Photochemical and Electrochemical Carbon Dioxide Utilization with Organic Compounds

Electrochemical hydrocarboxylation of enol derivatives with CO2: access to β-acetoxycarboxylic acids

Electrochemical hydrocarboxylation of enol acetates with CO2 is developed. The disclosed process provides β-acetoxycarboxylic acids in 25-66% yields, in contrast to the electrolysis of ketones, silyl enol ethers and vinyl tosylates with CO2, which leads mainly to alcohols.

Electroreductive carboxylation of benzylphosphonium salts with CO2 through the cleavage of the C(sp3)-P bond

Herein, a electroreductive carboxylation of benzylphosphonium salts was achieved by the cleavage of the C(sp3)-P bond, and various valuable arylacetic acids could be synthesized by this strategy. Also, based on control experiments and previous studies, a plausible reaction mechanism was proposed to explain the reaction process. The establishment of this procedure will provide a new paradigm for the functionalization of alkyl phosphonium salts.

Catalytic Regio- and Enantioselective Remote Hydrocarboxylation of Unactivated Alkenes with CO2

The catalytic regio- and enantioselective hydrocarboxylation of alkenes with carbon dioxide is a straightforward strategy to construct enantioenriched α-chiral carboxylic acids but remains a big challenge. Herein we report the first example of catalytic highly enantio- and site-selective remote hydrocarboxylation of a wide range of readily available unactivated alkenes with abundant and renewable CO2 under mild conditions enabled by the SaBOX/Ni catalyst. The key to this success is utilizing the chiral SaBOX ligand, which combines with nickel to simultaneously control both chain-walking and the enantioselectivity of carboxylation. This process directly furnishes a range of different alkyl-chain-substituted or benzo-fused α-chiral carboxylic acids bearing various functional groups in high yields and regio- and enantioselectivities. Furthermore, the synthetic utility of this methodology was demonstrated by the concise synthesis of the antiplatelet aggregation drug (R)-indobufen from commercial starting materials.

Electroreductive Arylcarboxylation of Styrenes with CO2 and Aryl Halides via a Radical-Polar Crossover Mechanism

2,3-Diaryl propanoic acids are important structures as a result of their widespread presence in numerous bioactive compounds. However, the limitations of existing synthetic techniques include the requirement for costly catalysts and limited substrates. Here, we developed a novel electroreductive arylcarboxylation of alkenes with CO2 based on a radical-polar crossover pathway assisted by easily accessible dimethyl terephthalate as a reductive mediator. This method will provide an efficient strategy for the synthesis of 2,3-diarylpropanoic acids.

Atropisomeric Carboxylic Acids Synthesis via Nickel-Catalyzed Enantioconvergent Carboxylation of Aza-Biaryl Triflates with CO2

Upgrading CO2 to value-added chiral molecules via catalytic asymmetric C-C bond formation is a highly important yet challenging task. Although great progress on the formation of centrally chiral carboxylic acids has been achieved, catalytic construction of axially chiral carboxylic acids with CO2 has never been reported to date. Herein, we report the first catalytic asymmetric synthesis of axially chiral carboxylic acids with CO2, which is enabled by nickel-catalyzed dynamic kinetic asymmetric reductive carboxylation of racemic aza-biaryl triflates. A variety of important axially chiral carboxylic acids, which are valuable but difficult to obtain via catalysis, are generated in an enantioconvergent version. This new methodology features good functional group tolerance, easy to scale-up, facile transformation and avoids cumbersome steps, handling organometallic reagents and using stoichiometric chiral materials. Mechanistic investigations indicate a dynamic kinetic asymmetric transformation process induced by chiral nickel catalysis.

Visible-light-driven alkene dicarboxylation with formate and CO2 under mild conditions

Low-cost formate salt was used as the reductant and part of the carboxyl source in a visible-light-driven dicarboxylation of diverse alkenes, including simple styrenes. The highly competing hydrocarboxylation side reaction was successfully overridden. Good yields of products were obtained under mild reaction conditions at ambient temperature and pressure of CO2. The dual role of formate salt may stimulate the discovery of a range of new transformations under mild and friendly conditions.

Photoelectrocatalytic Utilization of CO2 : A Big Show of Si-based Photoelectrodes

CO2 is a greenhouse gas that contributes to environmental deterioration; however, it can also be utilized as an abundant C1 resource for the production of valuable chemicals. Solar-driven photoelectrocatalytic (PEC) CO2 utilization represents an advanced technology for the resourcing of CO2 . The key to achieving PEC CO2 utilization lies in high-performance semiconductor photoelectrodes. Si-based photoelectrodes have attracted increasing attention in the field of PEC CO2 utilization due to their suitable band gap (1.1 eV), high carrier mobility, low cost, and abundance on Earth. There are two pathways to PEC CO2 utilization using Si-based photoelectrodes: direct reduction of CO2 into small molecule fuels and chemicals, and fixation of CO2 with organic substrates to generate high-value chemicals. The efficiency and product selectivity of PEC CO2 utilization depends on the structures of the photoelectrodes as well as the composition, morphology, and size of the catalysts. In recent years, significant and influential progress has been made in utilizing Si-based photoelectrodes for PEC CO2 utilization. This review summarizes the latest research achievements in Si-based PEC CO2 utilization, with a particular emphasis on the mechanistic understanding of CO2 reduction and fixation, which will inspire future developments in this field.

Et2 Zn-Mediated Gem-Dicarboxylation of Cyclopropanols with CO2

An unprecedented Et2 Zn-mediated gem-dicarboxylation of C─C/C─H single bond of cyclopropanols with CO2 is disclosed, which provides a straightforward and efficient methodology for the synthesis of a variety of structurally diverse and useful malonic acids in moderate to excellent yields. The protocol features mild reaction conditions, excellent functional group compatibility, broad substrate scope, and facile derivatization of the products. DFT calculations confirm that the transition-metal-free transformation proceeds through a novel ring-opening/α-functionalization/ring-closing/ring-opening/β-functionalization (ROFCOF) process, and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) plays dual important roles in the transformation.

Cu-Catalyzed Asymmetric Dicarboxylation of 1,3-Dienes with CO2

Dicarboxylic acids and derivatives are important building blocks in organic synthesis, biochemistry, and the polymer industry. Although catalytic dicarboxylation with CO2 represents a straightforward and sustainable route to dicarboxylic acids, it is still highly challenging and limited to generation of achiral or racemic dicarboxylic acids. To date, catalytic asymmetric dicarboxylation with CO2 to give chiral dicarboxylic acids has not been reported. Herein, we report the first asymmetric dicarboxylation of 1,3-dienes with CO2 via Cu catalysis. This strategy provides an efficient and environmentally benign route to chiral dicarboxylic acids with high regio-, chemo-, and enantioselectivities. The copper self-relay catalysis, that is, Cu-catalyzed boracarboxylation of 1,3-dienes to give carboxylated allyl boronic ester intermediates and subsequent carboxylation of C-B bonds to give dicarboxylates, is key to the success of this dicarboxylation. Moreover, this protocol exhibits broad substrate scope, good functional group tolerance, easy product derivatizations, and facile synthesis of chiral liquid crystalline polyester and drug-like scaffolds.

Mechanism of photocatalytic CO2 reduction to HCO2H by a robust multifunctional iridium complex

The tetradentate PNNP-type IrIII complex Mes-IrPCY2 ([Cl-IrIII-H]+) is reported to be an efficient catalyst for the reduction of CO2 to formate with excellent selectivity under visible light irradiation. Density functional calculations have been carried out to elucidate the mechanism and the origin of selectivity in the present work. Calculations suggest that the double-reduced complex 1-H (1[IrI-H]0) demonstrates higher activity than the single-reduced complex 2-H (2[IrIII(L˙-)-H]+), possibly owing to the higher hydride donor ability of the former compared to the latter; thus 1-H functions as the active species in the overall CO2 reduction reaction. In the HCOO- formation pathway, the hydride of 1-H performs a nucleophilic attack on CO2via an outer-sphere fashion to generate species 1-OCHO (1[IrI-OCHO]0), which then releases HCOO- to produce an IrI intermediate. A subsequent protonation and chloride coordination of the Ir center leads to the regeneration of catalyst 1[Cl-IrIII-H]+. For the CO production, a nucleophilic attack on CO2 takes place by the Ir atom of 1-Hvia an inner-sphere manner to afford complex O2C-3-H (1[O2C-IrIII-H]0), followed by a two-proton-one-electron reduction to furnish the OC-2-H complex (2[OC-IrIII(L˙-)-H]+) after liberating a H2O. Ultimately, CO is released to form 2-H. The stronger nucleophilicity as well as smaller steric hindrance of the hydride than the Ir atom of the active species 1-H (1[IrI-H]0) is found to account for the favoring of formate formation over CO formation. Meanwhile, the CO2 reduction reaction is calculated to be preferred over the hydrogen evolution reaction, and this is consistent with the experimental product distributions.

Electroreductive Carboxylation of Propargylic Acetates with CO2: Access to Tetrasubstituted 2,3-Allenoates

An electroreductive carboxylation of propargylic alcohols with CO2 and then workup with TMSCHN2 to construct tetrasubstituted 2,3-allenoates is developed. This method allows the incorporation of an external ester group into the resulting allene system through electroreduction, carboxylation, and deacetoxylation cascades. Mechanistically, electricity on/off experiments and cyclic voltammetry analysis support the preferential generation of the CO2 radical anion or the 3-aryl propargylic acetate radical anion based on the electron nature of the aryl rings.

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.

Arylcarboxylation of unactivated alkenes with CO2 via visible-light photoredox catalysis

Photocatalytic carboxylation of alkenes with CO₂ is a promising and sustainable strategy to synthesize high value-added carboxylic acids. However, it is challenging and rarely investigated for unactivated alkenes due to their low reactivities. Herein, we report a visible-light photoredox-catalyzed arylcarboxylation of unactivated alkenes with CO₂, delivering a variety of tetrahydronaphthalen-1-ylacetic acids, indan-1-ylacetic acids, indolin-3-ylacetic acids, chroman-4-ylacetic acids and thiochroman-4-ylacetic acids in moderate-to-good yields. This reaction features high chemo- and regio-selectivities, mild reaction conditions (1 atm, room temperature), broad substrate scope, good functional group compatibility, easy scalability and facile derivatization of products. Mechanistic studies indicate that in situ generation of carbon dioxide radical anion and following radical addition to unactivated alkenes might be involved in the process.

Photocatalytic Carboxylation of C-N Bonds in Cyclic Amines with CO2 by Consecutive Visible-Light-Induced Electron Transfer

Visible-light photocatalytic carboxylation with CO₂ is highly important. However, it still remains challenging for reluctant substrates with low reduction potentials. Herein, we report a novel photocatalytic carboxylation of C-N bonds in cyclic amines with CO₂ via consecutive photo-induced electron transfer (ConPET). It is also the first photocatalytic reductive ring-opening reaction of azetidines, pyrrolidines and piperidines. This strategy is practical to transform a variety of easily available cyclic amines to valuable β-, γ-, δ- and ϵ-amino acids in moderate-to-excellent yields. Moreover, the method also features mild and transition-metal-free conditions, high selectivity, good functional-group tolerance, facile scalability and product derivations. Mechanistic studies indicate that the ConPET might be the key to generating highly reactive photocatalysts, which enable the reductive activation of cyclic amines to generate carbon radicals and carbanions as the key intermediates.

Advancements and Challenges in Reductive Conversion of Carbon Dioxide via Thermo-/Photocatalysis

Carbon dioxide (CO₂) is the major greenhouse gas and also an abundant and renewable carbon resource. Therefore, its chemical conversion and utilization are of great attraction for sustainable development. Especially, reductive conversion of CO₂ with energy input has become a current hotspot due to its ability to access fuels and various important chemicals. Nowadays, the controllable CO₂ hydrogenation to formic acid and alcohols using sustainable H₂ resources has been regarded as an appealing solution to hydrogen storage and CO₂ accumulation. In addition, photocatalytic CO₂ reduction to CO also provides a potential way to utilize this greenhouse gas efficiently. Besides direct CO₂ hydrogenation, CO₂ reductive functionalization integrates CO₂ reduction with subsequent C-X (X = N, S, C, O) bond formation and indirect transformation strategies, enlarging the diverse products derived from CO₂ and promoting CO₂ reductive conversion into a new stage. In this Perspective, the progress and challenges of CO₂ reductive conversion, including hydrogenation, reductive functionalization, photocatalytic reduction, and photocatalytic reductive functionalization are summarized and discussed along with the key issues and future trends/directions in this field. We hope this Perspective can evoke intense interest and inspire much innovation in the promise of CO₂ valorization.

Electroreductive Ring-Opening Carboxylation of Cycloketone Oxime Esters with Carbon Dioxide

Electroreductive ring-opening carboxylation of cycloketone oxime esters with atmospheric carbon dioxide is reported. This reaction proceeded under simple constant current conditions in an undivided cell using glassy carbon as the cathode and magnesium as the sacrificial anode, providing substituted γ- and δ-cyanocarboxylic acids in moderate to good yields. Electrochemically generated cyanoalkyl radicals and cyanoalkyl anion are proposed as the key intermediates.

Photocatalytic CO2 conversion: from C1 products to multi-carbon oxygenates

Photocatalytic CO₂ conversion into high-value chemicals has been emerging as an attractive research direction in achieving carbon resource sustainability. The chemical products can be categorized into C1 and multi-carbon (C₂₊) products. In this review, we describe the recent research progress in photocatalytic CO₂ conversion systems from C1 products to multi-carbon oxygenates, and analyze the reasons related to their catalytic mechanisms, as the production of multi-carbon oxygenates is generally more difficult than that of C1 products. Then we discuss several examples in promoting the photoconversion of CO₂ to value-added multi-carbon products in the aspects of photocatalyst design, mass transfer control, determination of active sites, and intermediate regulation. Finally, we summarize perspectives on the challenges and propose potential directions in this fast-developing field, such as the prospect of CO₂ transformation to long-chain hydrocarbons like salicylic acid or even plastics.

Direct and Selective Electrocarboxylation of Styrene Oxides with CO2 for Accessing β-Hydroxy Acids

Highly selective and direct electroreductive ring-opening carboxylation of epoxides with CO2 in an undivided cell is reported. This reaction shows broad substrate scopes within styrene oxides under mild conditions, providing practical and scalable access to important synthetic intermediate β-hydroxy acids. Mechanistic studies show that CO2 functions not only as a carboxylative reagent in this reaction but also as a promoter to enable efficient and chemoselective transformation of epoxides under additive-free electrochemical conditions. Cathodically generated α-radical and α-carbanion intermediates lead to the regioselective formation of α-carboxylation products.

Visible light-driven carbamoyloxylation of the α-C(sp3)-H bond of arylacetones via radical-initiated hydrogen atom transfer

Photocatalytic synthesis has emerged as an efficient route to transform CO₂ into functionalized organic carbamates by photocatalysis. Herein, a catalyst-free carbamoyloxylation of arylacetones with CO₂ and amines under visible light was developed for the synthesis of O-β-oxoalkyl carbamates in yields up to 93%. This protocol proceeded smoothly with the assistance of inexpensive carbon tetrabromide at room temperature under atmospheric CO₂ pressure, leading to simultaneous construction of C-O and C-N bonds. Mechanism studies suggested the photoinduced hydrogen atom transfer (HAT) pathway followed by radical addition or single electron transfer (SET).

Electrochemical Ring-Opening Dicarboxylation of Strained Carbon-Carbon Single Bonds with CO2: Facile Synthesis of Diacids and Derivatization into Polyesters

Diacids are important monomers in the polymer industry to construct valuable materials. Dicarboxylation of unsaturated bonds, such as alkenes and alkynes, with CO₂ has been demonstrated as a promising synthetic method. However, dicarboxylation of C─C single bonds with CO₂ has rarely been investigated. Herein we report a novel electrochemical ring-opening dicarboxylation of C─C single bonds in strained rings with CO₂. Structurally diverse glutaric acid and adipic acid derivatives were synthesized from substituted cyclopropanes and cyclobutanes in moderate to high yields. In contrast to oxidative ring openings, this is also the first realization of an electroreductive ring-opening reaction of strained rings, including commercialized ones. Control experiments suggested that radical anions and carbanions might be the key intermediates in this reaction. Moreover, this process features high step and atom economy, mild reaction conditions (1 atm, room temperature), good chemoselectivity and functional group tolerance, low electrolyte concentration, and easy derivatization of the products. Furthermore, we conducted polymerization of the corresponding diesters with diols to obtain a potential UV-shielding material with a self-healing function and a fluorine-containing polyester, whose performance tests showed promising applications.

Technological Innovations in Photochemistry for Organic Synthesis: Flow Chemistry, High-Throughput Experimentation, Scale-up, and Photoelectrochemistry

Photoinduced chemical transformations have received in recent years a tremendous amount of attention, providing a plethora of opportunities to synthetic organic chemists. However, performing a photochemical transformation can be quite a challenge because of various issues related to the delivery of photons. These challenges have barred the widespread adoption of photochemical steps in the chemical industry. However, in the past decade, several technological innovations have led to more reproducible, selective, and scalable photoinduced reactions. Herein, we provide a comprehensive overview of these exciting technological advances, including flow chemistry, high-throughput experimentation, reactor design and scale-up, and the combination of photo- and electro-chemistry.

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.

Photocatalytic synthesis of tetra-substituted furans promoted by carbon dioxide

A CO2-promoted transition metal-free photocatalytic synthesis of tetra-substituted furan derivatives from 1,3-diketones as the only starting material.

Green Carbon Science: Efficient Carbon Resource Processing, Utilization, and Recycling Towards Carbon Neutrality

Green carbon science is defined as "Study and optimization of the transformation of carbon containing compounds and the relevant processes involved in the entire carbon cycle from carbon resource processing, carbon energy utilization, and carbon recycling to use carbon resources efficiently and minimize the net CO2 emission." [1] Green carbon science is related closely to carbon neutrality, and the relevant fields have developed quickly in the last decade. In this Minireview, we proposed the concept of carbon energy index, and the recent progresses in petroleum refining, production of liquid fuels, chemicals, and materials using coal, methane, CO2, biomass, and waste plastics are highlighted in combination with green carbon science, and an outlook for these important fields is provided in the final section.

Basicity as a Thermodynamic Descriptor of Carbanions Reactivity with Carbon Dioxide: Application to the Carboxylation of α,β-Unsaturated Ketones

The utilization of carbon dioxide as a raw material represents nowadays an appealing strategy in the renewable energy, organic synthesis, and green chemistry fields. Besides reduction strategies, carbon dioxide can be exploited as a single-carbon-atom building block through its fixation into organic scaffolds with the formation of new C-C bonds (carboxylation processes). In this case, activation of the organic substrate is commonly required, upon formation of a carbanion C^-, being sufficiently reactive toward the addition of CO₂. However, the prediction of the reactivity of C^- with CO₂ is often problematic with the process being possibly associated with unfavorable thermodynamics. In this contribution, we present a thermodynamic analysis combined with density functional theory calculations on 50 organic molecules enabling the achievement of a linear correlation of the standard free energy (ΔG⁰) of the carboxylation reaction with the basicity of the carbanion C^-, expressed as the pK_a of the CH/C^- couple. The analysis identifies a threshold pK_a of ca 36 (in CH₃CN) for the CH/C^- couple, above which the ΔG⁰ of the carboxylation reaction is negative and indicative of a favorable process. We then apply the model to a real case involving electrochemical carboxylation of flavone and chalcone as model compounds of α,β-unsaturated ketones. Carboxylation occurs in the β-position from the doubly reduced dianion intermediates of flavone and chalcone (calculated ΔG⁰ of carboxylation in β = -12.8 and -20.0 Kcalmol^-1 for flavone and chalcone, respectively, associated with pK_a values for the conjugate acids of 50.6 and 51.8, respectively). Conversely, the one-electron reduced radical anions are not reactive toward carboxylation (ΔG⁰ > +20 Kcalmol^-1 for both substrates, in either α or β position, consistent with pK_a of the conjugate acids < 18.5). For all the possible intermediates, the plot of calculated ΔG⁰ of carboxylation vs. pK_a is consistent with the linear correlation model developed. The application of the ΔG⁰ vs. pK_a correlation is finally discussed for alternative reaction mechanisms and for carboxylation of other C=C and C=O double bonds. These results offer a new mechanistic tool for the interpretation of the reactivity of CO₂ with organic intermediates.

Nickel-catalyzed electrochemical carboxylation of unactivated aryl and alkyl halides with CO2

Electrochemical catalytic reductive cross couplings are powerful and sustainable methods to construct C-C bonds by using electron as the clean reductant. However, activated substrates are used in most cases. Herein, we report a general and practical electro-reductive Ni-catalytic system, realizing the electrocatalytic carboxylation of unactivated aryl chlorides and alkyl bromides with CO₂. A variety of unactivated aryl bromides, iodides and sulfonates can also undergo such a reaction smoothly. Notably, we also realize the catalytic electrochemical carboxylation of aryl (pseudo)halides with CO₂ avoiding the use of sacrificial electrodes. Moreover, this sustainable and economic strategy with electron as the clean reductant features mild conditions, inexpensive catalyst, safe and cheap electrodes, good functional group tolerance and broad substrate scope. Mechanistic investigations indicate that the reaction might proceed via oxidative addition of aryl halides to Ni(0) complex, the reduction of aryl-Ni(II) adduct to the Ni(I) species and following carboxylation with CO₂.

Stereodivergent synthesis of β-iodoenol carbamates with CO2 via photocatalysis

Photocatalytic conversion of carbon dioxide (CO2) into value-added chemicals is of great significance from the viewpoint of green chemistry and sustainable development. Here, we report a stereodivergent synthesis of β-iodoenol carbamates through a photocatalytic three-component coupling of ethynylbenziodoxolones, CO2 and amines. By choosing appropriate photocatalysts, both Z- and E-isomers of β-iodoenol carbamates, which are difficult to prepare using existing methods, can be obtained stereoselectively. This transformation featured mild conditions, excellent functional group compatibility and broad substrate scope. The potential synthetic utility of this protocol was demonstrated by late-stage modification of bioactive molecules and pharmaceuticals as well as by elaborating the products to access a wide range of valuable compounds. More importantly, this strategy could provide a general and practical method for stereodivergent construction of trisubstituted alkenes such as triarylalkenes, which represents a fascinating challenge in the field of organic chemistry research. A series of mechanism investigations revealed that the transformation might proceed through a charge-transfer complex which might be formed through a halogen bond.

Desulfonylative Electrocarboxylation with Carbon Dioxide

Electrocarboxylation of organic halides is one of the most investigated electrochemical approaches for converting thermodynamically inert carbon dioxide (CO₂) into value-added carboxylic acids. By converting organic halides into their sulfone derivatives, we have developed a highly efficient electrochemical desulfonylative carboxylation protocol. Such a strategy takes advantage of CO₂ as the abundant C1 building block for the facile preparation of multifunctionalized carboxylic acids, including the nonsteroidal anti-inflammatory drug ibuprofen, under mild reaction conditions.

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.