Cobaltaelectro-Catalyzed C-H Annulation with Allenes for Atropochiral and P-Stereogenic Compounds: Late-Stage Diversification and Continuous Flow Scale-Up

Palladium-catalyzed regio- and enantio-selective trifluoromethylated allylic alkylation of diphenylphosphine oxides

The first convenient method for the allylic alkylation of diphenylphosphine oxides catalyzed by palladium has been designed and developed, affording various chiral allylphosphine oxides bearing CF3 groups with exclusive regio-selectivity and excellent enantioselective control. The reaction was performed in one pot under relatively mild reaction conditions using "easy-to-prepare" starting materials and relatively inexpensive chiral bis(phosphines) ligands. Further transformations of a representative product confirm the feasibility and potential applications of our synthetic strategies. In conclusion, this study provides a valuable method method for synthesizing chiral molecules bearing CF3 and allylphosphine oxide.

Parametrization of κ2-N,O-Oxazoline Preligands for Enantioselective Cobaltaelectro-Catalyzed C-H Activations

Enantioselective electrocatalyzed C-H activations have emerged as a transformative platform for the assembly of value-added chiral organic molecules. Despite the recent progress, the construction of multiple C(sp3)-stereogenic centers via a C(sp3)-C(sp3) bond formation has thus far proven to be elusive. In contrast, we herein report an annulative C-H activation strategy, generating chiral Fsp3-rich molecules with high levels of diastereo- and enantioselectivity. κ2-N,O-oxazoline preligands were effectively employed in enantioselective cobalt(III)-catalyzed C-H activation reactions. Using DFT-derived descriptors and regression statistical modeling, we performed a parametrization study on the modularity of chiral κ2-N,O-oxazoline preligands. The study resulted in a model describing ligands' selectivity characterized by key steric, electronic, and interaction behaviors.

Cobalt(III)-Catalyzed Enantioselective C-H Functionalization: Ligand Innovation and Reaction Development

ConspectusIn contrast to precious transition metals, such as palladium and rhodium, the development of novel chiral ligands for enantioselective C-H functionalizations catalyzed by earth-abundant, cost-effective, and environmentally friendly 3d metals poses substantial challenges, primarily due to the variable oxidation states, intricate coordination patterns, and limited mechanistic insights. In this Account, we summarize our research endeavors in the development of three novel types of Co(III) catalysis: pseudotetrahedral achiral Cp*Co(III)/chiral carbonyl acid (CCA) catalysis, in situ-generated chiral octahedral cobalt(III) via cobalt/salicyloxazoline (Salox) catalysis, and Co(II)/chiral phosphoric acid (CPA) cooperative catalysis, achieved through strategic chiral ligand design. Our initial objective was to achieve enantioselective C-H functionalization catalyzed by achiral Cp*Co(III) catalysts with external chiral ligands, aiming to circumvent the laborious preparation of chiral CpxCo(III) complexes. To this end, we developed several CCA ligands, incorporating non-covalent interactions (NCIs) as a crucial design element. Next, to address the limitations associated with the lengthy synthesis of Cp-ligated Co(III) complexes and the difficulties of modification, we explored the concept of the in situ generation of Co(III) catalysis using commercially available cobalt(II) salts with tailor-made chiral ligands. This exploration led to the development of two innovative catalytic systems, namely, Co(II)/Salox catalysis and Co(II)/CCA sequential catalysis. The Co(II)/Salox catalysis emerged as a versatile strategy, demonstrating excellent enantioselectivities across a range of asymmetric C-H functionalization reactions to construct various chiral molecules with central, axial, planar, and inherent chirality. The facile synthesis in a single step, along with ease of modification, further enhances the versatility and applicability of this approach. Moreover, we successfully applied cobalt/Salox catalysis in electro- and photochemical-catalyzed enantioselective C-H functionalization, using electrons or oxygen as traceless oxidant, thereby eliminating the need for stoichiometric chemical oxidants. Through mechanistic studies and reaction developments, we elucidated the detailed ligand structure-enantioselectivity relationships in cobalt/Salox catalysis, which are expected to inform future research endeavors. Finally, the Co(II)/CPA cooperative catalysis enabled the synthesis of chiral spiro-γ-lactams through sequential C-H olefination/asymmetric [4 + 1] spirocyclization. Mechanistically, the establishment of stereochemistry occurs during the cyclization step, where the CPA ligand serves as both a neutral ligand and a chiral Brønsted acid, with stereoinduction independent of the C-H cleavage step. We anticipate that the insights and advancements detailed in this Account will inspire further innovations in ligand development and drive progress in the exploration of 3d metal-catalyzed asymmetric C-H functionalization reactions.

Navigating electrochemical oxidative functionalization of olefins: selected mechanistic and synthetic examples

The functionalization of olefins to form added-value compounds is a cornerstone of modern organic chemistry, promoting the synthesis of complex molecules from simple feedstock materials. In parallel, electrochemistry has emerged as a powerful and sustainable technique for enabling challenging transformations under mild conditions by generating reactive intermediates in a controlled manner. This review highlights recent advances in oxidative electrochemical methods for olefin functionalization, showcasing key developments that underscore the versatility of this approach. Using selected representative examples, we explore diverse mechanistic pathways, bond-forming strategies, and the integration of electrochemical techniques with catalytic systems. By providing a concise overview of this rapidly evolving field, we aim to inspire further innovation in electrochemical methodologies to expand the frontiers of olefin chemistry.

Interweavable Metalloporphyrin-Based Fibers for Indirect Electrocatalysis

The applications of indirect electrocatalysis toward potential industrial processes are drastically limited by the utilization or processing forms of electrocatalysts. The remaining challenges of electrocatalysts like the recycling in homogeneous systems or pulverization in heterogeneous systems call for advanced processing forms to meet the desired requirements. Here, we report a series of metalloporphyrin-based polymer fibers (M-PF, M=Ni, Cu and Zn) through a rigid-flexible polymerization strategy based on rigid metalloporphyrin and flexible thiourea units that can be applied as heterogeneous redox-mediators in indirect electrocatalysis. These functional fibers with high strength and flexibility exhibit interweavable and designable functions that can be processed into different fiber-forms like knotted, two-spiral, three-ply, five-ply fibers or even interweaved networks. Interestingly, they can be readily applied in S-S bond cleaving/cyclization reaction or extended oxidative self-coupling reaction of thiols with high efficiency. Remarkably, it enables the scale-up production (1.25 g in a batch-experiment) under laboratory conditions.

Pd/Brønsted Acid Co-catalyzed Dehydrative Coupling of Propargylic Alcohols with Diarylphosphine Oxides

An efficient dehydrative coupling of propargylic alcohols with diarylphosphine oxides to construct tetrasubstituted allenylphosphoryl compounds in the presence of a Pd/Brønsted acid co-catalyst has been developed. As a benefit from the use of a Brønsted acid, this reaction could perform under mild conditions with excellent yields, accommodating a wide range of functional groups. The potential utility of this method has also been demonstrated.

Rhodaelectro-Catalyzed Synthesis of Pyrano[3,4-b]indol-1(9H)-ones via the Double Dehydrogenative Heck Reaction between Indole-2-carboxylic Acids and Alkenes

A rhodaelectro-catalyzed double dehydrogenative Heck reaction of indole-2-carboxylic acids with alkenes has been developed for the synthesis of pyrano[3,4-b]indol-1(9H)-ones. The weakly coordinating carboxyl group is utilized twice as a directing group to activate the C-H bonds throughout the reaction. This reaction precedes an acceptorless dehydrogenation under exogenous oxidant-free conditions in an undivided cell with a constant current.

Enhancing electrochemical reactions in organic synthesis: the impact of flow chemistry

Utilizing electrons directly offers significant potential for advancing organic synthesis by facilitating novel reactivity and enhancing selectivity under mild conditions. As a result, an increasing number of organic chemists are exploring electrosynthesis. However, the efficacy of electrochemical transformations depends critically on the design of the electrochemical cell. Batch cells often suffer from limitations such as large inter-electrode distances and poor mass transfer, making flow cells a promising alternative. Implementing flow cells, however, requires a foundational understanding of microreactor technology. In this review, we briefly outline the applications of flow electrosynthesis before providing a comprehensive examination of existing flow reactor technologies. Our goal is to equip organic chemists with the insights needed to tailor their electrochemical flow cells to meet specific reactivity requirements effectively. We also highlight the application of reactor designs in scaling up electrochemical processes and integrating high-throughput experimentation and automation. These advancements not only enhance the potential of flow electrosynthesis for the synthetic community but also hold promise for both academia and industry.

Metallaelectro-catalyzed alkyne annulations via C-H activations for sustainable heterocycle syntheses

Alkyne annulation represents a versatile and powerful strategy for the assembly of structurally complex compounds. Recent advances successfully enabled electrocatalytic alkyne annulations, significantly expanding the potential applications of this promising technique towards sustainable synthesis. The metallaelectro-catalyzed C-H activation/annulation stands out as a highly efficient approach that leverages electricity, combining the benefits of electrosynthesis with the power of transition-metal catalyzed C-H activation. Particularly attractive is the pairing of the electro-oxidative C-H activation with the valuable hydrogen evolution reaction (HER), thereby addressing the growing demand for green energy solutions. Herein, we provide an overview of the evolution of electrochemical C-H annulations with alkynes for the construction of heterocycles, with a topical focus on the underlying mechanism manifolds.

A Dual Cobalt-Photoredox Catalytic Approach for Asymmetric Dearomatization of Indoles with Aryl Amides via C-H Activation

In this study, we unveil a novel method for the asymmetric dearomatization of indoles under cobalt/photoredox catalysis. By strategically activating C-H bonds of amides and subsequent migratory insertion of π-bonds present in indole as reactive partner, we achieve syn-selective tetrahydro-5H-indolo[2,3-c]isoquinolin-5-one derivatives with excellent yields and enantiomeric excesses of up to >99 %. The developed method operates without a metal oxidant, relying solely on oxygen as the oxidant and employing an organic dye as a photocatalyst under irradiation. Control experiments and stoichiometric studies elucidate the reversible nature of the enantiodetermining C-H activation step, albeit not being rate-determining. This study not only expands the horizon of cobalt-catalyzed asymmetric C-H bond functionalization, but also showcases the potential synergy between cobalt and photoredox catalysis in enabling asymmetric synthesis of complex molecules.

Simultaneous construction of inherent and axial chirality by cobalt-catalyzed enantioselective C-H activation of calix[4]arenes

The simultaneous construction of multiple stereogenic elements in a single step is highly appealing and desirable in the field of asymmetric synthesis. Furthermore, the catalytic enantioselective synthesis of inherently chiral calix[n]arenes with high enantiopurity has long been a challenging endeavor. Herein, we report an enantioselective cobalt-catalyzed C-H activation/annulation for the efficient construction of inherently chiral calix[4]arenes bearing multiple C-N axially chiral element. By employing the benzamide tethered calix[4]arene as the substrate, the C-H annulation with alkynes can be successfully accomplished, leading to the generation of multiple stereogenic elements. A wide range of calix[4]arenes and alkynes are found to be well compatible, and exhibit good yields, high enantioselectivity and excellent diastereoselectivity. Notably, the gram-scale reaction, catalytic application, synthetic transformations, and chiral recognition further showcase the potential applications of this protocol.

Enantioselective Cobaltaphotoredox-Catalyzed C-H Activation

The quest for sustainable strategies in molecular synthesis has spurred the emergence of photocatalysis as a particularly powerful technique. In recent years, the application of photocatalysis in this context has greatly promoted the development of asymmetric catalysis. Despite the impressive advances, enantioselective photoinduced strong arene C-H activations by cobalt catalysis remain unexplored. Herein, we report a strategy that merges organic photoredox catalysis and enantioselective cobalt-catalyzed C-H activation, enabling the regio- and stereoselective dual functionalization of indoles in an enantioselective fashion. Thereby, the assembly of various chiral indolo[2,3-c]isoquinolin-5-ones was realized with high enantioselectivities of up to 99%. The robustness of the cobaltaphotoredox catalysis was demonstrated through enantioselective C-H activation and annulations in a continuous flow to provide straightforward access to central and axially chiral molecules.

Cobaltaelectro-Catalyzed C-H Activation for Central and Axial Double Enantio-Induction

In recent years, enantioselective electrocatalysis has surfaced as an increasingly-effective platform for sustainable molecular synthesis. Despite indisputable progress, strategies that allow the control of two distinct stereogenic elements with high selectivity remain elusive. In contrast, we, herein, describe electrochemical cobalt-catalyzed C-H activations that enable the installation of chiral stereogenic centers along with a chiral axis with high levels of enantio- and diastereoselectivities. The developed electrocatalysis strategy allowed for C-H/N-H activations/annulations with cyclic and non-cyclic alkenes providing expedient access to various central as well as atropo-chiral dihydroisoquinolinones paired to the valuable hydrogen evolution reaction. Studies on the atropo-stability of the obtained compounds demonstrated that the exceedingly mild conditions ensured by the electrocatalytic process were key for the achieved high stereoselectivities.

Electrochemical Enantioselective C-H Annulation by Achiral Rhodium(III)/Chiral Brønsted Base Domino Catalysis

Rhodium(III)-catalyzed enantioselective C-H activation has emerged as a powerful tool for assembling enabling chiral molecules. However, this approach is significantly hampered by the cumbersome synthetic routes for preparing chiral rhodium catalysts. In sharp contrast, we herein report on an electrochemical domino catalysis system that exploits an achiral Cp*-rhodium catalyst along with an easily accessible chiral Brønsted base for an enantioselective C-H activation/annulation reaction of alkenes by benzoic acids. Our strategy offers an environmentally benign and most user-friendly approach for assembling synthetically useful chiral phthalides in good enantioselectivity, employing electricity as the sustainable oxidant.

Nickel-Catalyzed Atroposelective C-H Alkylation Enabled by Bimetallic Catalysis with Air-Stable Heteroatom-Substituted Secondary Phosphine Oxide Preligands

The catalytic asymmetric construction of axially chiral C-N atropisomers remains a formidable challenge due to their low rotational barriers and is largely reliant on toxic, cost-intensive, and precious metal catalysts. In sharp contrast, we herein describe the first nickel-catalyzed atroposelective C-H alkylation for the construction of C-N axially chiral compounds with the aid of a chiral heteroatom-substituted secondary phosphine oxide (HASPO)-ligated Ni-Al bimetallic catalyst. A wide range of alkenes, including terminal and internal alkenes, were well compatible with the reaction, providing a variety of benzimidazole derivatives in high yields and enantioselectivities (up to 97:3 e.r.). The key to success was the identification of novel HASPOs as highly effective chiral preligands. Mechanistic studies revealed the catalyst mode of action, and in-depth data science analysis elucidated the key features of the responsible chiral preligands in controlling the enantioselectivity.

Pd-catalyzed divergent regioselective annulation of phosphinyl allenes accessing polyarylfurans and 2H-chromenes

A novel and efficient regioselective annulation of phosphinyl allenes with 2-bromophenol or 1-bromo-2-naphthol is achieved by palladium catalysis. The divergent pathway delivers structurally diverse polyarylfurans and 2H-chromene skeletons via two different Heck-type annulations. This protocol represents regioselectivity-tunable transformation of allenes into functionalized O-containing heterocycles with excellent group compatibility.

Metal-ligand cooperativity in chemical electrosynthesis

Electrochemistry has been an increasingly useful tool for organic synthesis, as it can selectively generate reactive intermediates under mild conditions using an applied potential. Concurrently, synergistic activity of a metal and a ligand has been used in thermal catalysis and electrocatalytic renewable fuel generation for substrate selectivity and improved catalyst activity. Combining these synthetic strategies is an attractive approach for mild, selective, and sustainable electrosynthesis. This perspective discusses examples of metal-ligand synergistic catalysis in electrochemical applications in organic and organometallic synthesis. The range of reactions and ligand design principles illustrates many opportunities for further discovery in this area and the potential for far-reaching synthetic benefits.

Enantioselective C-H bond functionalization under Co(III)-catalysis

While progress in enantioselective C-H functionalization has been accomplished by employing 4d and 5d transition metal-based catalysts, the rapid depletion of these metals in the earth's crust poses a serious threat to making these protocols sustainable. On the other hand, because of their unique reactivity, low toxicity, and high earth abundance, newer strategies utilizing affordable 3d transition metals have come to the forefront. Among the first-row transition metals, high-valent cobalt has recently attracted a lot of attention for catalytic C-H functionalization with mono and bidentate directing groups. This approach was extended for asymmetric catalysis due to a fairly thorough knowledge of its catalytic cycles. Four major themes have been investigated as a result of this insight: (1) rational design of a chiral Cp#Co(III)-catalyst, (2) chiral carboxylic acid with achiral Cp*Co(III)-catalysts using monodentate directing groups, (3) cobalt/salox-based systems, and (4) cobalt/chiral phosphoric acid-based hybrid systems with bidentate directing groups. Herein, we highlight the recent developments in high-valent cobalt-catalyzed enantioselective C-H functionalization up to October 2023, with the strong belief that the current state-of-the-art can attract considerable interest in the synthetic community, encouraging discoveries in the evolving landscape of asymmetric catalysis.

Reversal of Regioselectivity in Asymmetric C-H Bond Annulation with Bromoalkynes under Cobalt Catalysis

Metal-catalyzed asymmetric C-H bond annulation strategy offers a versatile platform, allowing the construction of complex P-chiral molecules through atom- and step-economical fashion. However, regioselective insertion of π-coupling partner between M-C bond with high enantio-induction remain elusive. Using commercially available Co(II) salt and chiral-Salox ligands, we demonstrate an unusual protocol for the regio-reversal, enantioselective C-H bond annulation of phosphinamide with bromoalkyne through desymmetrization. The reaction proceeds through ligand-assisted enantiodetermining cyclocobaltation followed by regioselective insertion of bromoalkyne between Co-C, subsequent reductive elimination, and halogen exchange with carboxylate resulted in P-stereogenic compounds in excellent ee (up to >99 %). The isolation of cobaltacycle involved in the catalytic cycle and the outcome of control experiments provide support for a plausible mechanism.