Ag(I)-Catalyzed C–H Carboxylation of Thiophene Derivatives

Immobilization of Silver(I) Ions on Amino-Functionalized Chromium(III) Terephthalate with Organophosphine and its C-H Carboxylation of a Heteroaromatic Compound

A newly designed heterogenized catalyst that incorporates silver(I) ions with 2-(dicyclohexylphosphaneyl)acetaldehyde (PCy2 aldehyde) into amino-functionalized chromium(III) terephthalate is developed. Silver(I) ions were robustly immobilized on the amino-functionalized chromium(III) terephthalate, which contains an imine bond formed by the reaction with PCy2 aldehyde. The Ag(I) ion is coordinated with the phosphine in the imine group to create MIL-101-AP(Ag). Characterizations were carefully carried out according to the synthetic steps. The catalytic performance of MIL-101-AP(Ag) was evaluated through the C-H carboxylation of thiophene-2-carbonitrile, achieving a 10 % yield with a turnover number of 1.0. The recyclability of the MIL-101-AP(Ag) catalyst was successfully demonstrated with five cycle, with no loss in activity and selectivity observed. This approach, which involves the formation of an imine bond to facilitate silver loading with phosphine on amino-functionalized MIL-101(Cr), exhibits significant potential for both CO2 fixation and C-H carboxylation, thereby highlighting the modified material's promise as a sustainable catalyst.

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.

Silver salt enabled H/D exchange at the β-position of thiophene rings: synthesis of fully deuterated thiophene derivatives

We disclose a silver catalyzed H/D exchange reaction, which can introduce the deuterium atom at the β position of thiophene rings without the assistance of any coordinating groups. The advantages of this reaction include operation in open air, usage of D₂O as the deuterium source, good tolerance to a range of functional groups and obtaining high atom% deuterium incorporation. In addition, this H/D exchange reaction is employed for direct deuteration of a thiophene based monomer, which is usually prepared by multistep synthesis from expensive deuterated starting materials.