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

Taming CO2•- via Synergistic Triple Catalysis in Anti-Markovnikov Hydrocarboxylation of Alkenes

The direct utilization of carbon dioxide as an ideal one-carbon source in value-added chemical synthesis has garnered significant attention from the standpoint of global sustainability. In this regard, the photo/electrochemical reduction of CO2 into useful fuels and chemical feedstocks could offer a great promise for the transition to a carbon-neutral economy. However, challenges in product selectivity continue to limit the practical application of these systems. A robust and general method for the conversion of CO2 to the polarity-reversed carbon dioxide radical anion, a C1 synthon, is critical for the successful valorization of CO2 to selective carboxylation reactions. We demonstrate herein a hydride and hydrogen atom transfer synergy driven general catalytic platform involving CO2•- for highly selective anti-Markovnikov hydrocarboxylation of alkenes via triple photoredox, hydride, and hydrogen atom transfer catalysis. Mechanistic studies suggest that the synergistic operation of the triple catalytic cycle ensures a low-steady-state concentration of CO2•- in the reaction medium. This method using a renewable light energy source is mild, robust, selective, and capable of accommodating a wide range of activated and unactivated alkenes. The highly selective nature of the transformation has been revealed through the synthesis of hydrocarboxylic acids from the substrates bearing a hydrogen atom available for intramolecular 1,n-HAT process as well as diastereoselective synthesis. This technology represents a general strategy for the merger of in situ formate generation with a synergistic photoredox and HAA catalytic cycle to provide CO2•- for selective chemical transformations.

Direct Excitation Strategy for Deacylative Couplings of Ketones

The homolysis of chemical bonds represents one of the most fundamental reactivities of excited molecules. Historically, it has been exploited to generate radicals under ultraviolet (UV) light irradiation. However, unlike most contemporary radical-generating mechanisms, the direct excitation to homolyze chemical bonds and produce aliphatic carbon-centered radicals under visible light remains rare, especially in metallaphotoredox cross couplings. Herein, we present our design of the dihydropyrimidoquinolinone (DHPQ) reagents derived from ketones, which can undergo formal deacylation and homolytic C-C bond cleavage to release alkyl radicals without external photocatalysts. Spectroscopic and computational analysis reveal unique optical and structural features of DHPQs, rationalizing their faster kinetics in alkyl radical generation than a structurally similar but visible-light transparent radical precursor. Such a capability allows DHPQ to facilitate a wide range of Ni-metallaphotoredox cross couplings with aryl, alkynyl and acyl halides. Other catalytic and non-catalyzed alkylative transformations of DHPQs are also feasible with various radical acceptors. We believe this work would be of broad interest, aiding the synthetic planning with simplified operation and expanding the synthetic reach of photocatalyst-free approaches in cutting-edge research.

Visible-Light Photoredox-Catalyzed Direct Carboxylation of Tertiary C(sp3)-H Bonds with CO2: Facile Synthesis of All-Carbon Quaternary Carboxylic Acids

Direct carboxylation of C-H bonds with CO2 represents an attractive strategy to synthesize valuable carboxylic acids with high atom, step, and redox economy. Although great progress has been achieved in this field, catalytic carboxylation of tertiary C(sp3)-H bonds still remains challenging due to their inherent inertness and significant steric hindrance. Herein, we report a direct carboxylation of tertiary benzylic C(sp3)-H bonds with CO2 via visible-light photoredox catalysis. Various all-carbon quaternary carboxylic acids, which are of significant importance in medicinal chemistry, are successfully obtained with high yields. This direct carboxylation is characterized by good functional group tolerance, broad substrate scope, and mild operational conditions. Furthermore, our methodology enables the efficient and rapid synthesis of key drug or bioactive molecules, such as carbetapentane, caramiphen, and PRE-084 (σ1 receptor agonist), and facilitates various functionalizations of C(sp2)-H bonds using the directing ability of target carboxylic acids, thus highlighting its practical applications. Mechanistic studies indicate that a carbanion, which serves as the key intermediate to react with CO2, is catalytically generated via a single electron reduction of a benzylic radical through a consecutive photoinduced electron transfer process.

Cross-Coupling of Gaseous Alkanes with (Hetero)Aryl Bromides via Dual Nickel/Photoredox Catalysis

Gaseous alkanes represent the most abundant carbon-based chemical feedstocks in our planet. However, the intrinsic inertness of their C-H bonds has rendered the use of these alkanes very difficult for purposes beyond aerobic combustion and energy intensive processes. Thus, clean and energy-efficient transformations for their use in synthetic organic chemistry are still rare. Here we report a catalytic methodology for the direct cross-coupling of gaseous alkanes with (hetero)aryl bromides through the combination of metallaphotoredox-mediated hydrogen atom transfer and nickel catalysis. This protocol provides an efficient platform for the addition of short alkyl groups into diverse (hetero) aromatic rings, providing a wide range of high-value alkyl(hetero)arenes, and bypassing the longstanding need of using preactivated alkylating agents in C(sp2)-C(sp3) cross-couplings. The method features high chemoselectivity, regioselectivity and a remarkable functional group tolerance, operates under mild conditions, and exhibits operational simplicity.

Recent Advances in Electrochemical Carboxylation with CO2

ConspectusCarbon dioxide (CO2) is recognized as a greenhouse gas and a common waste product. Simultaneously, it serves as an advantageous and commercially available C1 building block to generate valuable chemicals. Particularly, carboxylation with CO2 is considered a significant method for the direct and sustainable production of important carboxylic acids. However, the utilization of CO2 is challenging owing to its thermodynamic stability and kinetic inertness. Recently, organic electrosynthesis has emerged as a promising approach that utilizes electrons or holes as environmentally friendly redox reagents to produce reactive intermediates in a controlled and selective manner. This technique holds great potential for the CO2 utilization.Since 2015, our group has been dedicated to exploring the utilization of CO2 in organic synthesis with a particular focus on electrochemical carboxylation. Despite the significant advancements made in this area, there are still many challenges, including the activation of inert substrates, regulation of selectivity, diversity in electrolysis modes, and activation strategies. Over the past 7 years, our team, with many great experts, has presented findings on electrochemical carboxylation with CO2 under mild conditions. In this context, we primarily highlight our contributions to selective electrocarboxylations, encompassing new reaction systems, selectivity control methods, and activation approaches.We commenced our research by establishing a Ni-catalyzed electrochemical carboxylation of unactivated aryl halides and alkyl bromides in conjunction with a useful paired anodic reaction. This approach eliminates the need for sacrificial anodes, rendering the carboxylation process sustainable. To further utilize the widely existing yet cost-effective alkyl chlorides, we have developed a deep electroreductive system to achieve carboxylation of unactivated alkyl chlorides and poly(vinyl chloride), allowing the direct modification and upgrading of waste polymers.Through precise adjustment of the electroreductive conditions, we successfully demonstrated the dicarboxylation of both strained carbocycles and acyclic polyarylethanes with CO2 via C-C bond cleavage. Furthermore, we have realized the dicarboxylative cyclization of unactivated skipped dienes to produce the valuable ring-tethered adipic acids through single-electron reduction of CO2 to the CO2 radical anion (CO2•-). In terms of the asymmetric carboxylation, Guo's and our groups have recently achieved the nickel-catalyzed enantioselective electroreductive carboxylation reaction using racemic propargylic carbonates and CO2, paving the way for the synthesis of enantioenriched propargylic carboxylic acids.In addition to the aforementioned advancements, Lin's and our groups have also developed new electrolysis modes to achieve regiodivergent C-H carboxylation of N-heteroarenes dictated by electrochemical reactors. The choice of reactors plays a crucial role in determining whether the hydrogen atom transfer (HAT) reagents are formed anodically, consequently influencing the carboxylation pathways of N-heteroarene radical anions in the distinct electrolyzed environments.

Recent Progress on Multi-Component Reactions Involving Nucleophile, Arynes and CO2

Carbon dioxide (CO2) is a non-toxic, abundant and recoverable source of carbon monoxide. Despite its thermodynamically stable and kinetically inert nature, research on CO2 utilisation is ongoing. CO2-based aryne reactions, crucial for synthesising ortho-substituted benzoic acids and their cyclisation products, have garnered significant attention, and multi-component reactions (MCRs) involving CO2, aryne and nucleophilic reagents have been extensively studied. This review highlights recent advancements in CO2 capture reactions utilising phenylalkyne reactive intermediates. Mechanistic insights into these reactions are provided together with prospects for further development in this field.

Macrocyclization of carbon dioxide with 3-triflyloxybenzynes and tetrahydrofuran: straightforward access to 14-membered macrolactones

A novel [2+2+5+5] macrocyclization of carbon dioxide with 3-triflyloxybenzynes and tetrahydrofuran has been disclosed for the first time under transition metal-free conditions. The reaction provides a facile method for the synthesis of a rare type of 14-membered macrocyclic lactone, which is potentially useful but difficult to access by existing methods.

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.

Silver-Catalyzed Coupling of Ethynylbenziodoxolones with CO2 and Amines to Afford O-β-Oxoalkyl Carbamates

A novel three-component coupling reaction of ethynylbenziodoxolones (EBXs) with CO2 and amines has been achieved via silver catalysis, thereby providing an efficient method for the construction of a range of structurally diverse and valuable O-β-oxoalkyl carbamates. The transformation proceeds under mild reaction conditions and exhibits a wide substrate scope and good functional group compatibility. In addition, this strategy could be extended to the synthesis of α-acyloxyketones using carboxylic acids as the nucleophiles to react with EBXs.

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.

Antenna Effect in Noble Metal-Free Dye-Sensitized Photocatalytic Systems Enhances CO2 -to-CO Conversion

Dye-sensitized photocatalytic systems (DSPs) have been extensively investigated for solar-driven hydrogen (H2 ) evolution. However, their application in carbon dioxide (CO2 ) reduction remains limited. Furthermore, current solar-driven CO2 -to-CO DSPs typically employ rhenium complexes as catalysts. In this study, we have developed DSPs that incorporate noble metal-free components, specifically a zinc-porphyrin as photosensitizer (PS) and a cobalt-quaterpyridine as catalyst (CAT). Taking a significant stride forward, we have achieved an antenna effect for the first time in CO2 -to-CO DSPs by introducing a Bodipy as an additional chromophore to enhance light harvesting efficiency. The energy transfer from Bodipy to zinc porphyrin resulted in remarkable stability (turn over number (TON)=759 vs. CAT), and high CO evolution activity (42 mmol g-1  h-1 vs. CAT).

Visible Light-Induced One-Pot Carbonylation of Alkyl Halides with Aryl Formates

Described herein is the development of a visible-light-driven carbonylation of alkyl halides. The exploitation of visible light to activate Pd complexes and the use of formates to serve the dual role of a CO surrogate and a phenoxide source allow the preparation of esters in moderate to good yields. Its relatively mild reaction conditions and the ability to perform this transformation without direct handling of toxic CO gas provide a practical means to access esters from alkyl halides.

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.

Reductive Radical-Polar Crossover Enabled Carboxylative Alkylation of Aryl Thianthrenium Salts with CO2 and Styrenes

Carboxylic functionalities are among the pivotal groups in bioactive molecules and in the synthesis of new lead compounds because of their unique character in the formation of hydrogen bonds and the possibility of constructing molecular complexes via amide couplings. We adopt the reductive radical-polar crossover strategy to introduce carboxyalkyl groups into arenes with styrenes and CO2 via thianthrenium salts. This protocol exhibits excellent potential as a straightforward and modular platform for site-selective carboxylative derivation of bioactive molecules.

Divergent and Selective Light Alkene Cross-Coupling

Light olefins are abundantly manufactured in the petroleum industry and thus represent ideal starting materials for modern chemical synthesis. Selective and divergent transformations of feedstock light olefins to value-added chemicals are highly sought-after but remain challenging. Herein we report an exceptionally regioselective carbonickelation of light alkenes followed by in situ trapping with three types of nucleophiles, namely a reductant, base, or Grignard reagent. This protocol enables efficient 1,2-hydrofunctionalization, dicarbofunctionalization, and branched-selective Heck-type cross-coupling of light alkenes with aryl and alkenyl reagents to streamline access to diverse alkyl arenes and complex alkenes. Harnessing bulky N-heterocyclic carbene ligands with acenaphthyl backbones for nickel catalysts is crucial to attain high reactivity and selectivity. This strategy provides a rare, modular, and divergent platform for upgrading feedstock alkenes and is expected to find broad applications in medicinal chemistry and industrial processes.

Photoredox-Catalyzed Phosphonocarboxylation of Allenes with Phosphine Oxides and CO2

Phosphonocarboxylation of allenes with diarylphosphine oxides and CO2 via visible-light photoredox catalysis was developed for the first time. This work provided practical and sustainable access to highly valuable but otherwise difficult-to-access linear allylic β-phosphonyl carboxylic acids in moderate yields with exclusive regio- and stereoselectivity. This method was also characterized by step and atom economy and transition-metal free and mild conditions. Preliminary mechanistic studies suggested that allyl-methyl carbanion species are the key intermediates.

Asymmetric construction of phosphono dihydropyranones from α-ketophosphonates enabled by Pd/chiral isothiourea relay catalysis

A highly enantio- and diastereoselective approach has been developed for the synthesis of chiral phosphono dihydropyranones. This approach is enabled by Pd/chiral isothiourea relay catalysis under mild reaction conditions, starting from readily available benzyl bromides, CO, and α-ketophosphonates. The cascade reaction involves the generation of a ketene intermediate from Pd-catalyzed carbonylation of benzyl bromide and subsequent chiral Lewis base catalyzed formal [4 + 2] reaction. Phosphono lactone products can also be transformed to chiral 1,5-diester products in good yield and high stereoselectivity.

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.

Dual Nickel Photocatalysis for O-Aryl Carbamate Synthesis from Carbon Dioxide

We report the use of dual nickel photocatalysis in the synthesis of O-aryl carbamates from aryl iodides or bromides, amines, and carbon dioxide. The reaction proceeded in visible light, at ambient carbon dioxide pressure, and without stoichiometric activating reagents. Mechanistic analysis is consistent with a Ni(I-III) cycle, where the active species is generated by the photocatalyst. The rate-limiting steps were the photocatalyst-mediated reduction of Ni(II) to Ni(I) and subsequent oxidative addition of the aryl halide. The physical properties of the photocatalyst were critical for promoting formation of O-aryl carbamates over various byproducts. Nine new phthalonitrile photocatalysts were synthesized, which exhibited properties that were vital to achieve high selectivity and activity.

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.

A Proton-Coupled Electron Transfer Strategy to the Redox-Neutral Photocatalytic CO2 Fixation

Herein, we report our study on the design and development of a novel photocarboxylation method. We have used an organic photoredox catalyst (PC, 4CzIPN) and differently substituted dihydropyridines (DHPs) in combination with an organic base (1,5,7-triazabicyclodec-5-ene, TBD) to access a proton-coupled electron transfer (PCET) based manifold. In depth mechanistic investigations merging experimental analysis (NMR, IR, cyclic voltammetry) and density-functional theory (DFT) calculations reveal the key activity of a H-bonding complex between the DHP and the base. The thermodynamic and kinetic benefits of the PCET mechanism allowed the implementation of a redox-neutral fixation process leading to synthetically relevant carboxylic acids (18 examples with isolated yields up to 75%) under very mild reaction conditions. Finally, diverse product manipulations were performed to demonstrate the synthetic versatility of the obtained products.

Photocatalytic Late-Stage C-H Functionalization

The emergence of modern photocatalysis, characterized by mildness and selectivity, has significantly spurred innovative late-stage C-H functionalization approaches that make use of low energy photons as a controllable energy source. Compared to traditional late-stage functionalization strategies, photocatalysis paves the way toward complementary and/or previously unattainable regio- and chemoselectivities. Merging the compelling benefits of photocatalysis with the late-stage functionalization workflow offers a potentially unmatched arsenal to tackle drug development campaigns and beyond. This Review highlights the photocatalytic late-stage C-H functionalization strategies of small-molecule drugs, agrochemicals, and natural products, classified according to the targeted C-H bond and the newly formed one. Emphasis is devoted to identifying, describing, and comparing the main mechanistic scenarios. The Review draws a critical comparison between established ionic chemistry and photocatalyzed radical-based manifolds. The Review aims to establish the current state-of-the-art and illustrate the key unsolved challenges to be addressed in the future. The authors aim to introduce the general readership to the main approaches toward photocatalytic late-stage C-H functionalization, and specialist practitioners to the critical evaluation of the current methodologies, potential for improvement, and future uncharted directions.

Recent advancements in visible-light-driven carboxylation with carbon dioxide

Carbon dioxide as a classic C1 source has long been investigated in organic synthetic chemistry. Diverse catalytic methods for CO2 activation were reported in the past several decades. In this...

K2S2O8-mediated acylarylation of unactivated alkenes via acyl radical addition/C–H annulation cascade of N-allyl-indoles with silver cocatalysis

A silver-catalyzed, K2S2O8-mediated protocol to access the regioselective acylarylation of unactivated alkenes was reported.

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.

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.

Photoredox/nickel-catalyzed hydroacylation of ethylene with aromatic acids

We report a general, practical and scalable hydroacylation reaction of ethylene with aromatic carboxylic acids with the synergistic combination of nickel and photoredox catalysis. Under ambient temperature and pressure, feedstock chemicals such as ethylene can be converted into high-value-added aromatic ketones in moderate to good yields (up to 92%) with reaction time of 2-6 hours.

Visible-light photoredox-catalyzed umpolung carboxylation of carbonyl compounds with CO2

Photoredox-mediated umpolung strategy provides an alternative pattern for functionalization of carbonyl compounds. However, general approaches towards carboxylation of carbonyl compounds with CO₂ remain scarce. Herein, we report a strategy for visible-light photoredox-catalyzed umpolung carboxylation of diverse carbonyl compounds with CO₂ by using Lewis acidic chlorosilanes as activating/protecting groups. This strategy is general and practical to generate valuable α-hydroxycarboxylic acids. It works well for challenging alkyl aryl ketones and aryl aldehydes, as well as for α-ketoamides and α-ketoesters, the latter two of which have never been successfully applied in umpolung carboxylations with CO₂ (to the best of our knowledge). This reaction features high selectivity, broad substrate scope, good functional group tolerance, mild reaction conditions and facile derivations of products to bioactive compounds, including oxypheonium, mepenzolate bromide, benactyzine, and tiotropium. Moreover, the formation of carbon radicals and carbanions as well as the key role of chlorosilanes are supported by control experiments.

Compounds bearing a carbonyl group, such as aldehydes and ketones, are important industrial chemicals and widespread in pharmaceuticals and natural products. Here, the authors report a strategy for visible-light photoredox-catalyzed umpolung carboxylation of diverse carbonyl compounds with CO₂, to generate valuable α-hydroxycarboxylic acids.

Radical Carboxylative Cyclizations and Carboxylations with CO2

Carbon dioxide (CO₂) is not only a greenhouse gas and a common waste product but also an inexpensive, readily available, and renewable carbon resource. It is an important one-carbon (C1) building block in organic synthesis for the construction of valuable compounds. However, its utilization is challenging owing to its thermodynamic stability and kinetic inertness. Although significant progress has been achieved, many limitations remain in this field with regard to the substrate scope, reaction system, and activation strategies.Since 2015, our group has focused on CO₂ utilization in organic synthesis. We are also interested in the vast possibilities of radical chemistry, although the high reactivity of radicals presents challenges in controlling selectivity. We hope to develop highly useful CO₂ transformations involving radicals by achieving a balance of reactivity and selectivity under mild reaction conditions. Over the past 6 years, we along with other experts have disclosed radical-type carboxylative cyclizations and carboxylations using CO₂.We initiated our research by realizing the Cu-catalyzed radical-type oxytrifluoromethylation of allylamines and heteroaryl methylamines to generate valuable 2-oxazolidones with various radical precursors. Apart from Cu catalysis, visible-light photoredox catalysis is also a powerful method to achieve efficient carboxylative cyclization. In these cases, single-electron-oxidation-promoted C-O bond formation between benzylic radicals and carbamates is the key step.Since carboxylic acids exist widely in natural products and bioactive drugs and serve as important bulk chemicals in industry, we realized further visible-light-promoted carboxylations with CO₂ to construct such chemicals. We have achieved the selective umpolung carboxylations of imines, enamides, tetraalkylammonium salts, and oxime esters by successive single-electron-transfer (SSET) reduction. Using this strategy, we have also realized the dearomative arylcarboxylation of indoles with CO₂. In addition to the incorporation of 1 equiv of CO₂ per substrate, we have recently developed a visible-light photoredox-catalyzed dicarboxylation of alkenes, allenes, and (hetero)arenes via SSET reduction, which allows the incorporation of two CO₂ molecules into organic compounds to generate valuable diacids as polymer precursors.In addition to the two-electron activation of CO₂, we sought to develop new strategies to realize efficient and selective transformations via single-electron activation of CO₂. Inspired by the hypothetical electron-transfer mechanism of iron-sulfur proteins, we have realized the visible-light-driven thiocarboxylation of alkenes with CO₂ using catalytic iron salts as promoters. The in-situ-generated Fe/S complexes are likely able to reduce CO₂ to its radical anion, which could react with alkenes to give a stabilized carbon radical. Moreover, we have also disclosed charge-transfer complex (CTC) formation between thiolate and acrylate/styrene to realize the visible-light-driven hydrocarboxylation of alkenes with CO₂ via generation of a CO₂ or alkene radical anion. On the basis of this novel CTC, the visible-light-driven organocatalytic hydrocarboxylation of alkenes with CO₂ has also been realized using a Hantzsch ester as an effective reductant.

A highly selective decarboxylative deuteration of carboxylic acids

In this paper, we report a mild and practical method for precise deuteration of aliphatic carboxylic acids by synergistic photoredox and HAT catalysis. The reaction delivers excellent D-incorporation (up to 99%) at predicted sites even in substrates bearing reactive C-H bonds or versatile functional groups. The use of a recirculation reactor with a peristaltic pump supports a scalable preparative ability (up to 50 mmol) under very mild reaction conditions. The practical and precise deuteration of readily available complex carboxylic acids makes this protocol promising for the preparation of deuterium-labelled compounds.