Ni-Catalyzed Carboxylation of Unactivated Alkyl Chlorides with CO2

Ni-Catalyzed Direct Carboxylation of an Unactivated C-H Bond with CO2

The transition-metal-catalyzed direct carboxylation of an unactivated C-H bond is rarely reported, and no example of catalysis using abundant and cheap nickel has been reported. In this work, the first Ni-catalyzed direct carboxylation of an unactivated C-H bond under an atmospheric pressure of CO₂ is reported. This method affords moderate to high carboxylation yields of various methyl carboxylates under mild conditions. Preliminary mechanistic studies reveal that a Ni(0)-Ni(II)-Ni(I) catalytic cycle may be involved in this reaction.

Cross-Electrophile Coupling of Unactivated Alkyl Chlorides

Alkyl chlorides are bench-stable chemical feedstocks that remain among the most underutilized electrophile classes in transition metal catalysis. Overcoming intrinsic limitations of C(sp³)-Cl bond activation, we report the development of a novel organosilane reagent that can participate in chlorine atom abstraction under mild photocatalytic conditions. In particular, we describe the application of this mechanism to a dual nickel/photoredox catalytic protocol that enables the first cross-electrophile coupling of unactivated alkyl chlorides and aryl chlorides. Employing these low-toxicity, abundant, and commercially available organochloride building blocks, this methodology allows access to a broad array of highly functionalized C(sp²)-C(sp³) coupled adducts, including numerous drug analogues.

Ni(I)-Alkyl Complexes Bearing Phenanthroline Ligands: Experimental Evidence for CO2 Insertion at Ni(I) Centers

Although the catalytic carboxylation of unactivated alkyl electrophiles has reached remarkable levels of sophistication, the intermediacy of (phenanthroline)Ni(I)-alkyl species-complexes proposed in numerous Ni-catalyzed reductive cross-coupling reactions-has been subject to speculation. Herein we report the synthesis of such elusive (phenanthroline)Ni(I) species and their reactivity with CO₂, allowing us to address a long-standing question related to Ni-catalyzed carboxylation reactions.

Nickel-Catalyzed Cross-Electrophile Coupling of Aryl Chlorides with Primary Alkyl Chlorides

Alkyl chlorides and aryl chlorides are among the most abundant and stable carbon electrophiles. Although their coupling with carbon nucleophiles is well developed, the cross-electrophile coupling of aryl chlorides with alkyl chlorides has remained a challenge. We report here the first general approach to this transformation. The key to productive, selective cross-coupling is the use of a small amount of iodide or bromide along with a recently reported ligand, pyridine-2,6-bis(N-cyanocarboxamidine) (PyBCam^CN). The scope of the reaction is demonstrated with 35 examples (63 ± 16% average yield), and we show that the Br^- and I^- additives act as cocatalysts, generating a low, steady-state concentration of more-reactive alkyl bromide/iodide.

Amide Synthesis by Nickel/Photoredox-Catalyzed Direct Carbamoylation of (Hetero)Aryl Bromides

Herein, we report a one‐electron strategy for catalytic amide synthesis that enables the direct carbamoylation of (hetero)aryl bromides. This radical cross‐coupling approach, which is based on the combination of nickel and photoredox catalysis, proceeds at ambient temperature and uses readily available dihydropyridines as precursors of carbamoyl radicals. The method's mild reaction conditions make it tolerant of sensitive‐functional‐group‐containing substrates and allow the installation of an amide scaffold within biologically relevant heterocycles. In addition, we installed amide functionalities bearing electron‐poor and sterically hindered amine moieties, which would be difficult to prepare with classical dehydrative condensation methods.

Reaction scope and mechanistic insights of nickel-catalyzed migratory Suzuki-Miyaura cross-coupling

Cross-coupling reactions have developed into powerful approaches for carbon–carbon bond formation. In this work, a Ni-catalyzed migratory Suzuki–Miyaura cross-coupling featuring high benzylic or allylic selectivity has been developed. With this method, unactivated alkyl electrophiles and aryl or vinyl boronic acids can be efficiently transferred to diarylalkane or allylbenzene derivatives under mild conditions. Importantly, unactivated alkyl chlorides can also be successfully used as the coupling partners. To demonstrate the applicability of this method, we showcase that this strategy can serve as a platform for the synthesis of terminal, partially deuterium-labeled molecules from readily accessible starting materials. Experimental studies suggest that migratory cross-coupling products are generated from Ni(0/II) catalytic cycle. Theoretical calculations indicate that the chain-walking occurs at a neutral nickel complex rather than a cationic one. In addition, the original-site cross-coupling products can be obtained by alternating the ligand, wherein the formation of the products has been rationalized by a radical chain process.

Migratory cross-coupling reactions are powerful tools to form bonds at predictable positions. Here the authors report a nickel-catalyzed migratory Suzuki–Miyaura cross-coupling of unactivated alkyl electrophiles with aryl and vinyl boron reagents and provide experimental and computational mechanistic evidence.

Generation of Organozinc Reagents by Nickel Diazadiene Complex Catalyzed Zinc Insertion into Aryl Sulfonates

The generation of arylzinc reagents (ArZnX) by direct insertion of zinc into the C−X bond of ArX electrophiles has typically been restricted to iodides and bromides. The insertions of zinc dust into the C−O bonds of various aryl sulfonates (tosylates, mesylates, triflates, sulfamates), or into the C−X bonds of other moderate electrophiles (X=Cl, SMe) are catalyzed by a simple NiCl₂–1,4‐diazadiene catalyst system, in which 1,4‐diazadiene (DAD) stands for diacetyl diimines, phenanthroline, bipyridine and related ligands. Catalytic zincation in DMF or NMP solution at room temperature now provides arylzinc sulfonates, which undergo typical catalytic cross‐coupling or electrophilic substitution reactions.

Metal complex catalysis in the chemistry of lower diamondoids

The review presents the first survey of published data on the use of compounds, complexes and nanoparticles of transition metals (Fe, Co, Ni, Mn, V, Mo, Cu, Pd, Pt, Rh, Ru, Os, Au, Re and Th) in the catalytic transformations of lower diamondoids — adamantane, diamantane and their derivatives. Catalytic halogenation, oxidation, alkylation and cross-coupling reactions are considered, and the formation pathways of C–N, C–S and C–Se bonds in the series of adamantanoids are discussed. Reaction conditions, appropriate catalytic systems and the structures of products are presented.

The bibliography includes 242 references.

Selective C-F bond carboxylation of gem-difluoroalkenes with CO2 by photoredox/palladium dual catalysis

The catalytic C-F bond carboxylation of organofluorines with CO2 gas remains a challenging problem in synthetic chemistry. Here, we describe a selective defluorinative carboxylation of gem-difluoroalkenes through photoredox/palladium dual catalysis. The C-F bond activation is enabled by single electron reduction through photoredox catalysis to generate a fluorovinyl radical, which subsequently participates in an unprecedented palladium-catalyzed carboxylation. This novel C-F functionalization proved applicable to a wide range of substituted gem-difluoroalkenes, providing a rapid access to valuable α-fluoroacrylic acids.

Carboxylation of Aryl Triflates with CO2 Merging Palladium and Visible-Light-Photoredox Catalysts

We report herein a visible-light-promoted, highly practical carboxylation of readily accessible aryl triflates at ambient temperature and a balloon pressure of CO₂ by the combined use of palladium and photoredox Ir(III) catalysts. Strikingly, the stoichiometric metallic reductant is replaced by a nonmetallic amine reductant providing an environmentally benign carboxylation process. In addition, one-pot synthesis of a carboxylic acid directly from phenol and modification of estrone and concise synthesis of pharmaceutical drugs adapalene and bexarotene have been accomplished via late-stage carboxylation reaction. Furthermore, a parallel decarboxylation-carboxylation reaction has been demonstrated in an H-type closed vessel that is an interesting concept for the strategic sector. Spectroscopic and spectroelectrochemical studies indicated electron transfer from the Ir(III)/DIPEA combination to generate aryl carboxylate and Pd(0) for catalytic turnover.

Site-Selective, Remote sp3 C-H Carboxylation Enabled by the Merger of Photoredox and Nickel Catalysis

A photoinduced carboxylation of alkyl halides with CO₂ at remote sp³ C−H sites enabled by the merger of photoredox and Ni catalysis is described. This protocol features a predictable reactivity and site selectivity that can be modulated by the ligand backbone. Preliminary studies reinforce a rationale based on a dynamic displacement of the catalyst throughout the alkyl side chain.

Synthesis and Reactivity of Paramagnetic Nickel Polypyridyl Complexes Relevant to C(sp2 )-C(sp3 )Coupling Reactions

A number of new transition metal catalyzed methods for the formation of C(sp2 )-C(sp3 ) bonds have recently been described. These reactions often utilize bidentate polypyridyl-ligated Ni catalysts, and paramagnetic NiI halide or aryl species are proposed in the catalytic cycles. However, there is little knowledge about complexes of this type. Here, we report the synthesis of paramagnetic bidentate polypyridyl-ligated Ni halide and aryl complexes through elementary reactions proposed in catalytic cycles for C(sp2 )-C(sp3 ) bond formation. We investigate the ability of these complexes to undergo organometallic reactions that are relevant to C(sp2 )-C(sp3 ) coupling through stoichiometric studies and also explore their catalytic activity.

Toward ideal carbon dioxide functionalization

This Perspective recapitulates recent developments of carbon dioxide utilization in carbon-carbon bond formation reactions, with an intention of paving a way toward sustainable CO2-functionalization and its tangible applications in synthetic chemistry. CO2 functionalization reactions possess intrinsic drawbacks: the high kinetic inertness and thermodynamic stability of CO2. Numerous procedures for CO2 utilization depend on energy-intensive processes (i.e. high pressure and/or temperature), often solely relying on reactive substrates, hampering its general applications. Recent efforts thus have been dedicated to catalytic CO2-utilization under ambient reaction conditions, however, it is still limited to a few activation modes and the use of reactive substrates. Herein, ideal CO2-functionalization with particular emphasis on sustainability will be discussed based on the following sub-categories; (1) metal-catalyzed 'reductive' carboxylation reaction of halides, olefins and allyl alcohols, (2) photochemical CO2-utilization, (3) redox-neutral CO2-functionalization, and (4) enantioselective catalysis incorporating CO2 to form C-CO2 bonds (excluding strain mediated reactions with epoxide- and aziridine-based substrates). Recent progress in these fields will be discussed with the proposed reaction mechanisms and selected examples, highlighting redox-neutral, umpolung, and asymmetric carboxylation to postulate ideal CO2 functionalization reactions to be developed in the near future.

Development of an Improved System for the Carboxylation of Aryl Halides through Mechanistic Studies

The nickel-catalyzed carboxylation of organic halides or pseudohalides using carbon dioxide is an emerging method to prepare synthetically valuable carboxylic acids. Here, we report a detailed mechanistic investigation of these reactions using the carboxylation of aryl halides with (PPh₃)₂Ni^IICl₂ as a model reaction. Our studies allow us to understand several general features of nickel-catalyzed carboxylation reactions. For example, we demonstrate that both a Lewis acid and halide source are beneficial for catalysis. To this end, we establish that heterogeneous Mn(0) and Zn(0) reductants are multifaceted reagents that generate noninnocent Mn(II) or Zn(II) Lewis acids upon oxidation. In a key result, a rare example of a well-defined nickel(I) aryl complex is isolated, and it is demonstrated that its reaction with carbon dioxide results in the formation of a carboxylic acid in high yield (after workup). The carbon dioxide insertion product undergoes rapid decomposition, which ca These three oxidation states correspond to the onbe circumvented by a ligand metathesis reaction with a halide source. Our studies have led to both a revised mechanism and the development of a broadly applicable strategy to improve reductive carboxylation reactions. A critical component of this strategy is that we have replaced the heterogeneous Mn(0) reductant typically used in catalysis with a well-defined homogeneous organic reductant. Through its use, we have increased the range of ancillary ligands, additives, and substrates that are compatible with the reaction. This has enabled us to perform reductive carboxylations at low catalyst loadings. Additionally, we demonstrate that reductive carboxylations of organic (pseudo)halides can be achieved in high yields in more practically useful, non-amide solvents. Our results describe a mechanistically guided strategy to improve reductive carboxylations through the use of a homogeneous organic reductant, which may be broadly translatable to a wide range of cross-electrophile coupling reactions.

Reductive Cyclization of Unactivated Alkyl Chlorides with Tethered Alkenes under Visible-Light Photoredox Catalysis

The chemical inertness of abundant and commercially available alkyl chlorides precludes their widespread use as reactants in chemical transformations. Presented in this work is a metallaphotoredox methodology to achieve the catalytic intramolecular reductive cyclization of unactivated alkyl chlorides with tethered alkenes. The cleavage of strong C(sp3 )-Cl bonds is mediated by a highly nucleophilic low-valent cobalt or nickel intermediate generated by visible-light photoredox reduction employing a copper photosensitizer. The high basicity and multidentate nature of the ligands are key to obtaining efficient metal catalysts for the functionalization of unactivated alkyl chlorides.

The Dual Role of Benzophenone in Visible-Light/Nickel Photoredox-Catalyzed C-H Arylations: Hydrogen-Atom Transfer and Energy Transfer

A dual catalytic protocol for the direct arylation of non-activated C(sp³ )-H bonds has been developed. Upon photochemical excitation, the excited triplet state of a diaryl ketone photosensitizer abstracts a hydrogen atom from an aliphatic C-H bond. This inherent reactivity was exploited for the generation of benzylic radicals which subsequently enter a nickel catalytic cycle, accomplishing the benzylic arylation.

Direct Carbon Isotope Exchange through Decarboxylative Carboxylation

A two-step degradation-reconstruction approach to the carbon-14 radiolabeling of alkyl carboxylic acids is presented. Simple activation via redox-active ester formation was followed by nickel-mediated decarboxylative carboxylation to afford a range of complex compounds with ample isotopic incorporations for drug metabolism and pharmacokinetic studies. The practicality and operational simplicity of the protocol were demonstrated by its use in an industrial carbon-14 radiolabeling setting.

Computational exploration of substrate and ligand effects in nickel-catalyzed C–Si bond carboxylation with CO2

The ring strain of substrates and steric hindrance of NHC ligands are the key factors affecting the reactivity of CO₂ insertion into sila-nickelacycles.

A Core-Shell-Structured Silver Nanowire/Nitrogen-Doped Carbon Catalyst for Enhanced and Multifunctional Electrofixation of CO2

Numerous catalysts have been successfully introduced for CO₂ fixation in aqueous or organic systems. However, a single catalyst showing activity in both solvent types is still rare, to the best of our knowledge. We developed a core-shell-structured AgNW/NC700 composite using a Ag nanowire (NW) core encapsulated by a N-doped carbon (NC) shell at 700 °C. Through control experiments and density functional theory calculations, it was confirmed that Ag nanowires acted as the active sites for CO₂ fixation and the uniformly coating of N-doped carbon created a CO₂ -rich environment around the Ag nanowires, which could significantly improve the catalytic activity of Ag nanowires for electrochemical CO₂ fixation. Under mild conditions, up to 96 % faradaic efficiency of CO, 95 % yield of Ibuprofen and 92 % yield of propylene carbonate could be obtained in the electrochemical CO₂ direct reduction, carboxylation and cycloaddition, respectively, using the same AgNWs/NC700 catalyst. These results might provide an alternative strategy for efficient electrochemical fixation of CO₂ .

Ti-Catalyzed Radical Alkylation of Secondary and Tertiary Alkyl Chlorides Using Michael Acceptors

Alkyl chlorides are common functional groups in synthetic organic chemistry. However, the engagement of unactivated alkyl chlorides, especially tertiary alkyl chlorides, in transition-metal-catalyzed C-C bond formation remains challenging. Herein, we describe the development of a TiIII-catalyzed radical addition of 2° and 3° alkyl chlorides to electron-deficient alkenes. Mechanistic data are consistent with inner-sphere activation of the C-Cl bond featuring TiIII-mediated Cl atom abstraction. Evidence suggests that the active TiIII catalyst is generated from the TiIV precursor in a Lewis-acid-assisted electron transfer process.

sp3 C-H Arylation and Alkylation Enabled by the Synergy of Triplet Excited Ketones and Nickel Catalysts

Triplet ketone sensitizers are of central importance within the realm of photochemical transformations. Although the radical-type character of triplet excited states of diaryl ketones suggests the viability for triggering hydrogen-atom transfer (HAT) and single-electron transfer (SET) processes, among others, their use as multifaceted catalysts in C-C bond-formation via sp³ C-H functionalization of alkane feedstocks still remains rather unexplored. Herein, we unlock a modular photochemical platform for forging C( sp³)-C( sp²) and C( sp³)-C( sp³) linkages from abundant alkane sp³ C-H bonds as functional handles using the synergy between nickel catalysts and simple, cheap and modular diaryl ketones. This method is distinguished by its wide scope that is obtained from cheap catalysts and starting precursors, thus complementing existing inner-sphere C-H functionalization protocols or recent photoredox scenarios based on iridium polypyridyl complexes. Additionally, such a platform provides a new strategy for streamlining the synthesis of complex molecules with high levels of predictable site-selectivity and preparative utility. Mechanistic experiments suggest that sp³ C-H abstraction occurs via HAT from the ketone triplet excited state. We believe this study will contribute to a more systematic utilization of triplet excited ketones as catalysts in metallaphotoredox scenarios.

Enantioselective Synthesis of Chiral Cyclopent-2-enones by Nickel-Catalyzed Desymmetrization of Malonate Esters

The enantioselective synthesis of highly functionalized chiral cyclopent‐2‐enones by the reaction of alkynyl malonate esters with arylboronic acids is described. These desymmetrizing arylative cyclizations are catalyzed by a chiral phosphinooxazoline/nickel complex, and cyclization is enabled by the reversible E/Z isomerization of alkenylnickel species. The general methodology is also applicable to the synthesis of 1,6‐dihydropyridin‐3(2H)‐ones.

Mechanistic Insights into the Ni-Catalyzed Reductive Carboxylation of C-O Bonds in Aromatic Esters with CO2 : Understanding Remarkable Ligand and Traceless-Directing-Group Effects

The mechanism of the Ni⁰ -catalyzed reductive carboxylation reaction of C(sp² )-O and C(sp³ )-O bonds in aromatic esters with CO₂ to access valuable carboxylic acids was comprehensively studied by using DFT calculations. Computational results revealed that this transformation was composed of several key steps: C-O bond cleavage, reductive elimination, and/or CO₂ insertion. Of these steps, C-O bond cleavage was found to be rate-determining, and it occurred through either oxidative addition to form a Ni^II intermediate, or a radical pathway that involved a bimetallic species to generate two Ni^I species through homolytic dissociation of the C-O bond. DFT calculations revealed that the oxidative addition step was preferred in the reductive carboxylation reactions of C(sp² )-O and C(sp³ )-O bonds in substrates with extended π systems. In contrast, oxidative addition was highly disfavored when traceless directing groups were involved in the reductive coupling of substrates without extended π systems. In such cases, the presence of traceless directing groups allowed for docking of a second Ni⁰ catalyst, and the reactions proceed through a bimetallic radical pathway, rather than through concerted oxidative addition, to afford two Ni^I species both kinetically and thermodynamically. These theoretical mechanistic insights into the reductive carboxylation reactions of C-O bonds were also employed to investigate several experimentally observed phenomena, including ligand-dependent reactivity and site-selectivity.

Synthesis of Tetrasubstituted Alkenes by Tandem Metallacycle Formation/Cross-Electrophile Coupling

Nickel-catalyzed cross-electrophile couplings have recently emerged as highly effective and practical methods for the formation of C-C bonds. By merging this process with well-established π-π coupling chemistry, a new method for the synthesis of tetrasubstituted alkenes has been developed. The procedure relies on the use of chlorosilanes as a means of generating reactive vinylnickel intermediates, which are capable of undergoing a reductive cross-electrophile coupling with alkyl halides. The method not only generates highly substituted allylic alcohol derivatives but also obviates the need for stoichiometric organometallic nucleophiles and provides greatly improved scope and functional group tolerance compared with previously developed methods.

Nickel-catalyzed reductive thiolation and selenylation of unactivated alkyl bromides

Chalcogen-containing compounds have received considerable attention because of their manifold applications in agrochemicals, pharmaceuticals, and material science. While many classical methods have been developed for preparing organic sulfides, most of them exploited the transition-metal-catalyzed cross-couplings of aryl halides or pseudo halides with thiols or disulfides, with harsh reaction conditions usually being required. Herein, we present a user-friendly, nickel-catalyzed reductive thiolation of unactivated primary and secondary alkyl bromides with thiosulfonates as reliable thiolation reagents, which are easily prepared and bench-stable. Furthermore, a series of selenides is also prepared in a similar fashion with selenosulfonates as selenolation reagents. This catalytic method offers a facile synthesis of a wide range of unsymmetrical alkyl-aryl or alkyl-alkyl sulfides and selenides under mild conditions with an excellent tolerance of functional groups. Likewise, the use of sensitive and stoichiometric organometallic reagents can be avoided.

New trends and applications in carboxylation for isotope chemistry

Carboxylations are an important method for the incorporation of isotopically labeled ¹⁴CO₂ into molecules. This manuscript will review labeled carboxylations since 2010 and will present a perspective on the potential of recent unlabeled methodology for labeled carboxylations. The perspective portion of the manuscript is broken into 3 major sections based on product type, arylcarboxylic acids, benzylcarboxylic acids, and alkyl carboxylic acids, and each of those sections is further subdivided by substrate.

Ruthenium-catalyzed umpolung carboxylation of hydrazones with CO2

The first ruthenium-catalyzed umpolung carboxylation of hydrazones with CO₂ to generate important aryl acetic acids is reported.

The first ruthenium-catalyzed umpolung carboxylation of hydrazones with CO₂ to generate important aryl acetic acids is reported. Besides aldehyde hydrazones, a variety of ketone hydrazones, which have not been successfully applied in previous umpolung reactions with other reactive electrophiles, also show high reactivity and selectivity under mild conditions. Moreover, this operationally simple protocol features good functional group tolerance, is readily scalable, and offers easy derivation of important structures, including bioactive felbinac and adiphenine. Computational studies reveal that this umpolung reaction proceeds through the generation of a Ru-nitrenoid followed by concerted [4 + 2] cycloaddition with CO₂.

Nickel-Catalyzed Umpolung Arylation of Ambiphilic α-Bromoalkyl Boronic Esters

A nickel-catalyzed reductive arylation of ambiphilic α-bromoalkyl boronic esters with aryl halides is described. This platform provides an unrecognized opportunity to promote the catalytic umpolung reactivity of ambiphilic reagents with aryl halides, thus unlocking a new cross-coupling strategy that complements existing methods for the preparation of densely functionalized alkyl-substituted organometallic reagents from simple and readily accessible precursors.