Manganese-Catalyzed Z-Selective Allylation of Indoles with Allenyl Derivatives.

Herein, we report a manganese-catalyzed Z-selective hydroarylation of allenyl ethylene carbonates (AECs) under mild conditions. The methodology employs an earth-abundant Mn(I)-catalyst, which shows high functional group tolerance, performed at room temperature, resulting in good-to-excellent yields of the products with very high Z-selectivity. Besides, mechanistic insights reveal the substitution effects of the allenes over the control of Z-selectivity.

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  p-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.

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.

Efficient Adsorptive Removal of Methylene Blue Dye from Aqueous Solution Using Eragrostis Teff Biomass-Derived Nitrogen and Phosphorus-Codoped Carbon Quantum Dots.

Carbon quantum dots have a great application potential in environmental protection via adsorption technology due to their large specific surface area and negative zeta potential. In this work, nitrogen and phosphorus-codoped carbon quantum dots (NP-CQDs) with a large specific surface area and negative zeta potential were successfully synthesized by a single-step hydrothermal synthesis. Batch adsorption studies were utilized to assess the adsorbent's capacity to remove common methylene blue (MB) dye contaminants from an aqueous solution. The experiment showed that MB dye could be removed in 30 min under optimum experimental conditions, with a removal efficiency of 93.73%. The adsorbent's large surface area of 526.063 m2/g and negative zeta potential of -12.3 mV contribute to the high removal efficiency. The Freundlich isotherm model fits the adsorption process well at 298 K, with R2 and n values of 0.99678 and 4.564, respectively, indicating its applicability. A kinetic study demonstrated that the pseudo-second-order model, rather than the pseudo-first-order model, is more suited to represent the process of MB dye adsorption onto NP-CQDs. This research established a simple and cost-effective method for developing a highly efficient NP-CQD adsorbent for organic dye degradation by adsorption.

Hydrogenation and dehydrogenation of N-heterocycles under Cp*Co(III)-catalysis.

To realize the goal of a carbon-free energy economy, it is crucial to discover reactions that utilize sustainable resources as alternatives to fossil feedstocks. In this study, a well-defined, air-stable Cp*Co(III)-catalyst for transfer hydrogenation of quinoline derivatives and oxidative dehydrogenation of cyclic amines in water is developed. While the former reaction is promoted by formic acid as a transfer hydrogenation reagent, the latter is mediated by molecular oxygen as the sole oxidant. These processes provide new avenues for the investigation of air-stable cobalt catalysts for environmentally benign hydrogenation and dehydrogenation reactions.

Allenyl Carbonate as a Butadiene Surrogate in Cobalt-Catalyzed Crotylation of Aldehydes.

Allenyl carbonate was used as a 1,3-butadiene surrogate to develop a photocatalytically sustainable protocol for cobalt-catalyzed crotylation of aldehydes. The developed method tolerated a wide range of aromatic and aliphatic aldehydes with retention of functional groups under mild conditions and produced good-to-excellent yields of crotylated secondary alcohols. Based on preliminary mechanistic studies and literature precedents, a plausible mechanism is proposed.

Cp*Co(III)-Catalyzed C(8)-Nucleophilic Cascade Cyclization of Quinoline N-Oxide with 1,6-Enyne.

The C(8)-selective nucleophilic cascade cyclization of quinoline N-oxide with easily derived 1,6-enyne from phenol derivatives is demonstrated. A variety of quinoline N-oxide and alkynes are discovered to be suitable for producing a library of quinoline N-oxide tethered cis-hydrobenzofurans with high yields and excellent functional group tolerance. The utility of the protocol has been accomplished by post-synthetic modification of the cyclized product. The mechanistic studies indicate a base-assisted internal electrophilic-type substitution (BIES)-type pathway for C-H bond activation, and electrospray ionization mass spectrometry (ESI-MS) analysis of the stoichiometric reaction confirmed the formation of a key five-membered cobaltacycle.

Methanol as a hydrogen source: room-temperature highly-selective transfer hydrogenation of α,ß-unsaturated ketones.

The described system offers an ideal, user-friendly protocol for the chemoselective homogeneous hydrogenation of α,ß-unsaturated ketones at room temperature using methanol as a liquid organic hydrogen carrier. Excellent yields were achieved with an in situ-prepared phosphine-free Cp*Ir(III)/bipyridonate complex. Chemoselective reduction with other reducible functionalities and late-stage functionalization were also explored.

Iron-Catalyzed α-Methylation of Ketones Using Methanol as the C1 Source under Photoirradiation.

A mild, environmentally benign approach for α-methylation of ketones utilizing methanol as the C1 source under visible light has been developed. The reaction conditions were favorable for a wide range of ketones with both aromatic and aliphatic backbones, allowing for good-to-excellent yields of the respective products. The tentative mechanism is postulated after preliminary mechanistic and kinetic experiments.

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.

Synthesis and crystallographic studies of 2-(di-phenyl-phosphino-thio-yl)-2-(3-oxobut-1-en-yl)ferrocene.

As a follow-up to our research on the chemistry of disubstituted ferrocene derivatives, the synthesis and the structure of the title compound, 2-(di-phenyl-phosphino-thio-yl)-2-(3-oxobut-1-en-yl)ferrocene, [Fe(C5H5)(C21H18OPS)], are described. The mol-ecule is built up from a ferrocene unit disubstituted by an S-protected di-phenyl-phosphine group and by a methyl-vinyl-ketone chain. The crystal structure features weak C-H⋯O and C-H⋯S inter-actions, which build a two-dimensional network. This structure is compared to that of the related disubstituted di-phenyl-phosphino ferrocene.

Cp*CoIII-Catalyzed C(7)-H Bond Annulation of Indolines with Alkynes.

An efficient protocol for the synthesis of biologically essential pyrroloquinolinones has been developed under Cp*CoIII catalysis, which involves a cascade reaction of C(7)-H alkenylation with alkynes followed by nucleophilic addition. A wide variety of internal alkynes including enyne, diyne, and ynamide and more challenging terminal alkynes were successfully employed for the annulation in good to excellent yield with high regioselectivity.

C-H and N-H bond annulation of aryl amides with unactivated olefins by merging cobalt(iii) and photoredox catalysis.

A mild, environment-friendly protocol has been developed to carry out the [4+2] annulation of aryl amides with unactivated olefins. A range of sterically and electronically diverse aryl amides and unactivated olefins were successfully employed under the developed conditions to get a variety of dihydroisoquinolinones in good-to-excellent yields.

α-Alkylation of Ketones with Secondary Alcohols Catalyzed by Well-Defined Cp*CoIII -Complexes.

Although α-alkylation of ketones with primary alcohols by transition-metal catalysis is well-known, the same process with secondary alcohols is arduous and complicated by self-condensation. Herein a well-defined, high-valence cobalt(III)-catalyst was applied for successful α-alkylation of ketones with secondary alcohols. A wide-variety of secondary alcohols, which include cyclic, acyclic, symmetrical, and unsymmetrical compounds, was employed as alkylating agents to produce ß-alkyl aryl ketones.

Cobalt-Catalyzed Reductive Alkylation of Amines with Carboxylic Acids.

Direct reductive alkylation of amines with carboxylic acid is carried out by using an inexpensive, air-stable cobalt/triphos catalytic system with molecular hydrogen as the reductant. This efficient synthetic method proceeds through reduction and condensation, followed by reduction of the in situ-generated imine into the amine in a green catalytic process.

Cp*CoIII -Catalyzed Efficient Dehydrogenation of Secondary Alcohols.

A novel, well-defined molecular Cp*CoIII complex was isolated and structurally characterized for the first time. The efficiency of this cobalt catalyst was demonstrated in the alcohol dehydrogenation and dehydrative coupling of secondary alcohols under mild conditions into ketones and ethers, respectively.

Cp*Co(III)-Catalyzed C-H Alkylation with Maleimides Using Weakly Coordinating Carbonyl Directing Groups.

A novel protocol for ortho-C-H alkylation of aromatic and heteroaromatic ketones and esters under Cp*Co(III) catalysis has been developed for the first time. The reaction proceeds through initial cyclometalation via weak chelation-assisted C-H bond activation, followed by coordination of activated alkene, insertion between Co-C, and protodemetalation.

CoIII -Catalyzed Isonitrile Insertion/Acyl Group Migration Between C-H and N-H bonds of Arylamides.

A general efficient and site-selective cobalt-catalyzed insertion of isonitrile into C-H and N-H bonds of arylamides through C-H bond activation and alcohol assisted intramolecular trans-amidation is demonstrated. This straightforward approach overcomes the limitation by the presence of strongly chelating groups. Isolation of CoIII -isonitrile complex B has been achieved for the first time to understand the reaction mechanism.