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

Efficient palladium-catalyzed electrocarboxylation enables late-stage carbon isotope labelling

Carbon isotope labelling of bioactive molecules is essential for accessing the pharmacokinetic and pharmacodynamic properties of new drug entities. Aryl carboxylic acids represent an important class of structural motifs ubiquitous in pharmaceutically active molecules and are ideal targets for the installation of a radioactive tag employing isotopically labelled CO2. However, direct isotope incorporation via the reported catalytic reductive carboxylation (CRC) of aryl electrophiles relies on excess CO2, which is incompatible with carbon-14 isotope incorporation. Furthermore, the application of some CRC reactions for late-stage carboxylation is limited because of the low tolerance of molecular complexity by the catalysts. Herein, we report the development of a practical and affordable Pd-catalysed electrocarboxylation setup. This approach enables the use of near-stoichiometric 14CO2 generated from the primary carbon-14 source Ba14CO3, facilitating late-stage and single-step carbon-14 labelling of pharmaceuticals and representative precursors. The proposed isotope-labelling protocol holds significant promise for immediate impact on drug development programmes.

Visible-light-driven photocatalytic carboxylation to aromatic carboxylic acids with CO2

(Hetero)aromatic carboxylic acids and their derivatives attract attention due to their role in the synthesis of several biologically active molecules, active pharmaceutical ingredients, polymers, etc. Carbon dioxide (CO2) is a prime C1 source for the synthesis of aromatic carboxylic acids because of its nontoxicity, nonflammability, abundance and renewability. Owing to the thermodynamic and chemical inertness of CO2, traditional carboxylation to aromatic carboxylic acids with CO2 is always performed under harsh reaction conditions or using stoichiometric metallic reductants. Visible-light-driven carboxylation with CO2 provides an environmentally benign, mild, and high-efficiency route for the production of aromatic carboxylic acids. This review comprehensively introduces the visible-light-driven preparation of aromatic carboxylic acids through a visible-light-driven oxidative addition and reductive elimination mechanism, binding of aryl (radical) anions which are produced by photoinduced electron transfer (PET) to CO2, binding of carbon dioxide anion radicals (CO2˙-) which are formed by PET to aryl compounds, radical coupling between CO2˙- and aryl radicals, and other mechanisms. Finally, this review provides a summary and the future work direction. This article offers a theoretical guidance for efficient synthesis of aromatic carboxylic acids via photocatalysis.

Diethylzinc-promoted carboxylation of aryl/alkenyl boronic acids with CO2

The carboxylation of aryl and alkenyl boronic acids with CO2 is rarely studied and only achieved using copper salts as the catalyst in the presence of a strong base. Herein, we report a diethylzinc-promoted carboxylation of aryl or alkenyl boronic acids with carbon dioxide. The reaction does not require a transition-metal catalyst, and has simple and mild conditions and a broad substrate scope.

Recent progress in reductive carboxylation of C-O bonds with CO2

Transformation of carbon dioxide (CO2) into value-added organic compounds has attracted increasing interest of scientific community in the last few decades, not only because CO2 is the primary greenhouse gas that drives global climate change and ocean acidification, but also because it has been regarded as a plentiful, nontoxic, nonflammable and renewable one-carbon (C1) feedstock. Among the various CO2-conversion processes, carboxylation reactions represent one of the most beautiful and attractive research topics in the field, since it offers the possibility for the construction of synthetically and biologically important carboxylic acids from various easily accessible (pseudo)halides, organosilicon, and organoboron compounds. The purpose of this review is to summarize the available literature on deoxygenative carboxylation of alcohols and their derivatives utilizing CO2 as a carboxylative reagent. Depending on the C-O compounds employed, the paper is divided into five major sections. The direct dehydroxylative carboxylation of free alcohols is discussed first. This is followed by reductive carboxylation of carboxylates, triflates, and tosylates. In the final section, the only reported example on catalytic carboxylation of fluorosulfates will be covered. Notably, special attention has been paid on the mechanistic aspects of the reactions that may provide new insights into catalyst improvement and development, which currently mainly relies on the use of transition metal catalysts.

Palladium-Catalyzed Carbonylation of Aryl Bromides with Carbon Dioxide To Access Aryl Carboxylic Acids under Mild Conditions

A palladium-catalyzed direct carbonylation of aryl bromides with carbon dioxide as the carbonyl source has been developed by using Pd(dba)₂/DPEPhos as the catalyst under mild reaction conditions, providing an efficient route to a variety of aryl carboxylic acids in moderate to high yields. The methods have many advantages such as the use of a simple palladium catalyst system, wide substrate scope, good functional group tolerance, high yields, and easy scalability.

Protocol for chemo- and regioselective C(sp3)-H activation using a heterogeneous copper powder-catalyzed reaction

Here, we present a protocol for the synthesis of dibenzo[c,e]oxepin-5(7H)-ones starting from 2'-alkyl-[1,1'-biphenyl]-2-carboxylic acids. This technique uses two copper(0)-catalyzed benzylic C(sp³)-H activation strategies taking either di-tertbutyl peroxide or gaseous oxygen as an oxidant. We detail a photocatalytic thermal approach for copper powder-catalyzed reaction with oxygen. We also describe a procedure for catalyst recycling in both the strategies. The product has been successfully synthesized both in mmol and gram scale. For complete details on the use and execution of this protocol, please refer to Nandi et al. (2022).

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.

Chemo- and regioselective benzylic C(sp3)–H oxidation bridging the gap between hetero- and homogeneous copper catalysis

Selective C‒H functionalization in a pool of proximal C‒H bonds, predictably altering their innate reactivity is a daunting challenge. We disclose here, an expedient synthesis of privileged seven-membered lactones, dibenzo[c,e]oxepin-5(7H)-one through a highly chemoselective benzylic C(sp³)‒H activation. Remarkably, the formation of widely explored six-membered lactone via C(sp²)‒H activation is suppressed under the present conditions. The reaction proceeds smoothly on use of inexpensive metallic copper catalyst and di-tert-butyl peroxide (DTBP). Owing to the hazards of stoichiometric DTBP, further, we have developed a sustainable metallic copper/rose bengal dual catalytic system coupled with molecular oxygen replacing DTBP. A 1,5-aryl migration through Smiles rearrangement was realized from the corresponding diaryl ether substrates instead of expected eight-membered lactones. The present methodology is scalable, applied to the total synthesis of cytotoxic and neuroprotective natural product alterlactone. The catalyst is recyclable and the reaction can be performed in a copper bottle without any added catalyst.

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Catalytic strategy for chemo- and regioselective benzylic C–H activation

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Bulk copper catalysis merging with photocatalysis

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Reusable copper catalyst

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Reaction demonstrated in commercial copper bottle without external catalyst

Visible-Light-Enabled Carboxylation of Benzyl Alcohol Derivatives with CO2 Using a Palladium/Iridium Dual Catalyst

A highly efficient carboxylation of benzyl alcohol derivatives with CO₂ using a palladium/iridium dual catalyst under visible-light irradiation was developed. A wide range of benzyl alcohol derivatives could be employed to provide benzylic carboxylic acids in moderate to high yields. Mechanistic studies indicated that the oxidative addition of benzyl alcohol derivatives was possibly the rate-determining-step. It was also found that a switchable site-selective carboxylation between benzylic C-O and aryl C-Cl moieties could be achieved simply by changing the palladium catalyst.

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...

Advances in Palladium-Catalyzed Carboxylation Reactions

In this short review, we highlight the advancements in the field of palladium-catalyzed carbon dioxide utilization for the synthesis of high value added organic molecules. The review is structured on the basis of the kind of substrate undergoing the Pd-catalyzed carboxylation process. Accordingly, after the introductory section, the main sections of the review will illustrate Pd-catalyzed carboxylation of olefinic substrates, acetylenic substrates, and other substrates (aryl halides and triflates).

Cobalt-Catalyzed Highly Regioselective Three-Component Arylcarboxylation of Acrylate with Aryl Bromides and Carbon Dioxide

Cobalt-catalyzed regioselective three-component arylcarboxylation of acrylate with aryl bromides and carbon dioxide has been developed. The reaction is carried out by using cobalt chloride as a precatalyst and zinc powder as a reducing reagent under CO₂ (1 atm) at 40 °C. A range of aryl bromides are used for this reaction, leading to a series of valuable carboxylic acids with high regioselectivity and functional-group compatibility. Mechanistic experiments and DFT calculations indicate that this arylcarboxylation reaction involves the reaction of CO₂ with a cobalt enolate intermediate to form the C-C bond.

Metallaphotoredox: The Merger of Photoredox and Transition Metal Catalysis

The merger of photoredox catalysis with transition metal catalysis, termed metallaphotoredox catalysis, has become a mainstay in synthetic methodology over the past decade. Metallaphotoredox catalysis has combined the unparalleled capacity of transition metal catalysis for bond formation with the broad utility of photoinduced electron- and energy-transfer processes. Photocatalytic substrate activation has allowed the engagement of simple starting materials in metal-mediated bond-forming processes. Moreover, electron or energy transfer directly with key organometallic intermediates has provided novel activation modes entirely complementary to traditional catalytic platforms. This Review details and contextualizes the advancements in molecule construction brought forth by metallaphotocatalysis.

Carboxylation of a Palladacycle Formed via C(sp3 )-H Activation: Theory-Driven Reaction Design

Theory-driven organic synthesis is a powerful tool for developing new organic transformations. A palladacycle(II), generated from 8-methylquinoline via C(sp³ )-H activation, is frequently featured in the scientific literature, albeit that the reactivity toward CO₂ , an abundant, inexpensive, and non-toxic chemical, remains elusive. We have theoretically discovered potential carboxylation pathways using the artificial force induced reaction (AFIR) method, a density-functional-theory (DFT)-based automated reaction path search method. The thus obtained results suggest that the reduction of Pd(II) to Pd(I) is key to promote the insertion of CO₂ . Based on these computational findings, we employed various one-electron reductants, such as Cp*₂ Co, a photoredox catalyst under blue LED irradiation, and reductive electrolysis ((+)Mg/(-)Pt), which afforded the desired carboxylated products in high yields. After screening phosphine ligands under photoredox conditions, we discovered that bidentate ligands such as dppe promoted this carboxylation efficiently, which was rationally interpreted in terms of the redox potential of the Pd(II)-dppe complex as well as on the grounds of DFT calculations. We are convinced that these results could serve as future guidelines for the development of Pd(II)-catalyzed C(sp³ )-H carboxylation reactions with CO₂ .

Palladium Catalyzed Direct Carbonylative Thiomethylation of Aryldiazonium Salts and Amines with 4-(Methylthio)-2-Butanone as (Methylthio) Transfer Agent

Herein, an interesting palladium-catalyzed procedure for the direct carbonylative thiomethylation of aromatic amine derivatives with 4-methylthio-2-butanone is developed. Using 4-methylthio-2-butanone as (methylthio) transfer agent, a variety of corresponding thioesters are obtained with moderate to good yields under base-free condition. In addition, good functional group tolerance can be observed.

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.

Pivotal Electron Delivery Effect of the Cobalt Catalyst in Photocarboxylation of Alkynes: A DFT Calculation

Photocarboxylation of alkyne with carbon dioxide represents a highly attractive strategy to prepare functionalized alkenes with high efficiency and atomic economy. However, the reaction mechanism, especially the sequence of elementary steps (leading to different reaction pathways), reaction modes of the H-transfer step and carboxylation step, spin and charge states of the cobalt catalyst, etc., is still an open question. Herein, density functional theory calculations are carried out to probe the mechanism of the Ir/Co-catalyzed photocarboxylation of alkynes. The overall catalytic cycle mainly consists of four steps: reductive-quenching of the Ir catalyst, hydrogen transfer (rate-determining step), outer sphere carboxylation, and the final catalyst regeneration step. Importantly, the cobalt catalyst can facilitate the H-transfer by an uncommon hydride coupled electron transfer (HCET) process. The pivotal electron delivery effect of the Co center enables a facile H-transfer to the α-C(alkyne) of the aryl group, resulting in the high regioselectivity for β-carboxylation.

Cooperative photoredox and palladium catalysis: recent advances in various functionalization reactions

Cooperative photoredox and palladium catalysis for various functionalization reactions.

Functional Group-Directed Photochemical Reactions of Aromatic Alcohols, Amines, and Thiols Triggered by Excited-State Hydrogen Detachment: Additive-free Oligomerization, Disulfidation, and C(sp2)-H Carboxylation with CO2

Exploring new types of photochemical reactions is of great interest in the field of synthetic chemistry. Although excited-state hydrogen detachment (ESHD) represents a promising prospective template for additive-free photochemical reactions, applications of ESHD in a synthetic context remains scarce. Herein, we demonstrate the expansion of this photochemical reaction toward oligomerization, disulfidation, and regioselective C(sp²)-H carboxylation of aromatic alcohols, thiols, and amines. In the absence of any radical initiators in tetrahydrofuran upon irradiation with UV light (λ = 280 or 300 nm) under an atmosphere of N₂ or CO₂, thiols and catechol afforded disulfides and oligomers, respectively, as main products. Especially, the photochemical disulfidation proceeded highly selectively with the NMR and quantum yields of up to 69 and 0.46%, respectively. In stark contrast, the photolysis of phenylenediamines and aminophenols results in photocarboxylation in the presence of CO₂ (1 atm). p-Aminophenol was quantitatively carboxylated by photolysis for 17 h with a quantum yield of 0.45%. Furthermore, the photocarboxylation of phenylenediamines and aminophenols proceeds in a highly selective fashion on the aromatic C(sp²)-H bond next to a functional group, which is directed by the site-selective ESHD of the functional groups for the formation of aminyl and hydroxyl radicals.

Light-Mediated Carboxylation Using Carbon Dioxide

Carbon dioxide is a green and sustainable one-carbon source, which could be utilized in the production of various fine chemicals. In recent studies, the light-mediated carboxylation employing CO₂ has received considerable attention. The photocarboxylation of substrates with CO₂ to build novel C-C bonds is introduced in this Minireview. The article is arranged based on the light-driven reactive intermediates, including CO₂ radical anion, substrate radical anions, carbanions, and M-C species. Most of the cases are under the topic of photoredox catalysis, with single electron transfer as the main driving force. Some non-catalytic examples are also discussed to provide more mechanistic insights.

Visible-Light Photoredox-Catalyzed Ring-Opening Carboxylation of Cyclic Oxime Esters with CO2

The carboxylation of cyclic oxime esters with carbon dioxide via visible-light photoredox catalysis is demonstrated for the first time. A variety of cyclic oxime esters undergo ring-opening C-C bond cleavage and carboxylation to give cyanoalkyl-containing carboxylic acids in moderate to good yields. Moreover, this methodology features mild reaction conditions (room temperature, 1 atm), wide substrate scope, good functional group tolerance as well as facile derivations of products. Mechanistic studies indicate that the benzylic radicals and anions might be the key intermediates.

Utilization of CO2 Feedstock for Organic Synthesis by Visible-Light Photoredox Catalysis

CO₂ is a highly abundant, green, and sustainable carbon feedstock. Despite its kinetic inertness and thermodynamic stability, the development of various catalytic techniques has enabled the conversion of CO₂ to value-added products such as carboxylic acids, amino acids, and heterocyclic compounds, where visible-light photocatalysis has emerged to be an efficient promoter of these processes. This Minireview covers the progress in the areas of CO₂ incorporation onto organic matters based on the combined venture of renewable resources of CO₂ and light energy with significant emphasis on the last three years' developments.

Visible-light-mediated semi-heterogeneous black TiO2/nickel dual catalytic C (sp2)-P bond formation toward aryl phosphonates

The combination of black TiO₂ nanoparticles (NPs) with a nickel catalyst provides a low-cost, sustainable, and reusable alternative dual catalytic system to a homogeneous counterpart (noble metals). This black TiO₂-photoredox/nickel dual catalytic system has efficiently driven C-P bond formation between aryl iodides and diarylphosphine oxides under visible light, providing good to excellent yields as well as tolerating a variety of functional groups. The practical application of this semi-heterogeneous protocol has been highlighted by a sunlight experiment, a gram-scale reaction, and a reusability test.

Visible-Light Photoredox-Catalyzed Remote Difunctionalizing Carboxylation of Unactivated Alkenes with CO2

Remote difunctionalization of unactivated alkenes is challenging but a highly attractive tactic to install two functional groups across long distances. Reported herein is the first remote difunctionalization of alkenes with CO₂ . This visible-light photoredox catalysis strategy provides a facile method to synthesize a series of carboxylic acids bearing valuable fluorine- or phosphorus-containing functional groups. Moreover, this versatile protocol shows mild reaction conditions, broad substrate scope, and good functional-group tolerance. Based on DFT calculations, a radical adds to an unactivated alkene to smoothly form a new carbon radical, followed by a 1,5-hydrogen atom-transfer process, the rate-limiting step, generating a more stable benzylic radical. The reduction of the benzylic radicals by an Ir^II species generates the corresponding benzylic carbanions as the key intermediates, which further undergo nucleophilic attack with CO₂ to generate carboxylates.