Base-Mediated Carboxylation of Unprotected Indole Derivatives with Carbon Dioxide

A condition-tuned unorthodox approach to indole-3-carboxylic acids and anthranilic acids via carbon atom translocation

We describe a robust one-pot cascade method for the synthesis of indole-3-carboxylic acids using isatins and DMSO via a one-carbon translocation involving in situ generation of α,β-unsaturated methylvinylsulfoxide followed by amide bond cleavage and ring closure. The methodology has been extended to afford anthranilic acid derivatives by tuning the reaction conditions in the presence of molecular oxygen. Importantly, easy access to commercially available drugs, including tropisetron, is demonstrated.

Base-mediated carboxylation of C-nucleophiles with CO2

Carbon dioxide (CO2) is an available, abundant, and renewable C1 resource, which could be converted into value-added chemicals. Due to its inherent thermodynamic stability and kinetic inertness, it is difficult to realize its efficient utilization. Nevertheless, many elegant strategies for the utilization of CO2 have been developed using Lewis bases, frustrated Lewis pairs, hydroxyl-containing compounds, amino-group-containing compounds or transition metal catalysis. Among them, base-mediated carboxylation of C-nucleophiles is an environmentally friendly strategy for CO2 conversion, which is operationally simple, using low-toxicity bases and economical available promoters, without the use of complex ligands or cocatalysts. This review summarizes related work on the base-mediated carboxylation of C-nucleophiles with CO2, based on the effects of nucleophiles, promoters, additives, and solvents. The types of pronucleophile are categorized as follows: hydrocarbon with C(sp3)-H, C(sp2)-H or C(sp)-H bonds, organosilanes, organotin, organoboron, and N-tosylhydrazones. Typical mechanisms and applications of these carboxylation reactions are also depicted. Moreover, mechanistic comprehension of CO2 activation and conversion at a molecular level aims to further expand the repertoire of carboxylation transformations mediated by bases.

Carboxylic Acid Salts as Dual-Function Reagents for Carboxylation and Carbon Isotope Labeling

The potassium salts of carboxylic acids are developed as efficient carboxylating agents through CO₂ exchange. We describe these carboxylates as dual-function reagents because they function as a combined source of CO₂ and base/metalating agent. By using the salt of a commercially available carboxylic acid, this protocol overcomes difficulties when using CO₂ gas or organometallic reagents, such as pressurized containers or strictly inert conditions. The reaction proceeds under mild conditions, does not require transition metals or other additives, and shows broad substrate scope. Through the preparation of several biologically important molecules, we show how this strategy provides an opportunity for isotope labeling with low equivalents of labeled CO₂ .

Combined Brønsted Base-Promoted CO2 Fixation into Benzylic C-H Bonds of Alkylarenes

Interest in developing methods for direct CO₂ fixation into readily available unfunctionalized C-H bonds in organic substances has recently surged. In contrast to the well-studied carboxylations of alkynyl C(sp)-H and aromatic C(sp²)-H bonds, carboxylation of benzylic C(sp³)-H bonds to produce 2-arylacetic acids is limited to photoirradiation reactions and continues to be a challenging issue because of the low chemical reactivity. We herein describe that a combined Brønsted base (i.e., LiO-t-Bu/CsF and LiOCEt₃/CsF) achieves benzylic carboxylation of electron-deficient, -neutral, and -rich alkylarenes and enables various functionalities, including fragile ones such as bromide, alkene, alkyne, and carbonyl moieties. Dicarboxylation at the benzylic position is also established. Cs-alkoxide generated in situ acts as a reactive base, as demonstrated in experiments with independently prepared CsO-t-Bu and by ¹³³Cs nuclear magnetic resonance studies.

Silver-Catalyzed Carbon Dioxide Fixation on Alkynylindoles

A silver-catalyzed carbon dioxide fixation reaction into 2-alkynylindole derivatives was developed to afford tricyclic indoles. Carbon dioxide was selectively fixed on the N atom of the indole, and only 6-endo-dig cyclization proceeded under mild reaction conditions. Carboxylation on C3 of the indole was not observed. This method was applicable for a variety of 2-alkynylindoles, and the corresponding products were obtained in high yields without the production of side products.

Mechanistic Insights into Palladium(II)-Catalyzed Carboxylation of Thiophene and Carbon Dioxide

The mechanism in palladium-catalyzed carboxylation of thiophene and CO2 is investigated using the density functional theory (DFT) calculations, including three consecutive steps of the formation of carbanion through breaking the C–H bond(s) via the palladium acetate, the elimination of acetic acid and the nucleophile attacking the weak electrophile CO2 to form C–C bond. Results show that the C–C bond is formed through taking the three-membered cyclic conformation arrangement involving the interaction of the transition metal and the CO2, and the CO2 insertion step is the rate-determining step for this entire reaction process. Aiming to precisely disclose what factor determine the origin of the activation energy barrier in this carboxylation reaction, the distortion/interaction analysis is performed along with the entire reaction coordinate.

Carboxylate-Assisted Carboxylation of Thiophene with CO2 in the Solvent-Free Carbonate Medium

Direct carboxylation of thiophene with CO2 has been achieved under a relatively mild solvent-free carbonate and carboxylate medium. This base-mediated medium can cleave the very weakly acidic C–H bond without using other limiting reagents, which is one indispensable step in the carboxylation reaction. Product yield varies with different carboxylate salts, and cesium pivalate is the most suitable base additive among targeted simple carboxylate salts. Furthermore, the detailed mechanism of this carboxylation reaction is studied, which involves initial proton abstraction, rendered by carbonate and C–C bond formation, by inserting CO2. The activation energy barrier of the C–H activation step is higher than the following CO2 insertion step, whether for the formation of the mono- and/or di-carboxylate, which indicates that the C–H deprotonation induced by the base is slow and the resulting carbon-centered nucleophile reacts rapidly with CO2.

Electrochemical Dearomative Dicarboxylation of Heterocycles with Highly Negative Reduction Potentials

The dearomative dicarboxylation of stable heteroaromatics using CO₂ is highly challenging but represents a very powerful method for producing synthetically useful dicarboxylic acids, which can potentially be employed as intermediates of biologically active molecules such as natural products and drug leads. However, these types of transformations are still underdeveloped, and concise methodologies with high efficiency (e.g., high yield and high selectivity for dicarboxylations) have not been reported. We herein describe a new electrochemical protocol using the CO₂ radical anion (E_1/2 of CO₂ = -2.2 V in DMF and -2.3 V in CH₃CN vs SCE) that produces unprecedented trans-oriented 2,3-dicarboxylic acids from N-Ac-, Boc-, and Ph-protected indoles that exhibit highly negative reduction potentials (-2.50 to -2.94 V). On the basis of the calculated reduction potentials, N-protected indoles with reduction potentials up to -3 V smoothly undergo the desired dicarboxylation. Other heteroaromatics, including benzofuran, benzothiophene, electron-deficient furans, thiophenes, 1,3-diphenylisobenzofuran, and N-Boc-pyrazole, also exhibit reduction potentials more positive than -3 V and served as effective substrates for such dicarboxylations. The dicarboxylated products thus obtained can be derivatized into useful synthetic intermediates for biologically active compounds in few steps. We also show how the dearomative monocarboxylation can be achieved selectively by choice of the electrolyte, solvent, and protic additive; this strategy was then applied to the synthesis of an octahydroindole-2-carboxylic acid (Oic) derivative, which is a useful proline analogue.

Direct C-H Carboxylation Forming Polyfunctionalized Aromatic Carboxylic Acids by Combined Brønsted Bases

CO₂ fixation into electron-deficient aromatic C-H bonds proceeds with the combined Brønsted bases LiO-t-Bu and LiO-t-Am/CsF/18-crown-6 (t-Am = CEtMe₂) under a CO₂ atmosphere to afford a variety of polyfunctionalized aromatic carboxylic acids.

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.

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.

A General and Scalable Synthesis of Polysubstituted Indoles

A consecutive 2-step synthesis of N-unprotected polysubstituted indoles bearing an electron-withdrawing group at the C-3 position from readily available nitroarenes is reported. The protocol is based on the [3,3]-sigmatropic rearrangement of N-oxyenamines generated by the DABCO-catalyzed reaction of N-arylhydroxylamines and conjugated terminal alkynes, and delivers indoles endowed with a wide array of substitution patterns and topologies.

Carbonate-promoted C-H carboxylation of electron-rich heteroarenes

C-H carboxylation is an attractive transformation for both streamlining synthesis and valorizing CO₂. The high bond strength and very low acidity of most C-H bonds, as well as the low reactivity of CO₂, present fundamental challenges for this chemistry. Conventional methods for carboxylation of electron-rich heteroarenes require very strong organic bases to effect C-H deprotonation. Here we show that alkali carbonates (M₂CO₃) dispersed in mesoporous TiO₂ supports (M₂CO₃/TiO₂) effect CO₃ ^2--promoted C-H carboxylation of thiophene- and indole-based heteroarenes in gas-solid reactions at 200-320 °C. M₂CO₃/TiO₂ materials are strong bases in this temperature regime, which enables deprotonation of very weakly acidic bonds in these substrates to generate reactive carbanions. In addition, we show that M₂CO₃/TiO₂ enables C3 carboxylation of indole substrates via an apparent electrophilic aromatic substitution mechanism. No carboxylations take place when M₂CO₃/TiO₂ is replaced with un-supported M₂CO₃, demonstrating the critical role of carbonate dispersion and disruption of the M₂CO₃ lattice. After carboxylation, treatment of the support-bound carboxylate products with dimethyl carbonate affords isolable esters and the M₂CO₃/TiO₂ material can be regenerated upon heating under vacuum. Our results provide the basis for a closed cycle for the esterification of heteroarenes with CO₂ and dimethyl carbonate.

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

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

Synthesis, Thermal Properties, and Rheological Characteristics of Indole-Based Aromatic Polyesters

Currently, there is an intensive development of bio-based aromatic building blocks to replace fossil-based terephthalates used for poly(ethylene terephthalate) production. Indole is a ubiquitous aromatic unit in nature, which has great potential as a bio-based feedstock for polymers or plastics. In this study, we describe the synthesis and characterization of new indole-based dicarboxylate monomers with only aromatic ester bonds, which can improve the thermal stability and glass-transition temperature (T _g) of the resulting polyesters. The new dicarboxylate monomers were polymerized with five aliphatic diols to yield 10 new polyesters with tunable chemical structures and physical properties. Particularly, the T _g values of the obtained polyesters can be as high as 113 °C, as indicated by differential scanning calorimetry and dynamic mechanical analysis. The polyesters showed decent thermal stability and distinct flow transitions as revealed by thermogravimetric analysis and rheology measurements.

Kolbe-Schmitt type reaction under ambient conditions mediated by an organic base

The combined use of an organic base for resorcinols realized a Kolbe-Schmitt type reaction under ambient conditions. When resorcinols (3-hydroxyphenol derivatives) were treated with DBU under a carbon dioxide atmosphere, nucleophilic addition to carbon dioxide proceeded to afford the corresponding salicylic acid derivatives in high yields.

Iron-Catalyzed Regioselective Remote C(sp2)-H Carboxylation of Naphthyl and Quinoline Amides

Iron(III)-catalyzed regioselective direct remote C-H carboxylation of naphthyl and quinoline amides was developed using CBr₄ and alcohol. The reaction involves a radical pathway using a coordination activation strategy and single electron transfer process. The use of sustainable iron catalysis, selectivity, and the substrate scope are the important practical features.

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

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

Gold(III)-Catalyzed Decarboxylative C3-Benzylation of Indole-3-carboxylic Acids with Benzylic Alcohols in Water

A strategy for the gold(III)-catalyzed decarboxylative coupling reaction of indole-3-carboxylic acids with benzylic alcohols has been developed. This cascade reaction is devised as a straightforward and efficient synthetic route for 3-benzylindoles in moderate to excellent yields (50-93%). A Hammett study of the protodecarboxylation gives a negative ρ value, suggesting that there is a buildup of positive charge on the indole ring in the transition state. Furthermore, comparison of initial rates in H₂O and in D₂O reveals an observed kinetic solvent isotope effect (KSIE = 2.7). This simple protocol, which affords the desired products with CO₂ and water as the coproducts, can be achieved under mild conditions without the need for base or other additives in water.

Direct Carboxylation of Electron-Rich Heteroarenes Promoted by LiO-tBu with CsF and [18]Crown-6

We herein demonstrate that the combination of LiO-tBu, CsF, and [18]crown-6 efficiently promotes the direct C-H carboxylation of electron-rich heteroarenes (benzothiophene, thiophene, benzofuran, and furan derivatives). A variety of functional groups, including methyl, methoxy, halo, cyano, amide, and keto moieties, are compatible with this system. The reaction proceeds via the formation of a tert-butyl carbonate species.

C-H Bond Carboxylation with Carbon Dioxide

Carbon dioxide is a nontoxic, renewable, and abundant C₁ source, whereas C-H bond functionalization represents one of the most important approaches to the construction of carbon-carbon bonds and carbon-heteroatom bonds in an atom- and step-economical manner. Combining the chemical transformation of CO₂ with C-H bond functionalization is of great importance in the synthesis of carboxylic acids and their derivatives. The contents of this Review are organized according to the type of C-H bond involved in carboxylation. The primary types of C-H bonds are as follows: C(sp)-H bonds of terminal alkynes, C(sp² )-H bonds of (hetero)arenes, vinylic C(sp² )-H bonds, the ipso-C(sp² )-H bonds of the diazo group, aldehyde C(sp² )-H bonds, α-C(sp³ )-H bonds of the carbonyl group, γ-C(sp³ )-H bonds of the carbonyl group, C(sp³ )-H bonds adjacent to nitrogen atoms, C(sp³ )-H bonds of o-alkyl phenyl ketones, allylic C(sp³ )-H bonds, C(sp³ )-H bonds of methane, and C(sp³ )-H bonds of halogenated aliphatic hydrocarbons. In addition, multicomponent reactions, tandem reactions, and key theoretical studies related to the carboxylation of C-H bonds are briefly summarized. Transition-metal-free, organocatalytic, electrochemical, and light-driven methods are highlighted.

Recent advances in nucleophile-triggered CO2-incorporated cyclization leading to heterocycles

CO₂, as a sustainable, feasible, abundant one-carbon synthon, has been utilized in carboxylative cyclization, carbonylative cyclization, and reductive cyclization.

Pd-Catalyzed Dearomative Carboxylation of Indolylmethanol Derivatives

By using a new catalytic system (PdCl₂[P( n-Bu)₃]₂ in combination with ZnEt₂), various 3-indolylmethanol derivatives were successfully carboxylated with CO₂ (1 atm) via dearomatization of the indole nucleus, affording 3-methyleneindoline-2-carboxylates. In contrast, carboxylation of 2-indolylmethanol derivatives afforded unexpected doubly carboxylated products, which are useful synthetic precursors for biologically active compounds.

First catalyst-free CO2 trapping of N-acyliminium ions under ambient conditions: sustainable multicomponent synthesis of thia- and oxazolidinyl carbamates

The first trapping of N-acyliminium ions by in situ generated carbaminic acid (product of carbon dioxide (CO2) and amine) is reported. This catalyst-free reaction provides a convenient and feasible approach to prepare N-acyl thia- and oxazolidinyl carbamates with good functional-group compatibility and high efficiency under green conditions. Furthermore, the multicomponent method features a broad substrate scope, facile product diversification, smooth scale-up and notable potential for polymer applications.

Transition Metal-Free CO2 Fixation into New Carbon-Carbon Bonds

CO₂ is the ultimate renewable carbon source on Earth and the essential C₁ building block for carbohydrate biosynthesis in photosynthetic organisms. Modern synthetic chemistry is facing the key challenge of developing fundamental transformations, such as C-C bond formation, in a sustainable and efficient manner from renewable sources. In this Minireview, the most significant methods recently reported for CO₂ fixation under transition metal-free conditions are summarized, organized into three different chapters according to the nature of the chemical transformation that forges the new C-C bond. The focus is on the mechanistic aspects of the different CO₂ activation modes, with specific attention to those systems that operate under catalytic conditions.

Catalytic Lactonization of Unactivated Aryl C-H Bonds with CO2: Experimental and Computational Investigation

The first catalytic lactonization of unactivated aryl C-H bonds with CO₂ to afford important phthalides is reported. Notably, this method features high selectivity, excellent functional group tolerance, smooth scalability, and facile product diversification. DFT calculations reveal that a novel insertion of two CO₂ into the O-Pd bond of a palladacycle might be the key step, providing great potential and a different perspective for carbonylation with CO₂.

Palladium-Catalyzed Four-Component Cascade Reaction for the Synthesis of Highly Functionalized Acyclic O,O-Acetals

A palladium-catalyzed four-component cascade reaction of carbon dioxide, amines, allenyl ethers, and aryl iodides has been developed for the first time. The novel reaction allows simultaneous construction of three different new bonds (C-N, C-O, and C-C) in a single step, affording an efficient method for the synthesis of a variety of highly functionalized acyclic O,O-acetals. Excellent chemo- and regioselectivity, wide substrate scope, and good functional group tolerance are features of the method.

Pd-catalyzed carbonylation of aryl C–H bonds in benzamides with CO2

Herein, we report the first Pd-catalyzed carbonylation of unactivated aryl C–H bonds in benzamides under 1 atm of CO₂ to directly afford important phthalimides.

C-H Carboxylation of Aromatic Compounds through CO2 Fixation

Carbon dioxide (CO2 ) represents the most abundant and accessible carbon source on Earth. Thus the ability to transform CO2 into valuable commodity chemicals through the construction of C-C bonds is an invaluable strategy. Carboxylic acids and derivatives, the main products obtained by carboxylation of carbon nucleophiles by reaction of CO2 , have wide application in pharmaceuticals and advanced materials. Among the variety of carboxylation methods currently available, the direct carboxylation of C-H bonds with CO2 has attracted much attention owing to advantages from a step- and atom-economical point of view. In particular, the prevalence of (hetero)aromatic carboxylic acids and derivatives among biologically active compounds has led to significant interest in the development of methods for their direct carboxylation from CO2 . Herein, the latest achievements in the area of direct C-H carboxylation of (hetero)aromatic compounds with CO2 will be discussed.

tert-Butyl Nitrite-Mediated Synthesis of N-Nitrosoamides, Carboxylic Acids, Benzocoumarins, and Isocoumarins from Amides

This work reports tert-butyl nitrite (TBN) as a multitask reagent for (1) the controlled synthesis of N-nitrosoamide from N-alkyl amides, (2) hydrolysis of N-methoxyamides to carboxylic acids, (3) metal- and oxidant-free benzocoumarin synthesis from ortho-aryl-N-methoxyamides via N-H, C-N, and C-H bond activation, and (4) isocoumarin synthesis using Ru(II)/PEG as a recyclable catalytic system via ortho-C-H activation and TBN as an oxygen source. The sequential functional group interconversion of amide to acid has also been examined using IR spectroscopic analysis. Additionally, this methodology is highly advantageous due to short reaction time, gram scale synthesis, and broad substrate scope.