Sequential protocol for C(sp)–H carboxylation with CO<Subscript>2</Subscript>: KO<Superscript><Emphasis Type="Italic">t</Emphasis></Superscript>Bu-catalyzed C(sp)–H silylation and KO<Superscript><Emphasis Type="Italic">t</Emphasis></Superscript>Bu-mediated carboxylation

CO₂ incorporation into C–H bonds is an important and interesting topic. Herein a sequential protocol for C(sp)–H carboxylation by employing a metal-free C–H activation/catalytic silylation reaction in conjunction with KO^tBu-mediated carboxylation with CO₂ was established, in which KO^tBu catalyzes silylation of terminal alkynes to form alkynylsilanes at low temperature, and simultaneously mediates carboxylation of the alkynesilanes with atmospheric CO₂. Importantly, the carboxylation further promotes the silylation, which makes the whole reaction proceed very rapidly. Moreover, this methodology is simple and scalable, which is characterized by short reaction time, wide substrate scope, excellent functional-group tolerance and mild reaction conditions, affording a range of corresponding propiolic acid products in excellent yields in most cases. In addition, it also allows for a convenient ¹³C-labeling through the use of ¹³CO₂.     

Carboxylation Reactions with Carbon Dioxide Using N‐Heterocyclic Carbene‐Copper Catalysts

The development of versatile catalyst systems and new transformations for the utilization of carbon dioxide (CO₂) is of great interest and significance. This Personal Account reviews our studies on the exploration of the reactions of CO₂ with various substrates by the use of N‐heterocyclic carbene (NHC)‐copper catalysts. The carboxylation of organoboron compounds gave access to a wide range of carboxylic acids with excellent functional group tolerance. The C?H bond carboxylation with CO₂ emerged as a straightforward protocol for the preparation of a series of aromatic carboxylic esters and butenoates from simple substrates. The hydrosilylation of CO₂ with hydrosilanes provided an efficient method for the synthesis of silyl formate on gram scale. The hydrogenative or alkylative carboxylation of alkynes, ynamides and allenamides yielded useful α,β‐unsaturated carboxylic acids and α,β‐dehydro amino acid esters. The boracarboxylation of alkynes or aldehydes afforded the novel lithium cyclic boralactone or boracarbonate products, respectively. The NHC‐copper catalysts generally featured excellent functional group compatibility, broad substrate scope, high efficiency, and high regio‐ and stereoselectivity. The unique electronic and steric properties of the NHC‐copper units also enabled the isolation and structural characterization of some key intermediates for better understanding of the catalytic reaction mechanisms.     

Transition metal promoted carboxylation of unsaturated substrates with carbon dioxide

The use of CO₂ as a C1 building block for the synthesis of useful chemicals is of great significance, and has attracted increasing attention in recent years. The transition metal catalyzed or mediated addition of CO₂ to unsaturated chemical bonds has proved to be a powerful and versatile protocol for the incorporation of CO₂ into various unsaturated organic substrates such as alkynes, alkenes, allenes, aldehydes, and 1,3-dienes. The hydrogenative, alkylative and arylative carboxylation, heterocarboxylation, and carboxylative cyclization with CO₂ have led to efficient and selective formation of various functionalized carboxylic acids and derivatives. This review focuses on recent advances in this area with emphasis on conceptual reaction design.     

An efficient Ag/MIL-100(Fe) catalyst for photothermal conversion of CO2 at ambient temperature

The conversion of CO₂ under mild condition is of great importance because these reactions involving CO₂ can not only produce value-added chemicals from abundant and inexpensive CO₂ feedstock but also close the carbon cycle. However, the chemical inertness of CO₂ requires the development of high-performance catalysts. Herein, Ag nanoparticles/MIL-100(Fe) composites were synthesized by simple impregnation-reduction method and employed as catalysts for the photothermal carboxylation of terminal alkynes with CO₂. MIL-100(Fe) could stabilize Ag nanoparticles and prevent them from aggregation during catalytic process. Taking the advantages of photothermal effects and catalytic activities of both Ag nanoparticles and MIL-100(Fe), various aromatic alkynes could be converted to corresponding carboxylic acid products (86%–92% yields) with 1 atm CO₂ at room temperature under visible light irradiation when using Ag nanoparticles/MIL-100(Fe) as photothermal catalysts. The catalysts also showed good recyclability with almost no loss of catalytic activity for three consecutive runs. More importantly, the catalytic performance of Ag nanoparticles/MIL-100(Fe) under visible light irradiation at room temperature was comparable to that upon heating, showing that the light source could replace conventional heating method to drive the reaction. This work provided a promising strategy of utilizing solar energy for achieving efficient CO₂ conversion to value-added chemicals under mild condition.     

Cu‐Catalyzed Formal Methylative and Hydrogenative Carboxylation of Alkynes with Carbon Dioxide: Efficient Synthesis of α,β‐Unsaturated Carboxylic Acids

The sequential hydroalumination or methylalumination of various alkynes catalyzed by different catalyst systems, such those based on Sc, Zr, and Ni complexes, and the subsequent carboxylation of the resulting alkenylaluminum species with CO₂ catalyzed by an N‐heterocyclic carbene (NHC)–copper catalyst have been examined in detail. The regio‐ and stereoselectivity of the overall reaction relied largely on the hydroalumination or methylalumination reactions, which significantly depended on the catalyst and alkyne substrates. The subsequent Cu‐catalyzed carboxylation proceeded with retention of the stereoconfiguration of the alkenylaluminum species. All the reactions could be carried out in one‐pot to afford efficiently a variety of α,β‐unsaturated carboxylic acids with well‐controlled configurations, which are difficult to construct by previously reported methods. This protocol could be practically useful and attractive because of its high regio‐ and stereoselectivity, simple one‐pot reaction operation, and the use of CO₂ as a starting material.     

Green Catalytic Process for Cyclic Carbonate Synthesis from Carbon Dioxide under Mild Conditions

As a renewable and abundant C₁ resource possessing multiple attractive characteristics, such as low cost, nontoxicity, non‐flammability, and easy accessibility, CO₂ conversion into value‐added chemicals and fuels can contribute to green chemistry and sustainable development. Since CO₂ is a thermodynamically inert molecule, the activation of CO₂ is pivotal for its effective conversion. In this regard, the formation of a transition‐metal CO₂ complex through direct coordination is one of the most powerful ways to induce the inert CO₂ molecule to undergo chemical reactions. To date, numerous processes have been developed for efficient synthesis of cyclic carbonates from CO₂. On the basis of mechanistic understanding, we have developed efficient metal catalysts and green processes, including heterogeneous catalysis, and metal‐free systems, such as ionic liquids, for cyclic carbonate synthesis. The big challenge is to develop catalysts that promote the reaction under low pressure (preferably at 1 bar). In this context, bifunctional catalysis is capable of synergistic activation of both the substrate and CO₂ molecule, and thus, could render CO₂ conversion smoothly under mild conditions. Alternatively, converting CO₂ derivatives, that is, the captured CO₂ as an activated species, would more easily take place at low pressure in comparison with gaseous CO₂. The aim of this Personal Account is to summarize versatile catalytic processes for cyclic carbonate synthesis from CO₂, including epoxide/CO₂ coupling reaction, carboxylation of 1,2‐diol with CO₂, oxidative cyclization of olefins with CO₂, condensation of vicinal halohydrin with CO₂, carboxylative cyclization of propargyl alcohols with CO₂, and conversion of the CO₂ derivatives.

     

Porous Carbon Nitride Frameworks Derived from Covalent Triazine Framework Anchored Ag Nanoparticles for Catalytic CO2 Conversion

Porous carbon nitride frameworks (PCNFs) with uniform and rich nitrogen dopants and abundant porosity were successfully fabricated through the direct carbonization of the covalent triazine frameworks (CTFs) at different pyrolysis temperatures and used as supports to anchor and stabilize Ag nanoparticles (NPs) for catalytic CO₂ conversion. Importantly, the pyrolysis temperature plays a crucial role in the properties of porous carbon nitride frameworks. The material carbonized at 700 °C showed the highest surface area and micro- and mesoporous structure with a certain interlayer distance. Taking advantage of their unique surface characteristics, PCNF-supported Ag NP catalysts (Ag/PCNF-T, T=pyrolysis temperature) were prepared by a simple chemical method. A series of characterizations revealed that Ag NPs are embedded in the porous carbon nitride frameworks and confined to a relatively small size with high dispersion owing to the assistance of the abundant surface groups and porous structures. The as-obtained Ag/PCNF-T catalysts, especially Ag/PCNF-700, showed excellent catalytic activity, selectivity, and stability for the carboxylation of CO₂ with terminal alkynes under mild conditions. This can be due to the existence of abundant nitrogen atoms and diverse porosity, which resulted in highly efficient catalytic activity and stability.     

Alkylative Carboxylation of Ynamides and Allenamides with Functionalized Alkylzinc Halides and Carbon Dioxide by a Copper Catalyst

The alkylative carboxylation of ynamides and allenamides with CO₂ and alkylzinc halides catalyzed by a copper catalyst was developed. A variety of alkylzinc halides bearing functional groups were used for this transformation to afford α,β-unsaturated carboxylic acids, which contain the α,β-dehydroamino acid skeleton, introducing the corresponding alkyl group and CO₂ across the carbon–carbon triple or double bond. This alkylative carboxylation formally consists of Cu-catalyzed carbozincation of ynamides or allenamides with alkylzinc halides and the subsequent nucleophilic carboxylation of the resulting alkenylzinc species with CO₂. This protocol would be a useful method for the synthesis of α,β-dehydroamino acid derivatives possessing a functionalized alkyl group due to the high regio- and stereoselectivity, simple one-pot procedure as well as the use of CO₂ as a starting material.     

Carboxylation of Phenols with CO2 at Atmospheric Pressure

A convenient and efficient method for the ortho‐carboxylation of phenols under atmospheric CO₂ pressure has been developed. This method provides an alternative to the previously reported Kolbe–Schmitt method, which requires very high pressures of CO₂. The addition of a trisubstituted phenol has proved essential for the successful carboxylation of phenols with CO₂ at standard atmospheric pressure, allowing the efficient preparation of a broad variety of salicylic acids.     

Nickel‐Catalyzed Direct Carboxylation of Olefins with CO2: One‐Pot Synthesis of α,β‐Unsaturated Carboxylic Acid Salts

The nickel‐catalyzed direct carboxylation of alkenes with the cheap and abundantly available C₁ building block carbon dioxide (CO₂) in the presence of a base has been achieved. The one‐pot reaction allows for the direct and selective synthesis of a wide range of α,β‐unsaturated carboxylates (TON>100, TOF up to 6 h^?1, TON=turnover number, TOF=turnover frequency). Thus, it is possible, in one step, to synthesize sodium acrylate from ethylene, CO₂, and a sodium salt. Acrylates are industrially important products, the synthesis of which has hitherto required multiple steps.     

温和条件下金属双氮杂环卡宾配合物催化CO2直接转化为炔酸的高效均相羧化反应

近年来,催化CO2合成精细化学品的研究备受关注。本研究在温和条件下利用金属双氮杂环卡宾催化剂实现CO2与末端炔烃的直接羧化反应,并提出合理的催化机理。首先,合成制备了铜基、银基两种金属双氮杂环卡宾催化剂,实验证明银双氮杂环卡宾配合物具有较好催化活性。通过改变环境条件和底物种类,对反应条件及催化剂底物适应性进行了探究,利用核磁共振谱仪表征产物分子结构并计算直接羧化反应的催化产率。结果表明,适宜催化条件为:1.2eq Cs2CO3作为添加剂、1大气压、室温、无水溶剂和1(mmol)%催化剂用量。银基金属催化剂活性较铜基催化剂高并具有广泛的底物适应性,对苯乙炔的催化产率高达93%;对乙炔气体同样具有良好的催化活性。此类催化剂具有优良的催化活性,能催化合成丙炔酸等重要医药中间体,在工业应用上具有极大潜力和广阔前景。     

Modellkomplexe des nickels für die [2+2+2′]-cycloaddition von alkinen mit kohlendioxid

The oxanickelacyclopentenone (I), formed from 2-butyne, CO₂ and nickel(0), reacts with activated alkynes by insertion to give oxanickelacycloheptadienones (II). It is suggested that these novel nickela complexes are intermediates in the nickel-catalysed synthesis of 2-pyrones from alkynes and CO₂.     

A Light/Ketone/Copper System for Carboxylation of Allylic C−H Bonds of Alkenes with CO2

A photo‐induced carboxylation reaction of allylic C?H bonds of simple alkenes with CO₂ is prompted by means of a ketone and a copper complex. The unique carboxylation reaction proceeds through a sequence of an endergonic photoreaction of ketones with alkenes forming homoallyl alcohol intermediates and a thermal copper‐catalyzed allyl transfer reaction from the homoallyl alcohols to CO₂ through C?C bond cleavage.     

Cu‐Catalyzed Alkylative Carboxylation of Ynamides with Dialkylzinc Reagents and Carbon Dioxide

Alkylative carboxylation of ynamides with CO₂ and dialkylzinc reagents using a N‐heterocyclic carbene (NHC)–copper catalyst has been developed. A variety of ynamides, both cyclic and acyclic, undergo this transformation under mild conditions to afford the corresponding α,β‐unsaturated carboxylic acids, which contain the α,β‐dehydroamino acid skeleton. The present alkylative carboxylation formally consists of Cu‐catalyzed carbozincation of ynamides with dialkylzinc reagents with the subsequent nucleophilic carboxylation of the resulting alkenylzinc species with CO₂. Dialkylzinc reagents bearing a β‐hydrogen atom such as Et₂Zn and Bu₂Zn still afford the alkylated products despite the potential for β‐hydride elimination. This protocol would be a desirable method for the synthesis of highly substituted α,β‐ dehydroamino acid derivatives due to its high regio‐ and stereoselectivity, simple one‐pot procedure, and its use of CO₂ as a starting material.     

Insertion of CO2 Mediated by a (Xantphos)NiI–Alkyl Species

The incorporation of CO₂ into organometallic and organic molecules represents a sustainable way to prepare carboxylates. The mechanism of reductive carboxylation of alkyl halides has been proposed to proceed through the reduction of Ni^II to Ni^I by either Zn or Mn, followed by CO₂ insertion into Ni^I‐alkyl species. No experimental evidence has been previously established to support the two proposed steps. Demonstrated herein is that the direct reduction of (tBu‐Xantphos)Ni^IIBr₂ by Zn affords Ni^I species. (tBu‐Xantphos)Ni^I‐Me and (tBu‐Xantphos)Ni^I‐Et complexes undergo fast insertion of CO₂ at 22 °C. The substantially faster rate, relative to that of Ni^II complexes, serves as the long‐sought‐after experimental support for the proposed mechanisms of Ni‐catalyzed carboxylation reactions.     

Photosynthetic Fixation of CO2 in Alkenes by Heterogeneous Photoredox Catalysis with Visible Light

Light-driven fixation of CO₂ in organics has emerged as an appealing alternative for the synthesis of value-added fine chemicals. Challenges remain in the transformation of CO₂ as well as product selectivity due to its thermodynamic stability and kinetic inertness. Here we develop a boron carbonitride (BCN) with the abundant terminal B/N defects around the mesoporous walls, which essentially enhances surface active sites as well as charge transfer kinetics, boosting the overall rate of CO₂ adsorption and activation. In this protocol, anti-Markovnikov hydrocarboxylation of alkenes with CO₂ to an extended carbon chain is achieved with good functional group tolerance and specific regioselectivity under visible-light irradiation. The mechanistic studies demonstrate the formation of CO₂ radical anion intermediate on defective boron carbonitride, leading to the anti-Markovnikov carboxylation. Gram-scale reaction, late-stage carboxylation of natural products and synthesis of anti-diabetic GPR40 agonists reveal the utility of this method. This study sheds new insight on the design and application of metal-free semiconductors for the conversion of CO₂ in an atom-economic and sustainable manner.     

Direct Synthetic Processes for Cyclic Carbonates from Olefins and CO<Subscript>2</Subscript>

This short review presents the recent developments in the direct synthesis of cyclic carbonates from olefins and CO₂. The straightforward synthesis of cyclic carbonates from olefins instead of epoxides, also called one-pot “oxidative carboxylation” of olefins, can be viewed as the coupling of two sequential reactions of epoxidation of olefins and CO₂ cycloaddition to epoxides formed. The facile synthetic approach would make carbonate synthesis simpler and even cheaper with industrial potential from environmental and economic points of view. Some progresses have been made on this direct synthetic reaction for cyclic carbonates with homogeneous and heterogeneous catalysts, however, this reaction system is still at a preliminary stage. Among the catalysts reported, only a few can be considered as effective for the direct oxidative carboxylation of olefins to cyclic carbonates. Thus active and selective catalysts should be explored to put the direct synthesis of cyclic carbonates into practical applications.     

Proline-Catalyzed Synthesis of 5-Aryl-2-oxazolidinones from Carbon Dioxide and Aziridines Under Solvent-Free Conditions

Natural α-amino acids were proven to be ecofriendly and recyclable catalysts for the carboxylation of aziridines with CO₂ without utilization of any organic solvents or additives. Notably, a series of 5-aryl-2-oxazolidinones were obtained in good yield together with excellent chemo- and regioselectivity under mild conditions using proline as the catalyst. Notably, the catalyst could be recycled more than five times after a simple separation procedure without appreciable loss of catalytic activity. This process represents a promising strategy for homogeneous catalyst recycling.     

Carboxylation of Aromatic and Aliphatic Bromides and Triflates with CO2 by Dual Visible‐Light–Nickel Catalysis

We report the efficient carboxylation of bromides and triflates with K₂CO₃ as the source of CO₂ in the presence of an organic photocatalyst in combination with a nickel complex under visible light irradiation at room temperature. The reaction is compatible with a variety of functional groups and has been successfully applied to the synthesis and derivatization of biologically active molecules. In particular, the carboxylation of unactivated cyclic alkyl bromides proceeded well with our protocol, thus extending the scope of this transformation. Spectroscopic and spectroelectrochemical investigations indicated the generation of a Ni⁰ species as a catalytic reactive intermediate.