Cobalt-Carbon Bonding in a Salen-Supported Cobalt(IV) Alkyl Complex Postulated in Oxidative MHAT Catalysis

Fe/Thiol Cooperative Hydrogen Atom Transfer Olefin Hydrogenation: Mechanistic Insights That Inform Enantioselective Catalysis

Asymmetric hydrogenation of activated olefins using transition metal catalysis is a powerful tool for the synthesis of complex molecules, but traditional metal catalysts have difficulty with enantioselective reduction of electron-neutral, electron-rich, and minimally functionalized olefins. Hydrogenation based on radical, metal-catalyzed hydrogen atom transfer (mHAT) mechanisms offers an outstanding opportunity to overcome these difficulties, enabling the mild reduction of these challenging olefins with selectivity that is complementary to traditional hydrogenations with H2. Further, mHAT presents an opportunity for asymmetric induction through cooperative hydrogen atom transfer (cHAT) using chiral thiols. Here, we report insights from a mechanistic study of an iron-catalyzed achiral cHAT reaction and leverage these insights to deliver stereocontrol from chiral thiols. Kinetic analysis and variation of silane structure point to the transfer of hydride from silane to iron as the likely rate-limiting step. The data indicate that the selectivity-determining step is quenching of the alkyl radical by thiol, which becomes a more potent H atom donor when coordinated to iron(II). The resulting iron(III)-thiolate complex is in equilibrium with other iron species, including FeII(acac)2, which is shown to be the predominant off-cycle species. The enantiodetermining nature of the thiol trapping step enables enantioselective net hydrogenation of olefins through cHAT using a commercially available glucose-derived thiol catalyst with up to 80:20 enantiomeric ratio. To the best of our knowledge, this is the first demonstration of asymmetric hydrogenation via iron-catalyzed mHAT. These findings advance our understanding of cooperative radical catalysis and act as a proof of principle for the development of enantioselective iron-catalyzed mHAT reactions.

Generation, Characterization and Reactivity of a High-Valent Mononuclear Cobalt(IV)-Diazide Complex

High-valent Fe(IV)=O intermediates of metalloenzymes have inspired numerous efforts to generate synthetic analogs to mimic and understand their substrate oxidation reactivities. However, high-valent M(IV) complexes of late transition metals are rare. We have recently reported a novel Co(IV)-dinitrate complex (1-NO3) that activates sp3 C-H bonds up to 87 kcal/mol. In this work, we have shown that the nitrate ligands in 1-NO3 can be replaced by azide, a more basic coordinating base, resulting in the formation of a more potent Co(IV)-diazide species (1-N3) that reacts with substrates (hydrocarbons and phenols) at faster rate constants and activates stronger C-H bonds than the parent complex 1-NO3. We have characterized 1-N3 employing a combination of spectroscopic and computational approaches. Our results clearly show that the coordination of azide leads to the modulation of the Co(IV) electronic structure and the Co(IV/III) redox potential. Together with the higher basicity of azide, these thermodynamic parameters contribute to the higher driving forces of 1-N3 than 1-NO3 for C-H bond activation. Our discoveries are thus insightful for designing more reactive bio-inspired high-valent late transition metal complexes for activating inert aliphatic hydrocarbons.

Cobalt-Catalyzed Photo-Semipinacol Rearrangement of Unactivated Allylic Alcohols

We report a photochemical method for the semipinacol rearrangement of unactivated allylic alcohols. Aliphatic as well as aromatic groups participate as migrating groups, yielding a variety of α,α-disubstituted ketones. The reaction proceeds under mild conditions and is compatible with ethers, esters, halides, nitriles, carbamates, and substituted arenes. The operationally simple and fully catalytic conditions prescribe 1 mol % benzothiazinoquinoxaline as organophotocatalyst, 0.5 mol % Co-salen, and 10 mol % lutidinium triflate and, importantly, display reactivity complementary to procedures employing Brønsted acid. We showcase the utility of the protocol in late-stage drug diversifications.

Engaging Alkenes in Metallaphotoredox: A Triple Catalytic, Radical Sorting Approach to Olefin-Alcohol Cross-Coupling

Metallaphotoredox cross-coupling is a well-established strategy for generating clinically privileged aliphatic scaffolds via single-electron reactivity. Correspondingly, expanding metallaphotoredox to encompass new C(sp3)-coupling partners could provide entry to a novel, medicinally relevant chemical space. In particular, alkenes are abundant, bench-stable, and capable of versatile C(sp3)-radical reactivity via metal-hydride hydrogen atom transfer (MHAT), although metallaphotoredox methodologies invoking this strategy remain underdeveloped. Importantly, merging MHAT activation with metallaphotoredox could enable the cross-coupling of olefins with feedstock partners such as alcohols, which undergo facile open-shell activation via photocatalysis. Herein, we report the first C(sp3)-C(sp3) coupling of MHAT-activated alkenes with alcohols by performing deoxygenative hydroalkylation via triple cocatalysis. Through synergistic Ir photoredox, Mn MHAT, and Ni radical sorting pathways, this branch-selective protocol pairs diverse olefins and methanol or primary alcohols with remarkable functional group tolerance to enable the rapid construction of complex aliphatic frameworks.

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.

Evaluating Diazene to N2 Interconversion at Iron-Sulfur Complexes

Biological N2 reduction occurs at sulfur-rich multiiron sites, and an interesting potential pathway is concerted double reduction/ protonation of bridging N2 through PCET. Here, we test the feasibility of using synthetic sulfur-supported diiron complexes to mimic this pathway. Oxidative proton transfer from μ-η1 : η1-diazene (HN=NH) is the microscopic reverse of the proposed N2 fixation pathway, revealing the energetics of the process. Previously, Sellmann assigned the purple metastable product from two-electron oxidation of [{Fe2+(PPr3)L1}2(μ-η1 : η1-N2H2)] (L1=tetradentate SSSS ligand) at -78 °C as [{Fe2+(PPr3)L1}2(μ-η1 : η1-N2)]2+, which would come from double PCET from diazene to sulfur atoms of the supporting ligands. Using resonance Raman, Mössbauer, NMR, and EPR spectroscopies in conjunction with DFT calculations, we show that the product is not an N2 complex. Instead, the data are most consistent with the spectroscopically observed species being the mononuclear iron(III) diazene complex [{Fe(PPr3)L1}(η2-N2H2)]+. Calculations indicate that the proposed double PCET has a barrier that is too high for proton transfer at the reaction temperature. Also, PCET from the bridging diazene is highly exergonic as a result of the high Fe3+/2+ redox potential, indicating that the reverse N2 protonation would be too endergonic to proceed. This system establishes the "ground rules" for designing reversible N2/N2H2 interconversion through PCET, such as tuning the redox potentials of the metal sites.

Cobalt-Catalyzed Carbon-Heteroatom Transfer Enables Regioselective Tricomponent 1,4-Carboamination

Tricomponent cobalt(salen)-catalyzed carbofunctionalization of unsaturated substrates by radical-polar crossover has the potential to streamline access to broad classes of heteroatom-functionalized synthetic targets, yet the reaction platform has remained elusive, despite the well-developed analogous hydrofunctionalizations mediated by high-valent alkylcobalt intermediates. We report herein the development of a cobalt(salen) catalytic system that enables carbofunctionalization. The reaction entails a tricomponent decarboxylative 1,4-carboamination of dienes and provides a direct route to aromatic allylic amines by obviating preformed allylation reagents and protection of oxidation-sensitive aromatic amines. The catalytic system merges acridine photocatalysis with cobalt(salen)-catalyzed regioselective 1,4-carbofunctionalization that facilitates the crossover of the radical and polar phases of the tricomponent coupling process, revealing critical roles of the reactants, as well as ligand effects and the nature of the formal high-valent alkylcobalt species on the chemo- and regioselectivity.

Co-Catalyzed Hydrofluorination of Alkenes: Photocatalytic Method Development and Electroanalytical Mechanistic Investigation

The hydrofluorination of alkenes represents an attractive strategy for the synthesis of aliphatic fluorides. This approach provides a direct means to form C(sp3)-F bonds selectively from readily available alkenes. Nonetheless, conducting hydrofluorination using nucleophilic fluorine sources poses significant challenges due to the low acidity and high toxicity associated with HF and the poor nucleophilicity of fluoride. In this study, we present a new Co(salen)-catalyzed hydrofluorination of simple alkenes utilizing Et3N·3HF as the sole source of both hydrogen and fluorine. This process operates via a photoredox-mediated polar-radical-polar crossover mechanism. We also demonstrated the versatility of this method by effectively converting a diverse array of simple and activated alkenes with varying degrees of substitution into hydrofluorinated products. Furthermore, we successfully applied this methodology to 18F-hydrofluorination reactions, enabling the introduction of 18F into potential radiopharmaceuticals. Our mechanistic investigations, conducted using rotating disk electrode voltammetry and DFT calculations, unveiled the involvement of both carbocation and CoIV-alkyl species as viable intermediates during the fluorination step, and the contribution of each pathway depends on the structure of the starting alkene.

A Dual Cobalt and Photoredox Catalysis for Hydrohalogenation of Alkenes

We demonstrate hydrohalogenation of aliphatic alkenes with collidine·HX salts through dual photoredox/cobalt catalysis. The dual catalysis enables conversion of a proton and a halide anion from collidine·HX salt to a nucleophilic hydrogen radical equivalent and an electrophilic halogen radical equivalent and delivery of them to an alkene moiety. This protocol allows for introduction of fluorine, chlorine, bromine, or iodine atom to alkene, producing highly functionalized alkyl halides.

Mechanism of Alkene Hydrofunctionalization by Oxidative Cobalt(salen) Catalyzed Hydrogen Atom Transfer

Oxidative MHAT hydrofunctionalization of alkenes provides a mild cobalt-catalyzed route to forming C-N and C-O bonds. Here, we characterize relevant salen-supported cobalt complexes and their reactions with alkenes, silanes, oxidant, and solvent. These stoichiometric investigations are complemented by kinetic studies of the catalytic reaction and catalyst speciation. We describe the solution characterization of an elusive cobalt(III) fluoride complex, which surprisingly is not the species that reacts with silane under catalytic conditions; rather, a cobalt(III) aquo complex is more active. Accordingly, the addition of water (0.15 M) speeds the catalytic reaction, and kinetic studies show that water addition enables catalytic product formation in 2 h at -50 °C in acetone. Under these conditions, cobalt(III) resting states can be observed by UV-vis spectrophotometry, including a cobalt(III)-alkyl complex. It comes from a transient cobalt(III) hydride complex that is formed in the turnover-limiting step of the catalytic cycle. This hydride readily degrades but not to H2; it releases H+ through a bimetallic pathway that explains the [Co]2 dependence of the off-cycle reaction. In contrast, the rate of the catalytic reaction follows the power law kobs[Co]1[silane]1. Because of the different [Co] dependence of the catalytic reaction and the degradation reaction, lower catalyst loading improves the yield of the catalytic reaction by reducing the relative rate of unproductive silane/oxidant consumption. These studies illuminate mechanistic details of oxidative MHAT hydrofunctionalization of alkenes and lay the groundwork for understanding other catalytic reactions mediated by cobalt hydride and cobalt alkyl complexes.

Dual-Catalysed Intermolecular Reductive Coupling of Dienes and Ketones

We report a mild, catalytic method for the intermolecular reductive coupling of feedstock dienes and styrenes with ketones. Our conditions allow concomitant formation of a cobalt hydride species and single-electron reduction of ketones. Subsequent selective hydrogen-atom transfer from the cobalt hydride generates an allylic radical which can selectively couple with the persistent radical-anion of the ketone. This radical-radical coupling negates unfavourable steric interactions of ionic pathways and avoids the unstable alkoxy radical of previous radical olefin-carbonyl couplings, which were limited, as a result, to aldehydes. Applications of this novel and straightforward approach include the efficient synthesis of drug molecules, key intermediates in drug synthesis and site-selective late-stage functionalisation.

Cobalt-Catalyzed Wagner-Meerwein Rearrangements with Concomitant Nucleophilic Hydrofluorination

We report a cobalt-catalyzed Wagner-Meerwein rearrangement of gem-disubstituted allylarenes that generates fluoroalkane products with isolated yields up to 84 %. Modification of the counteranion of the N-fluoropyridinium oxidant suggests the substrates undergo nucleophilic fluorination during the reaction. Subjecting the substrates to other known metal-mediated hydrofluorination procedures did not lead to observable 1,2-aryl migration. Thus, indicating the unique ability of these cobalt-catalyzed conditions to generate a sufficiently reactive electrophilic intermediate capable of promoting this Wagner-Meerwein rearrangement.

Nitrogen Trifluoride Complexes of Group 10 Transition Metals M(NF3) (M = Pd, Pt)

The group 10 transition metal atoms Pd and Pt react with nitrogen trifluoride (NF3) forming N-coordination M(NF3) complexes in solid neon and argon matrices. The M(NF3) complexes isomerize to more stable fluoronitrenoid FNMF2 isomers via fluorine migration upon blue LED (λ = 470 nm) light irradiation. These products are characterized on the basis of infrared absorption spectroscopy with isotopic substitutions and theoretical frequency calculations. The analysis of the electronic structure of nitrogen trifluoride complexes indicates that the bonding between metal and nitrogen trifluoride can be described as σ donation from the HOMO of nitrogen trifluoride to the empty metal dz2 orbital and π back-donation from the metal dxz/yz orbitals to the LUMO of nitrogen trifluoride, the latter of which stabilized the metal ligand bond and destabilized the ligand N-F bond. In FNMF2, the FN ligand doubly bonded to the metal and bear imido character.

Electrocatalytic Access to Azetidines via Intramolecular Allylic Hydroamination: Scrutinizing Key Oxidation Steps through Electrochemical Kinetic Analysis

Azetidines are prominent structural scaffolds in bioactive molecules, medicinal chemistry, and ligand design for transition metals. However, state-of-the-art methods cannot be applied to intramolecular hydroamination of allylic amine derivatives despite their underlying potential as one of the most prevalent synthetic precursors to azetidines. Herein, we report an electrocatalytic method for intramolecular hydroamination of allylic sulfonamides to access azetidines for the first time. The merger of cobalt catalysis and electricity enables the regioselective generation of key carbocationic intermediates, which could directly undergo intramolecular C-N bond formation. The mechanistic investigations including electrochemical kinetic analysis suggest that either the catalyst regeneration by nucleophilic cyclization or the second electrochemical oxidation to access the carbocationic intermediate is involved in the rate-determining step (RDS) of our electrochemical protocol and highlight the ability of electrochemistry in providing ideal means to mediate catalyst oxidation.

Aldehydes as O-Nucleophiles in Cobalt Hydride Hydrogen Atom Transfer Catalysis: Overriding the Innate Somophilicity

In metal hydride-catalyzed alkene hydrofunctionalization reactions via hydrogen atom transfer, simple carbonyl groups have been well-recognized as good somophiles at the carbon for C-C bond formation. Here we report an alternative pathway exploring the carbonyl as an O-nucleophile to make new C-O bonds during the CoH-catalyzed oxidative cyclization of alkenyl aldehydes. This reaction provides a rapid, mild, modular, and stereoselective (up to >20:1) entry to saturated O-heterocycles via nucleophilic trapping of an in situ-formed oxocarbenium intermediate. The key to overriding the carbonyl's innate somophilicity was found to be promoting the formation of organocobalt species and suppressing the radical exchange.

Cobalt-Catalyzed Enantioselective Hydroamination of Arylalkenes with Secondary Amines

Catalytic asymmetric hydroamination of alkenes with Lewis basic amines is of great interest but remains a challenge in synthetic chemistry. Here, we developed a Co-catalyzed asymmetric hydroamination of arylalkenes directly using commercially accessible secondary amines. This process enables the efficient access to valuable α-chiral tertiary amines in good to excellent yields and enantioselectivities. Mechanistic studies suggest that the reaction includes a CoH-mediated hydrogen atom transfer (MHAT) with arylalkenes, followed by a pivotal catalyst controlled S_N 2-like pathway between in situ generated electrophilic cationic alkylcobalt(IV) species and free amines. This radical-polar crossover strategy not only provides a straightforward and alternative approach for the synthesis of enantioenriched α-tertiary amines, but also underpins the substantial opportunities in developing asymmetric radical functionalization of alkenes with various free nucleophiles in oxidative MHAT catalysis.