Nickel-catalyzed reductive carboxylation of styrenes using CO2

Revisiting the Mechanism of Asymmetric Ni-Catalyzed Reductive Carbo-Carboxylation with CO2: The Additives Affect the Product Selectivity

The mechanistic details of the asymmetric Ni-catalyzed reductive cyclization/carboxylation of alkenes with CO2 have been revisited using DFT methods. Emphasis was put on the enantioselectivity and the mechanistic role of Lewis acid additives and in situ formed salts. Our results show that oxidative addition of the substrate is rate-limiting, with the formed Ni(II)-aryl intermediate preferring a triplet spin state. After reduction to Ni(I), enantioselective cyclization of the substrate occurs, followed by inner sphere carboxylation. Our proposed mechanism reproduces the experimentally observed enantiomeric excess and identifies critical C-H/O and C-H/N interactions that affect the selectivity. Further, our results highlight the beneficial effect of Lewis acids on CO2 insertion and suggest that in situ formed salts influence if the 5-exo or 6-endo product will be formed.

Metal-Free Heterogeneous Photocatalysis for Carbocarboxylation of Alkenes: Efficient Synthesis of γ-Amino Carboxylic Derivatives

A metal-free heterogeneous protocol is established herein for the synthesis of value-added γ-amino acid scaffolds via carbocarboxylation of alkenes with CO2 and alkylamines under visible light irradiation. The protocol shows broad substrate scope under mild reaction conditions and good stability of the catalyst for recycle tests. Moreover, the methodology could be feasible to the late-stage derivatization of several natural products, enriching the chemical arsenal for practical application.

Deconstructive Carboxylation of Activated Alkenes with Carbon Dioxide

Carboxylation with carbon dioxide (CO2 ) represents one notable methodology to produce carboxylic acids. In contrast to carbon-heteroatom bonds, carbon-carbon bond cleavage for carboxylation with CO2 is far more challenging due to their inherent and less favorable orbital directionality for interacting with transition metals. Here we report a photocatalytic protocol for the deconstructive carboxylation of alkenes with CO2 to generate carboxylic acids in the absence of transition metals. It is emphasized that our protocol provides carboxylic acids with obviously unchanged carbon numbers when terminal alkenes were used. To show the power of this strategy, a variety of pharmaceutically relevant applications including the modular synthesis of propionate nonsteroidal anti-inflammatory drugs and the late-stage carboxylation of bioactive molecule derivatives are demonstrated.

Magnesium-Mediated Umpolung Carboxylation of p-Quinone Methides with CO2

Magnesium-mediated reductive carboxylation of p-QMs with CO2 via an Umpolung strategy has been developed, which can be used for the preparation of various aryl acetic acids. This protocol featured high atom economy, mild conditions, and operational simplicity. The creation of this Umpolung carboxylation of p-QMs will unprecedentedly extend the application of p-QMs to nucleophilic reagents.

Ni-Catalyzed Regioselective Reductive 1,3-Dialkenylation of Alkenes

Dicarbofunctionalization is an important efficient synthetic technique for adding two chemical moieties across an alkene. Here, a novel method of reductive dicarbofunctionalization has been developed using a single alkenyl triflate as the electrophile, combined with an unactivated alkene. The reaction does not require an external auxiliary and proceeds with complete regioselectivity.

Nickel-Catalyzed Enantioconvergent Carboxylation Enabled by a Chiral 2,2'-Bipyridine Ligand

In contrast to previous approaches to chiral α-aryl carboxylic acids that based on reactions using hazardous gases, pressurized setup and mostly noble metal catalysts, in this work, a nickel-catalyzed general, efficient and highly enantioselective carboxylation reaction of racemic benzylic (pseudo)halides under mild conditions using atmospheric CO₂ has been developed. A unique chiral 2,2'-bipyridine ligand named Me-SBpy featuring compact polycyclic skeleton enabled both high reactivity and stereoselectivity. The utility of this method has been demonstrated by synthesis of various chiral α-aryl carboxylic acids (30 examples, up to 95 % yield and 99 : 1 er), including profen family anti-inflammatory drugs and transformations using the acids as key intermediates. Based on mechanistic experimental results, a plausible catalytic cycle involving Ni-complex/radical equilibrium and Lewis acid-assisted CO₂ activation has been proposed.

Catalytic direct hydrocarboxylation of styrenes with CO2 and H2

A three-component hydrocarboxylation of an olefin with CO2 and H2 could be regarded as a dream reaction, since it would provide a straightforward approach for the synthesis of aliphatic carboxylic acids in perfect atom economy. However, this transformation has not been realized in a direct manner under mild conditions, because boosting the carboxylation with thermodynamically stable CO2 while suppressing the rapid hydrogenation of olefin remains a challenging task. Here, we report a rhodium-catalysed reductive hydrocarboxylation of styrene derivatives with CO2 and H2 under mild conditions, in which H2 served as the terminal reductant. In this approach, the carboxylation process was largely accelerated by visible light irradiation, which was proved both experimentally and by computational studies. Hydrocarboxylation of various kinds of styrene derivatives was achieved in good yields without additional base under ambient pressure of CO2/H2 at room temperature. Mechanistic investigations revealed that use of a cationic rhodium complex was critical to achieve high hydrocarboxylation selectivity.

Visible-Light-Driven Reductive Carboxylation of Benzyl Bromides with Carbon Dioxide Using Formate as Terminal Reductant

Cheap and available formate can be seen formally as a carbon dioxide radical anion (CO₂^•-) combined with a hydrogen atom, where the CO₂^•- is not only a highly active radical but also a very powerful reductant. In this paper, we successfully realized a visible-light-driven carboxylation of benzyl bromides with carbon dioxide to prepare high-value arylacetic acids using potassium formate as a terminal reductant. This reaction is characterized by mild reaction conditions and a wide range of substrates. Moreover, under nitrogen atmosphere, the reaction can also achieve the carboxylation of benzyl bromides utilizing an excess of potassium formate. Mechanistic experiments indicate this carboxylation proceeded through CO₂^•-, which was generated from the oxidation of 1,4-diazabicyclo[2.2.2]octane with excited photosensitizer Ir(ppy)₂(dtbbpy)PF₆ in the presence of the potassium formate.

Boryl Radical Activation of Benzylic C-OH Bond: Cross-Electrophile Coupling of Free Alcohols and CO2 via Photoredox Catalysis

A new strategy for the direct cleavage of the C(sp³)-OH bond has been developed via activation of free alcohols with neutral diphenyl boryl radical generated from sodium tetraphenylborate under mild visible light photoredox conditions. This strategy has been verified by cross-electrophile coupling of free alcohols and carbon dioxide for the synthesis of carboxylic acids. Direct transformation of a range of primary, secondary, and tertiary benzyl alcohols to acids has been achieved. Control experiments and computational studies indicate that activation of alcohols with neutral boryl radical undergoes homolysis of the C(sp³)-OH bond, generating alkyl radicals. After reducing the alkyl radical into carbon anion under photoredox conditions, the following carboxylation with CO₂ affords the coupling product.

Challenges and recent advancements in the transformation of CO2 into carboxylic acids: straightforward assembly with homogeneous 3d metals

Transformation of carbon dioxide (CO2) into valuable organic carboxylic acids is essential for maintaining sustainability. In this review, such CO2 thermo-, photo- and electrochemical transformations under 3d-transition metal catalysis are described from 2017 until 2022.

Benzannulation and Hydrocarboxylation Methods for the Synthesis of a Neopentylene-Fused Analogue of Ibuprofen

Neopentylene ring fusions (ring-fused 4,4-dimethylcyclopentane polycycles) are found in many natural products, but they are largely absent from synthetic compound libraries and focused medicinal chemistry research. Here is reported a synthetic approach to one of the few non-natural product-based target compounds from medicinal chemistry that includes a neopentylene ring fusion: an analogue of ibuprofen referred to herein as "neoprofen". The approach features ring-opening fragmentation reactions of dimedone derivatives coupled with transition metal-catalyzed benzannulation and hydrocarboxylation methods.

DFT insights into the Ni-catalyzed regioselective hydrocarboxylation of unsaturated alkenes with CO2

The nickel-catalyzed hydrocarboxylation of alkenes using carbon dioxide has recently become an appealing method to prepare functionalized carboxylic acids with high efficiency and regioselectivity. Herein, density functional theory (DFT) calculations were conducted on the Ni-catalyzed hydrocarboxylation of aryl-/alkyl-substituted alkenes with CO₂. The α- and β-carboxylation of aromatic and aliphatic olefins originate from distinct catalytic cycles: H-transfer-carboxylation and carboxylation-H-transfer pathways. The typical hydrometallation-carboxylation mechanism is unlikely because water/carbonic acid (H-resource) are inferior hydride donors.

Selective Electrosynthetic Hydrocarboxylation of α,β-Unsaturated Esters with Carbon Dioxide*

The carboxylation of low-value commodity chemicals to provide higher-value carboxylic acids is of significant interest. Recently alternative routes to the traditional hydroformylation processes that used potentially toxic carbon monoxide and a transition metal catalyst have appeared. A significant challenge has been the selectivity observed for olefin carboxylation. Photochemical methods have shown a viable route towards the hydrocarboxylation of α,β-unsaturated alkenes but rely on the use of an excess reducing or amine reagent. Herein we report our investigations of an electrochemical approach that is able to hydrocarboxylate α,β-unsaturated alkenes with excellent regioselectivity and the ability to carboxylate hindered substrates to afford α-quaternary center carboxylic acids. The reported process requires no chromatography and the products are purified by simple crystallization from the reaction mixture after work-up.

Photoinduced Hydrocarboxylation via Thiol-Catalyzed Delivery of Formate Across Activated Alkenes

Herein we disclose a new photochemical process to prepare carboxylic acids from formate salts and alkenes. This redox-neutral hydrocarboxylation proceeds in high yields across diverse functionalized alkene substrates with excellent regioselectivity. This operationally simple procedure can be readily scaled in batch at low photocatalyst loading (0.01% photocatalyst). Furthermore, this new reaction can leverage commercially available formate carbon isotologues to enable the direct synthesis of isotopically labeled carboxylic acids. Mechanistic studies support the working model involving a thiol-catalyzed radical chain process wherein the atoms from formate are delivered across the alkene substrate via CO₂^•- as a key reactive intermediate.

Photocatalytic carbocarboxylation of styrenes with CO2 for the synthesis of γ-aminobutyric esters

Metal-free photoredox-catalyzed carbocarboxylation of various styrenes with carbon dioxide (CO2) and amines to obtain γ-aminobutyric ester derivatives has been developed (up to 91% yield, 36 examples). The radical anion of (2,3,4,6)-3-benzyl-2,4,5,6-tetra(9H-carbazol-9-yl)benzonitrile (4CzBnBN) possessing a high reduction potential (-1.72 V vs. saturated calomel electrode (SCE)) easily reduces both electron-donating and electron-withdrawing group-substituted styrenes.

CuH-Catalyzed Regio- and Enantioselective Hydrocarboxylation of Allenes: Toward Carboxylic Acids with Acyclic Quaternary Centers

We report a method to prepare α-chiral carboxylic acid derivatives, including those bearing all-carbon quaternary centers, through an enantioselective CuH-catalyzed hydrocarboxylation of allenes with a commercially available fluoroformate. A broad range of heterocycles and functional groups on the allenes were tolerated in this protocol, giving enantioenriched α-quaternary and tertiary carboxylic acid derivatives in good yields with exclusive branched regioselectivity. The synthetic utility of this approach was further demonstrated by derivatization of the products to afford biologically important compounds, including the antiplatelet drug indobufen.

Light-Promoted Organic Transformations Utilizing Carbon-Based Gas Molecules as Feedstocks

Carbon-based gas molecules are readily available feedstocks and are widely used in industry as building blocks or fuels. However, their application in the synthesis of fine chemicals has been hampered due to operational complexity, poor reaction efficiency and selectivity. Recent development of photoredox-promoted transformations using such gaseous reagents has received considerable attention from the synthetic community. In this review, efforts in developing light-promoted organic transformations using carbon-based natural gases as C1 or C2 feedstocks and to overcome the associated challenges are briefly summarized.

Carbon dioxide based methodologies for the synthesis of fine chemicals

Rapid environmental changes triggered by the increase in the concentration of heat-absorbing gases such as CO2 in the atmosphere have become a major cause of concern. One of the ways to counter this growing threat will be to efficiently convert atmospheric CO2 into value-added products via the development of efficient transition-metal-catalyzed processes. Conversion of CO2 into bulk products such as CH3OH and methane as well as its incorporation into commercial polyurethane synthesis has been achieved and reviewed extensively. However, the efficient transformation of CO2 into fine chemicals and value-added chemicals has many fold advantages. Recent years have seen a rapid rise in the number of metal-mediated protocols to achieve this goal of converting CO2 into fine chemicals. These are essential developments given the requirement of several commodities and fine chemicals in various industrial processes and the utilization of atmospheric CO2 will help provide a sustainable solution to the current environmental problems. Accordingly, we present here a comprehensive compilation of catalytic processes, involving CO2 as the C1 source for reacting with substrates such as alkanes, alkenes, alkynes, amines, acid chlorides, alcohols, allyl boronates, alkenyl triflates, and many others to provide easy access to a wide variety of useful molecules. Such a technology would certainly prove to be beneficial in solving the problems associated with the environmental accumulation of CO2.

Control of the synthesis and morphology of nano dendritic CuAl2O4 as a nanocatalyst for photoredox-catalyzed dicarbofunctionalization of styrenes with amines and CO2

Various morphologies of a nano CuAl₂O₄ fiber microsphere were produced through diverse synthesis situations.

Transition Metal-Free Synthesis of Carbamates Using CO2 as the Carbon Source

Utilization of carbon dioxide as a C1 synthon is highly attractive for the synthesis of valuable chemicals. However, activation of CO₂ is highly challenging, owing to its thermodynamic stability and kinetic inertness. With this in mind, several strategies have been developed for the generation of carbon-heteroatom bonds. Among these, formation of C-N bonds is highly attractive, especially, when carbamates can be synthesized directly from CO₂ . This Minireview focuses on transition metal-free approaches for the fixation of CO₂ to generate carbamates for the production of fine chemicals and pharmaceuticals. Within the past decade, transition metal-free approaches have gained increasing attention, but traditional reviews have rarely focused on these approaches. Direct comparisons between such methods have been even more scarce. This Minireview seeks to address this discrepancy.

Theoretical Characterization of Catalytically Active Species in Reductive Hydroxymethylation of Styrene with CO2 over a Bisphosphine-Ligated Copper Complex

In this work, a density functional theory (DFT) study was performed to identify the catalytically active species in the copper-catalyzed three-component reductive hydroxymethylation of styrene with CO₂ and hydrosilane. The calculations reveal that the dimeric copper(I) hydride species, formed in a mixture of the bisphosphine ligand, Cu(OAc)₂, and hydrosilane, probably acts as the catalyst precursor. In the beginning, this species is catalytically competent to trigger the hydrocupration of styrene, along with the formation of the dimeric copper(I) alkyl intermediate. Subsequently, CO₂ insertion into the dimeric copper(I) alkyl intermediate occurs, which is accompanied by the cleavage of the Cu-Cu bond and the generation of the monomeric copper(I) carboxylate intermediate. In the end, the sequential reduction of the monomeric copper(I) carboxylate intermediate with the hydrosilane produces the monomeric copper(I) hydride species as the actual catalyst and turns on the catalytic cycle. On the other hand, the monomeric copper(II) hydride species, yielded as the kinetic product in the initial reaction of the bisphosphine ligand, Cu(OAc)₂, and hydrosilane, is also reactive for the hydrocupration of styrene. However, the resulting monomeric copper(II) alkyl intermediate is found to be the catalyst resting state, because of the much higher energy barrier demanded for the subsequent nucleophilic attack toward CO₂. On the basis of the results of an activation-strain model (ASM) analysis and charge decomposition analysis (CDA), the low activity of the monomeric copper(II) alkyl intermediate can be ascribed to the more crowded environment around the central copper(II) ion and the weaker nucleophilicity of the alkyl moiety. Furthermore, all of the possible CuH species generated in the system are competent to promote the two-component hydrosilylation of CO₂ with hydrosilane, which is an inevitable side reaction along with the reductive hydroxymethylation of styrene with CO₂ and hydrosilane.

Exploration of New Biomass-Derived Solvents: Application to Carboxylation Reactions

A range of hitherto unexplored biomass-derived chemicals have been evaluated as new sustainable solvents for a large variety of CO2 -based carboxylation reactions. Known biomass-derived solvents (biosolvents) are also included in the study and the results are compared with commonly used solvents for the reactions. Biosolvents can be efficiently applied in a variety of carboxylation reactions, such as Cu-catalyzed carboxylation of organoboranes and organoboronates, metal-catalyzed hydrocarboxylation, borocarboxylation, and other related reactions. For many of these reactions, the use of biosolvents provides comparable or better yields than the commonly used solvents. The best biosolvents identified are the so far unexplored candidates isosorbide dimethyl ether, acetaldehyde diethyl acetal, rose oxide, and eucalyptol, alongside the known biosolvent 2-methyltetrahydrofuran. This strategy was used for the synthesis of the commercial drugs Fenoprofen and Flurbiprofen.

Evaluation of Influencing Factors in Tetravalent Uranium Complex-Mediated CO2 Functionalization by Density Functional Theory

The functionalization of CO₂ mediated by a series of U(IV) mixed-sandwich compounds, (COT^TIPS2)Cp*UR (R = -CH₃, -CH₂Ph, -CH₂TMS, -CH(TMS)₂, -NHPh, -OPh, -SPh, -SePh; COT^TIPS2 = C₈H₆(SiⁱPr₃-1,4)₂; Cp* = C₅Me₅; TMS = SiMe₃), was investigated by the density functional theory method. A two-step mechanism was revealed, in which the insertion of CO₂ into the U-C bond was identified as the rate-determining step via a transition state featured by a four-membered ring with a free-energy barrier of 18.8 kcal/mol to the reaction of the (COT^TIPS2)Cp*UCH₃ system. The whole reaction was strongly exothermic by 45.0 kcal/mol. Substitution effect was discussed, including the bulkiness of the R group and the nature of the ligating atom, and steric hindrance and electrostatic interactions were found to be responsible for the observed variation in reactivity. The reactivity of U(III) and U(IV) complexes in CO₂ functionalization was also compared and discussed. The results were consistent with experimental studies and complemented with molecular level of understanding on the mechanisms of CO₂ functionalization promoted by tetravalent U complexes.

Electrochemical β-Selective Hydrocarboxylation of Styrene Using CO2 and Water

The carboxylation of hydrocarbons using CO2 as a one-carbon building block is an attractive route for the synthesis of carboxylic acids and their derivatives. Until now, chemical carboxylation catalyzed by organometallic nucleophiles and reductants has been generally adopted particularly for the precise selectivity control of carboxylation sites. As another approach, electrochemical carboxylation has been attempted but these carboxylation reactions are limited to only a few pathways. In the case of styrene, dicarboxylation at the α- and β-positions is mostly observed with electrochemical carboxylation while site-selective hydrocarboxylations are hardly achieved. In this study, electrochemical β-selective hydrocarboxylation of styrene using CO2 and water is developed, in which the site selectivity can be precisely controlled between β-hydrocarboxylation and dicarboxylation without the aid of homogeneous catalysts. In this platform, water is used as proton source in the β-hydrocarboxylation of styrene where its addition results in significant enhancement of the selectivity toward β-hydrocarboxylation. This work provides insights into new strategies for site-selectivity-controllable carboxylation with CO2 using an electrochemical platform.

Carboxylation of Benzylic and Aliphatic C-H Bonds with CO2 Induced by Light/Ketone/Nickel

A photoinduced carboxylation reaction of benzylic and aliphatic C-H bonds with CO₂ is developed. Toluene derivatives capture gaseous CO₂ at the benzylic position to produce phenylacetic acid derivatives when irradiated with UV light in the presence of an aromatic ketone, a nickel complex, and potassium tert-butoxide. Cyclohexane reacts with CO₂ to furnish cyclohexanecarboxylic acid under analogous reaction conditions. The present photoinduced carboxylation reaction provides a direct access from readily available hydrocarbons to the corresponding carboxylic acids with one carbon extension.