Double-Carboxylation of Two C–H Bonds in 2-Alkylheteroarenes Using LiO-t-Bu/CsF

One-Pot Synthesis of Indolin-3-ones Mediated by LiN(SiMe3)2/CsF

Indolin-3-ones are essential heterocycles with wide-ranging biological activities and medicinal values, and therefore, efficient approaches to their synthesis remain in demand. Herein, a novel and operationally simple method to generate indolin-3-ones is reported by using a tandem reaction of N-methylbenzylamines and methyl 2-fluorobenzoates mediated by the LiN(SiMe3)2 and CsF system (34 examples, 30-85% yields). The synthesis of C2-quaternary indolin-3-one further demonstrated the potential practicability of these tandem reactions.

Skeletal Formation of Carbocycles with CO2: Selective Synthesis of Indolo[3,2-b]carbazoles or Cyclophanes from Indoles, CO2, and Phenylsilane

The catalytic reactions of indoles with CO2 and phenylsilane afforded indolo[3,2-b]carbazoles, where the fused benzene ring was constructed by forming two C-H bonds and four C-C bonds with two CO2 molecules via deoxygenative conversions. Nine-membered cyclophanes made up of three indoles and three CO2 molecules were also obtained, where the cyclophane framework was constructed by forming six C-H bonds and six C-C bonds. These multicomponent cascade reactions giving completely different carbocycles were switched simply by choosing the solvent, acetonitrile or ethyl acetate.

Electrochemical E-Selective Semireductive Dicarboxylation of Aryl Alkynes with CO2

Herein, we report an electrochemical protocol for the dicarboxylation of aryl alkynes using CO2. With a graphite rod as the cathode and Al as the sacrificial anode, a series of valuable butenedioic acids are obtained in moderate to excellent yields with an E/Z ratio up to 50:1. This method features high E-selectivity, high step and atom economy, easy scalability, and a nice substrate scope, which renders it appealing for promising applications in organic synthesis and materials chemistry.

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.

A Tandem Madelung Indole Synthesis Mediated by a LiN(SiMe3)2/CsF System

A tandem Madelung indole synthesis by the reaction of methyl benzoate and N-methyl-o-toluidine has been discovered. The combination of LiN(SiMe3)2 with CsF is the key factor, which secures the high efficiency of such tandem transformations. Simply combining methyl benzoate, N-methyl-o-toluidine LiN(SiMe3)2, and CsF generated a diverse array of N-methyl-2-phenylindoles (31 examples, 50-90% yields). Furthermore, the scalability and the poststructural modifications of this indole synthesis were demonstrated.

Cu-catalyzed carboxylation of organoboronic acid pinacol esters with CO2

Chemical fixation of CO2 has received much attention. In particular, catalytic C-C bond formation with CO2 giving carboxylic acids is of great significance. Among the CO2 fixation methods, multiple carboxylation is one of the challenging subjects. Here we investigated the Cu-catalyzed carboxylation of a variety of boronic acid pinacol esters (C(sp2)-, C(sp3)-, and C(sp)-B compounds) with CO2, which efficiently provided the corresponding products, including aryl, alkenyl, alkyl, and alkynyl carboxylic acids. This carboxylation was also applicable to multiple CO2 fixation giving di- and tri-carboxylic acids under robust reaction conditions (totally 29 examples).

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 .

LiHMDS-Mediated Deprotonative Coupling of Toluenes with Ketones

We demonstrate that lithium hexamethyldisilazide (LiHMDS) acts as an effective base for deprotonative coupling reactions of toluenes with ketones to afford stilbenes. Various functionalities (halogen, OCF₃ , amide, Me, aryl, alkenyl, alkynyl, SMe, and SPh) are allowed on the toluenes. Notably, this system proved successful with low-reactive toluenes bearing a large pKa value compared to that of the conjugate acid of LiHMDS (hexamethyldisilazane, 25.8, THF), as demonstrated by 4-phenyltoluene (38.57, THF) and toluene itself (∼43, DMSO).

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.

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.

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.

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.

Palladium-Catalyzed Carboxylate-Assisted Ethoxycarboxylation of Aromatic Acids To Synthesize Monoethyl Phthalate Derivatives with Ethyl Bromodifluoroacetate

A novel and efficient approach for direct carbonation of aromatic acids with ethyl bromodifluoroacetate as the carbonyl source is reported. A broad range of substrates bearing various functional groups were tolerated, leading to monoalkyl phthalate derivatives in moderate to good yields.

Alkaline-Metal-Catalyzed One-Pot Aminobenzylation of Aldehydes with Toluenes

A novel and easily accessible MN(SiMe₃)₂ (M = Li or Na)/Cs₂CO₃ co-catalyzed benzylation of in situ generated N-(trimethylsilyl) aldimines with toluene derivatives has been successfully developed. The catalyst exhibits high chemoselectivity for deprotonation of toluenes at the benzylic position. The utility of this system is exemplified by the one-pot synthesis of a diverse array of bioactive 1,2-diarylethylamines with excellent efficiency and broad functional group tolerance.

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.