2)─C(sp3) bonds in confined macrocyclic environments of N-confused porphyrin and azuliporphyrin. Such modified ligands allow for following altered keto-enol equilibrium. They reversibly form unique structural motifs, azuleno[1,2-c]pyran and 2,4-dihydropyrano[3,4-c]pyrrole modulated by acid-base conditions and macrocyclic aromaticity. The coordination of a modified macrocycle to a rhodium(I) center leads to a series of complexes with a final form with rhodium(III) in the cavity. The rhodium(III) center cleaved the C(21)─C bond and transferred the acetone fragment back to the metal ion from the azulene fragment.

Hydrogen-Localization Transfer Regulation in 3D COFs Enhances Photocatalytic Acetylene Semi-Hydrogenation to Ethylene

In this work, a series of new crystalline three-dimensional covalent organic frameworks (3D COFs) based on [8+4] construction was designed and successfully realized efficient photocatalytic acetylene (C2H2) hydrogenation to ethylene (C2H4). By regulating the hydrogen-localization transfer effect in these 3D COFs, the Cz-Co-COF-H containing cobalt glyoximate active centers exhibited excellent C2H2-to-C2H4 performance, with an average C2H4 yield of 1755.33 μmol g-1 h-1 in pure C2H2, also showed near 100 % conversion of C2H2 in 1 % C2H2 contained crude C2H4 mixtures (industry-relevant conditions), and finally obtain polymer grade C2H4. In contrast, the Cz-Co-COF-BF2 only showed one fifth activity due to lack of hydrogen-localization transfer. The density functional theory (DFT), projected density of states (PDOS) and molecular dynamics "slow-growth" kinetic calculations based on precise 3D COF structures confirmed that the rapid hydrogen species transfer, enhanced water dissociation and suitable C2H2 adsorption in COFs jointly contributed efficient photocatalytic acetylene hydrogenation (PAH). This work provides new opportunity towards rational design and development of crystalline photocatalysts for C2H2 hydrogenation.

Metallaphotoredox Enabled Single Carbon Atom Insertion into Alkenes for Allene Synthesis

Efficient methods for synthesizing allenes from readily available starting materials pose a persistent challenge in organic chemistry. In this work, we present a novel two-stage protocol for allene synthesis involving the single-atom insertion into alkenes, facilitated by synergistic photoredox and cobalt catalysis. Diverging from conventional methods such as the Doering-LaFlamme reaction, this photochemical rearrangement approach operates efficiently under mild conditions in a radical-based manner. The protocol exhibits a broad substrate scope and demonstrates applicability in the late-stage diversification of alkene-containing natural products and bioactive molecules. Preliminary mechanistic studies and density functional theory (DFT) calculations offer insights into the reaction pathway, indicating a radical mechanism involving fleeting cyclopropyl carbene intermediates followed by rapid ring opening to form allenes.

Cobalt-Hydride-Catalyzed Alkene-Carboxylate Transposition (ACT) of Allyl Carboxylates

The alkene-carboxylate transposition (ACT) of allyl carboxylates is one of the most atom-economic and synthetically reliable transformations in organic chemistry, as allyl carboxylates are versatile synthetic intermediates. Classic ACT transformations, including [3,3]-sigmatropic rearrangement and transition metal-catalyzed allylic rearrangement, typically yield 1,2-alkene/1,3-acyloxy shifted products through a two-electron process. However, position-altered ACT to produce distinct 1,3-alkene/1,2-acyloxy shifted products remains elusive. Here, we report the first cobalt-hydride-catalyzed ACT of allyl carboxylates, enabling access to these unprecedented 1,3-alkene/1,2-acyloxy shifted products via a 1,2-radical migration (RaM) strategy. This transformation demonstrates broad functional group tolerance, is suitable for late-stage modification of complex molecules, and is amenable to gram-scale synthesis. It also expands the reaction profiles of both allyl carboxylates and cobalt catalysis. Preliminary experimental and computational studies suggest a mechanism involving metal-hydride hydrogen atom transfer (MHAT) and the 1,2-RaM process. This reaction is expected to serve as the basis for the development of versatile Co-H-catalyzed transformations of allyl carboxylates, generating a wide array of valuable building blocks for synthetic, medicinal, and materials chemistry.

When transition-metal catalysis meets electrosynthesis: a recent update

While aiming at sustainable synthesis, organic electrosynthesis has attracted increasing attention in the past few years. In parallel, with a deeper understanding of catalyst and ligand design, 3d transition-metal catalysis allows the conception of more straightforward synthetic routes in a cost-effective fashion. Owing to their intrinsic advantages, the merger of organic electrosynthesis with 3d transition-metal catalysis has offered huge opportunities for conceptually novel transformations while limiting ecological footprint. This review summarizes the key advancements in this direction published in the recent two years, with specific focus placed on strategy design and mechanistic aspects.

Combined Computational and Experimental Study Reveals Complex Mechanistic Landscape of Brønsted Acid-Catalyzed Silane-Dependent P═O Reduction

The reaction mechanism of Brønsted acid-catalyzed silane-dependent P═O reduction has been elucidated through combined computational and experimental methods. Due to its remarkable chemo- and stereoselective nature, the Brønsted acid/silane reduction system has been widely employed in organophosphine-catalyzed transformations involving P(V)/P(III) redox cycle. However, the full mechanistic profile of this type of P═O reduction has yet to be clearly established to date. Supported by both DFT and experimental studies, our research reveals that the reaction likely proceeds through mechanisms other than the widely accepted "dual activation mode by silyl ester" or "acid-mediated direct P═O activation" mechanism. We propose that although the reduction mechanisms may vary with the substitution patterns of silane species, Brønsted acid generally activates the silane rather than the P═O group in transition structures. The proposed activation mode differs significantly from that associated with traditional Brønsted acid-catalyzed C═O reduction. The uniqueness of P═O reduction originates from the dominant Si/O═P orbital interactions in transition structures rather than the P/H-Si interactions. The comprehensive mechanistic landscape provided by us will serve as a guidance for the rational design and development of more efficient P═O reduction systems as well as novel organophosphine-catalyzed reactions involving P(V)/P(III) redox cycle.

Catalytic Tetrazole Synthesis via [3+2] Cycloaddition of NaN3 to Organonitriles Promoted by Co(II)-complex: Isolation and Characterization of a Co(II)-diazido Intermediate

The [3+2] cycloaddition of sodium azide to nitriles to give 5-substituted 1H-tetrazoles is efficiently catalyzed by a Cobalt(II) complex (1) with a tetradentate ligand N,N-bis(pyridin-2-ylmethyl)quinolin-8-amine. Detailed mechanistic investigation shows the intermediacy of the cobalt(II) diazido complex (2), which has been isolated and structurally characterized. Complex 2 also shows good catalytic activity for the synthesis of 5-substituted 1H-tetrazoles. These are the first examples of cobalt complexes used for the [3+2] cycloaddition reaction for the synthesis of 1H-tetrazoles under homogeneous conditions.

Photo- and Cobalt-Catalyzed Synthesis of Heterocycles via Cycloisomerization of Unactivated Olefins

We report a general, intramolecular cycloisomerization of unactivated olefins with pendant nucleophiles. The reaction proceeds under mild conditions and tolerates ethers, esters, protected amines, acetals, pyrazoles, carbamates, and arenes. It is amenable to N-, O-, as well as C-nucleophiles, yielding a number of different heterocycles including, but not limited to, pyrrolidines, piperidines, oxazolidinones, and lactones. Use of both a benzothiazinoquinoxaline as organophotocatalyst and a Co-salen catalyst obviates the need for stoichiometric oxidant or reductant. We showcase the utility of the protocol in late-stage drug diversification and synthesis of several small natural products.

Cobalt-Catalyzed Electroreductive Alkylation of Unactivated Alkyl Chlorides with Conjugated Olefins

Reactions of unactivated alkyl chlorides under mild and sustainable conditions are rare compared to those of alkyl bromides or iodides. As a result, synthetic methods capable of modifying the vast chemical space of chloroalkane reagents, wastes, and materials are limited. We report the cobalt-catalyzed reductive addition of unactivated alkyl chlorides to conjugated alkenes. Co-catalyzed activation of alkyl chlorides is performed under electroreductive conditions, and the resulting reactions constitute formal alkyl-alkyl bond formation. In addition to developing an operationally simple methodology, detailed mechanistic studies provide insights into the elementary steps of a proposed catalytic cycle. In particular, we propose a switch in the mechanism of C-Cl bond activation from nucleophilic substitution to halogen atom abstraction, which is critical for efficiently generating alkyl radicals. These mechanistic insights were leveraged in designing ligands that enable couplings of primary, secondary, and tertiary alkyl chlorides.

Multimetallic-Catalyzed C-C Bond-Forming Reactions: From Serendipity to Strategy

The use of two or more metal catalysts in a reaction is a powerful synthetic strategy to access complex targets efficiently and selectively from simple starting materials. While capable of uniting distinct reactivities, the principles governing multimetallic catalysis are not always intuitive, making the discovery and optimization of new reactions challenging. Here, we outline our perspective on the design elements of multimetallic catalysis using precedent from well-documented C-C bond-forming reactions. These strategies provide insight into the synergy of metal catalysts and compatibility of the individual components of a reaction. Advantages and limitations are discussed to promote further development of the field.

C-H bond activations by the HO˙/(Salophent-Bu)Co(II) radical pair generated via homolysis of a terminal Co(III)-OH bond

The reactive HO˙/(Salophen^t-Bu)Co(II) radical pair was observed to be generated via homolysis of the terminal Co(III)-OH bond in transient (Salophen^t-Bu)(L)Co(III)(OH) (L = Py, MeOH) complexes as indicated by UV-Vis and EPR measurements. Based on this elementary process, C-H bond activations in acetone, 2-butanone, acetonitrile and benzene were achieved under ambient conditions. For the reactions of the first three substrates, the alkylcobalt(III) complexes were formed as the products.

Metalated covalent organic frameworks: from synthetic strategies to diverse applications

Covalent organic frameworks (COFs) are a class of organic crystalline porous materials discovered in the early 21st century that have become an attractive class of emerging materials due to their high crystallinity, intrinsic porosity, structural regularity, diverse functionality, design flexibility, and outstanding stability. However, many chemical and physical properties strongly depend on the presence of metal ions in materials for advanced applications, but metal-free COFs do not have these properties and are therefore excluded from such applications. Metalated COFs formed by combining COFs with metal ions, while retaining the advantages of COFs, have additional intriguing properties and applications, and have attracted considerable attention over the past decade. This review presents all aspects of metalated COFs, from synthetic strategies to various applications, in the hope of promoting the continued development of this young field.

Asymmetric C-H Dehydrogenative Allylic Alkylation by Ternary Photoredox-Cobalt-Chiral Primary Amine Catalysis under Visible Light

We report herein an asymmetric C-H dehydrogenative allylic alkylation by a synergistic catalytic system involving a chiral primary amine, a photoredox catalyst, and a cobaloxime cocatalyst. The ternary catalytic system enables the coupling of β-ketocarbonyls and olefins with good yields and high enantioselectivities. Mechanism studies disclosed a cooperative radical addition process with a chiral α-imino radical and Co(II)-metalloradical wherein the chiral primary aminocatalyst and the cobaloxime catalyst work in concert to control the stereoinduction.

Selective visible-light photocatalysis of acetylene to ethylene using a cobalt molecular catalyst and water as a proton source

The production of polymers from ethylene requires the ethylene feed to be sufficiently purified of acetylene contaminant. Accomplishing this task by thermally hydrogenating acetylene requires a high temperature, an external feed of H₂ gas and noble-metal catalysts. It is not only expensive and energy-intensive, but also prone to overhydrogenating to ethane. Here we report a photocatalytic system that reduces acetylene to ethylene with ≥99% selectivity under both non-competitive (no ethylene co-feed) and competitive (ethylene co-feed) conditions, and near 100% conversion under the latter industrially relevant conditions. Our system uses a molecular catalyst based on earth-abundant cobalt operating under ambient conditions and sensitized by either [Ru(bpy)₃]²⁺ or an inexpensive organic semiconductor (metal-free mesoporous graphitic carbon nitride) under visible light. These features and the use of water as a proton source offer advantages over current hydrogenation technologies with respect to selectivity and sustainability.

A Triple Photoredox/Cobalt/Brønsted Acid Catalysis Enabling Markovnikov Hydroalkoxylation of Unactivated Alkenes

We demonstrate Markovnikov hydroalkoxylation of unactivated alkenes using alcohols through a triple catalysis consisting of photoredox, cobalt, and Brønsted acid catalysts under visible light irradiation. The triple catalysis realizes three key elementary steps in a single catalytic cycle: (1) Co(III) hydride generation by photochemical reduction of Co(II) followed by protonation, (2) metal hydride hydrogen atom transfer (MHAT) of alkenes by Co(III) hydride, and (3) oxidation of the alkyl Co(III) complex to alkyl Co(IV). The precise control of protons and electrons by the three catalysts allows the elimination of strong acids and external reductants/oxidants that are required in the conventional methods.

Electrocatalytic Allylic C-H Alkylation Enabled by a Dual-Function Cobalt Catalyst

The direct functionalization of allylic C-H bonds with nucleophiles minimizes pre-functionalization and converts inexpensive, abundantly available materials to value-added alkenyl-substituted products but remains challenging. Here we report an electrocatalytic allylic C-H alkylation reaction with carbon nucleophiles employing an easily available cobalt-salen complex as the molecular catalyst. These C(sp³ )-H/C(sp³ )-H cross-coupling reactions proceed through H₂ evolution and require no external chemical oxidants. Importantly, the mild conditions and unique electrocatalytic radical process ensure excellent functional group tolerance and substrate compatibility with both linear and branched terminal alkenes. The synthetic utility of the electrochemical method is highlighted by its scalability (up to 200 mmol scale) under low loading of electrolyte (down to 0.05 equiv) and its successful application in the late-stage functionalization of complex structures.

Semisynthesis of (-)-Bufospirostenin A Enabled by Photosantonin Rearrangement Reaction

An enantioselective semisynthesis of (-)-bufospirostenin A is described. The key steps in the synthesis involve use of our proposed biomimetic and diastereoselective photosantonin rearrangement reaction for construction of the 5/7 bicyclic motif, and a Co-catalyzed reversible double-bond isomerization reaction for installing the double bond in the seven-membered ring.

Cobalt Catalysts for [2+2+2] Cycloaddition Reactions: Isolated Precatalysts and in situ Generated Catalysts

[2+2+2] Cycloaddition reactions belong to the not even large class of reactions, which can be catalyzed or mediated by a significant number of different transition metals. Cobalt complexes belong to the catalysts that paved the way for the extensive use and profound mechanistic knowledge on this particular transformation. Beside the established role, cyclopentadienyl (Cp) cobalt complexes inherit in synthetic applications of the cyclotrimerization reaction, modification of the precatalysts opened up novel reactivities and versatility for such catalytic initiators. At the same time the development of in situ generated cobalt catalysts allowed the conversion of novel substrates as well as novel reaction modes to be realized. In this personal account recent developments will be presented and the possibilities of catalysts containing cobalt atoms in different oxidation states be discussed.

Visible-Light-Induced Homolysis of Earth-Abundant Metal-Substrate Complexes: A Complementary Activation Strategy in Photoredox Catalysis

The mainstream applications of visible-light photoredox catalysis predominately involve outer-sphere single-electron transfer (SET) or energy transfer (EnT) processes of precious metal RuII or IrIII complexes or of organic dyes with low photostability. Earth-abundant metal-based Mn Ln -type (M=metal, Ln =polydentate ligands) complexes are rapidly evolving as alternative photocatalysts as they offer not only economic and ecological advantages but also access to the complementary inner-sphere mechanistic modes, thereby transcending their inherent limitations of ultrashort excited-state lifetimes for use as effective photocatalysts. The generic process, termed visible-light-induced homolysis (VLIH), entails the formation of suitable light-absorbing ligated metal-substrate complexes (Mn Ln -Z; Z=substrate) that can undergo homolytic cleavage to generate Mn-1 Ln and Z. for further transformations.