Current Mechanistic Understanding of Cobalt-Catalyzed C–H Functionalization

The Recent Advances in Iron-Catalyzed C(sp3 )-H Functionalization

The use of iron as a core metal in catalysis has become a research topic of interest over the last few decades. The reasons are clear. Iron is the most abundant transition metal on Earth's crust and it is widely distributed across the world. It has been extracted and processed since the dawn of civilization. All these features render iron a noncontaminant, biocompatible, nontoxic, and inexpensive metal and therefore it constitutes the perfect candidate to replace noble metals (rhodium, palladium, platinum, iridium, etc.). Moreover, direct C-H functionalization is one of the most efficient strategies by which to introduce new functional groups into small organic molecules. The majority of organic compounds contain C(sp3 )-H bonds. Given the enormous importance of organic molecules in so many aspects of existence, the utilization and bioactivity of C(sp3 )-H bonds are of the utmost importance. This review sheds light on the substrate scope, selectivity, benefits, and limitations of iron catalysts for direct C(sp3 )-H bond activations. An overview of the use of iron catalysis in C(sp3 )-H activation protocols is summarized herein up to 2022.

C-H Activation by Isolable Cationic Bis(phosphine) Cobalt(III) Metallacycles

Five- and six-coordinate cationic bis(phosphine) cobalt(III) metallacycle complexes were synthesized with the general structures, [(depe)Co(cycloneophyl)(L)(L')][BArF4] (depe = 1,2-bis(diethylphosphino)ethane; cycloneophyl = [κ-C:C-(CH2C(Me)2)C6H4]; L/L' = pyridine, pivalonitrile, or the vacant site, BAr4F = B[(3,5-(CF3)2)C6H3]4). Each of these compounds promoted facile directed C(sp2)-H activation with exclusive selectivity for ortho-alkylated products, consistent with the selectivity of reported cobalt-catalyzed arene-alkene-alkyne coupling reactions. The direct observation of C-H activation by cobalt(III) metallacycles provided experimental support for the intermediacy of these compounds in this class of catalytic C-H functionalization reaction. Deuterium labeling and kinetic studies provided insight into the nature of C-H bond cleavage and C-C bond reductive elimination from isolable cobalt(III) precursors.

Three-Component Coupling of Arenes, Ethylene, and Alkynes Catalyzed by a Cationic Bis(phosphine) Cobalt Complex: Intercepting Metallacyclopentenes for C-H Functionalization

A cobalt-catalyzed intermolecular three-component coupling of arenes, ethylene, and alkynes was developed using the well-defined air-stable cationic bis(phosphine) cobalt(I) complex, [(dcype)Co(η⁶-C₇H₈)][BAr^F₄] (dcype = 1,2-bis(dicyclohexylphosphino)ethane; BAr^F₄ = B[(3,5-(CF₃)₂)C₆H₃]₄), as the precatalyst. All three components were required for turnover and formation of ortho-homoallylated arene products. A range of directing groups including amide, ketone, and 2-pyridyl substituents on the arene promoted the reaction. The cobalt-catalyzed method exhibited broad functional group tolerance allowing for the late-stage functionalization of two drug molecules, fenofibrate and haloperidol. A series of control reactions, deuterium labeling studies, resting state analysis, as well as synthesis of substrate- and product-bound η⁶-arene complexes supported a pathway involving C(sp²)–H activation from a cobalt(III) metallacycle.

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.

Insights into Cobalta(III/IV/II)-Electrocatalysis: Oxidation-Induced Reductive Elimination for Twofold C-H Activation

The merger of cobalt‐catalyzed C−H activation and electrosynthesis provides new avenues for resource‐economical molecular syntheses, unfortunately their reaction mechanisms remain poorly understood. Herein, we report the identification and full characterization of electrochemically generated high‐valent cobalt(III/IV) complexes as crucial intermediates in electrochemical cobalt‐catalyzed C−H oxygenations. Detailed mechanistic studies provided support for an oxidatively‐induced reductive elimination via highly‐reactive cobalt(IV) intermediates. These key insights set the stage for unprecedented cobaltaelectro two‐fold C−H/C−H activation.

Cp*Co(III)-Catalyzed C-7 C-C Coupling of Indolines with Aziridines: Merging C-H Activation and Ring Opening

A Cp*Co(III)-catalyzed directing group-assisted C7 C-C coupling of indolines with aziridines has been developed by merging C-H activation and ring opening. The use of cobalt catalyst, detection of a Co(III) intermediate, and late-stage removal of the directing group are important practical features.

Unravelling the mechanism of cobalt-catalysed remote C-H nitration of 8-aminoquinolinamides and expansion of substrate scope towards 1-naphthylpicolinamide

Previously, an unexpected Co-catalysed remote C-H nitration of 8-aminoquinolinamide compounds was developed. This report provided a novel reactivity for Co which was assumed to proceed through the mechanistic pathway already known for analogous Cu-catalysed remote couplings of the same substrates. In order to shed light into this intriguing, and previously unobserved reactivity for Co, a thorough computational study has now been performed, which has allowed for a full understanding of the operative mechanism. This study demonstrates that the Co-catalysed remote coupling does not occur through the previously proposed Single Electron Transfer (SET) mechanism, but actually operates through a high-spin induced remote radical coupling mechanism, through a key intermediate with significant proportion of spin density at the 5- and 7-positions of the aminoquinoline ring. Additionally, new experimental data provides expansion of the synthetic utility of the original nitration procedure towards 1-naphthylpicolinamide which unexpectedly appears to operate via a subtly different mechanism despite having a similar chelate environment.

Synthesis of Homoallylic Alcohols with Acyclic Quaternary Centers through CoIII -Catalyzed Three-Component C-H Bond Addition to Internally Substituted Dienes and Carbonyls

An efficient CoIII -catalyzed three-component strategy to prepare homoallylic alcohols containing acyclic quaternary centers is disclosed. This transformation enables the introduction of two C-C σ bonds through C-H bond activation and sequential addition to internally substituted dienes and a wide range of aldehydes and activated ketones. Isoprene and other internally monosubstituted dienes are effective inputs, with the reaction proceeding with high diastereoselectivity for those substrate combinations that result in more than one stereogenic center. Moreover, the opposite relative stereochemistry can be achieved by employing 1,2-disubstituted dienes. A mechanism for the transformation is proposed based upon the relative stereochemistry of the products and studies with isotopically labeled starting materials.

Weinreb Amides as Directing Groups for Transition Metal-Catalyzed C-H Functionalizations

Weinreb amides are a privileged, multi-functional group with well-established utility in classical synthesis. Recently, several studies have demonstrated the use of Weinreb amides as interesting substrates in transition metal-catalyzed C-H functionalization reactions. Herein, we review this part of the literature, including the metal catalysts, transformations explored so far and specific insights from mechanistic studies.

A challenging redox neutral Cp*Co(III)-catalysed alkylation of acetanilides with 3-buten-2-one: synthesis and key insights into the mechanism through DFT calculations

Traditional, established palladium cross-coupling procedures are widely applied in complex molecule synthesis; however, there is a significant disadvantage in the requirement for pre-functionalised substrates (commonly halides/triflates). Direct C–H activation protocols provide the opportunity for a novel approach to synthesis, although this field is still in its relative infancy and often transferability between substrate classes remains unresolved and limitations not fully understood. This study focuses on the translation of an established Cp*Co(III)-catalysed alkylation of benzamides to related acetanilides using 3-buten-2-one as coupling partner. The developed procedure provides a wide substrate scope in terms of substituted acetanilides, although the optimised conditions were found to be more forcing than those for the corresponding benzamide substrates. Interestingly, density functional theory (DFT) studies reveal that the major impediment in the mechanism is not the C–H activation step, but instead and unexpectedly, effective competition with more stable compounds (resting states) not involved in the catalytic cycle.

Mechanistic insights into the SN2-type reactivity of aryl-Co(iii) masked-carbenes for C-C bond forming transformations

Herein we describe the synthesis and characterization of a family of C-metalated aryl-Co(iii) enolates, which can be considered as masked-carbenes, using diazoacetates as coupling partners. These species have been proved to be necessary intermediates in the C(sp2)-C(sp3) bond forming event to obtain cyclic amides, taming the elusive Co(iii)-carbene species. The scope of diazoacetates has been exhaustively examined, varying the nature of the ester and the α-substitution, and a clear preference for electron-poor carbene precursors is observed. Exhaustive experimental and theoretical studies indicate that an unprecedented intramolecular SN2-type process governs the formation of the newly formed C-C bond. Furthermore, the key role of several Lewis acids as carboxylate-activating reagents is further explored by DFT calculations.