Regioselective Alkylative Carboxylation of Allenamides with Carbon Dioxide and Dialkylzinc Reagents Catalyzed by an N‐Heterocyclic Carbene–Copper Complex

The alkylative carboxylation of allenamide catalyzed by an N‐heterocyclic carbene (NHC)–copper(I) complex [(IPr)CuCl] with CO₂ and dialkylzinc reagents was investigated. The reaction of allenamides with dialkylzinc reagents (1.5 equiv) and CO₂ (1 atm.) proceeded smoothly in the presence of a catalytic quantity of [(IPr)CuCl] to afford (Z)‐α,β‐dehydro‐β‐amino acid esters in good yields. The reaction is regioselective, with the alkyl group introduced onto the less hindered γ‐carbon, and the carboxyl group introduced onto the β‐carbon atom of the allenamides. The first step of the reaction was alkylative zincation of the allenamides to give an alkenylzinc intermediate followed by nucleophilic addition to CO₂. A variety of cyclic and acyclic allenamides were found to be applicable to this transformation. Dialkylzinc reagents bearing β‐hydrogen atoms, such as Et₂Zn or Bu₂Zn, also gave the corresponding alkylative carboxylation products without β‐hydride elimination. The present methodology provides an easy route to alkyl‐substituted α,β‐dehydro‐β‐amino acid ester derivatives under mild reaction conditions with high regio‐ and stereoselectivtiy.     

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.     

Access to α‐Arylglycines by Umpolung Carboxylation of Aromatic Imines with Carbon Dioxide

A straightforward and transition‐metal‐free approach for the efficient synthesis of α‐arylglycine derivatives from aromatic imines and carbon dioxide was enabled by an umpolung carboxylation reaction. Various substituted diphenylmethimines underwent the carboxylation smoothly with carbon dioxide in the presence of potassium tert‐butoxide and 18‐crown‐6 to give the corresponding carboxylated products in good to high yields. Besides the enhancement of the solubility of potassium tert‐butoxide in THF, 18‐crown‐6 also plays key roles in suppressing the reverse protonation or 1, 3‐proton shift isomerization as well as by stabilizing the carboxylated intermediate.     

Highly Regioselective Palladium‐Catalyzed Carboxylation of Allylic Alcohols with CO2

Various allylic alcohols were carboxylated in the presence of a catalytic amount of PdCl₂ and PPh₃ using ZnEt₂ as a stoichiometric transmetalation agent under a CO₂ atmosphere (1 atm). This carboxylation proceeded in a highly regioselective manner to afford branched carboxylic acids predominantly. The β,γ‐unsaturated carboxylic acid thus obtained was successfully converted into an optically active γ‐butyrolactone, a known intermediate of (R)‐baclofen.     

Direct Assembly of 2‐Oxazolidinones by Chemical Fixation of Carbon Dioxide

The reaction of β‐ and γ‐haloamines with carbon dioxide to give pharmaceutically relevant 2‐oxazolidinones and 1,3‐dioxazin‐2‐ones, was found to proceed efficiently in the presence of a base and in the absence of catalyst. After optimization of reaction conditions, the system was successfully expanded to a variety of haloamines, even at multigram scale. The reaction was further studied in silico by DFT calculations.     

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.     

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.     

Dinitrosyl Iron Complex [K‐18‐crown‐6‐ether][(NO)2Fe(MePyrCO2)]: Intermediate for Capture and Reduction of Carbon Dioxide

Continued efforts are made for the utilization of CO₂ as a C₁ feedstock for regeneration of valuable chemicals and fuels. Mechanistic study of molecular (electro‐/photo‐)catalysts disclosed that initial step for CO₂ activation involves either nucleophilic insertion or direct reduction of CO₂. In this study, nucleophilic activation of CO₂ by complex [(NO)₂Fe(μ‐^MePyr)₂Fe(NO)₂]^2? ( 2 , ^MePyr=3‐methylpyrazolate) results in the formation of CO₂‐captured complex [(NO)₂Fe(^MePyrCO₂)]^? ( 2‐CO₂ , ^MePyrCO₂=3‐methyl‐pyrazole‐1‐carboxylate). Single‐crystal structure, spectroscopic, reactivity, and computational study unravels 2‐CO₂ as a unique intermediate for reductive transformation of CO₂ promoted by Ca²⁺. Moreover, sequential reaction of 2 with CO₂, Ca(OTf)₂, and KC₈ established a synthetic cycle, 2 → 2‐CO₂ → [(NO)₂Fe(μ‐^MePyr)₂Fe(NO)₂] ( 1 ) → 2 , for selective conversion of CO₂ into oxalate. Presumably, characterization of the unprecedented intermediate 2‐CO₂ may open an avenue for systematic evaluation of the effects of alternative Lewis acids on reduction of CO₂.     

Nickel‐Promoted Highly Regioselective Carboxylation of Aryl Ynol Ether and Its Application to the Synthesis of Chiral β‐Aryloxypropionic Acid Derivatives

Nickel(0)‐promoted carboxylation of aryl ynol ether proceeded in a highly regioselective manner to produce α‐substituted‐β‐aryloxyacrylic acid derivatives. The α‐substituted‐β‐aryloxyacrylic acids were transformed into the corresponding β‐aryloxypropionic acid derivative as an optically active form via rhodium‐catalyzed asymmetric hydrogenation.     

N‐Heterocyclic Olefin–Carbon Dioxide and –Sulfur Dioxide Adducts: Structures and Interesting Reactivity Patterns

Depending on the amount of methanol present in solution, CO₂ adducts of N‐heterocyclic carbenes (NHCs) and N‐heterocyclic olefins (NHOs) have been found to be in fully reversible equilibrium with the corresponding methyl carbonate salts [EMIm][OCO₂Me] and [EMMIm][OCO₂Me]. The reactivity pattern of representative 1‐ethyl‐3‐methyl‐NHO–CO₂ adduct 4 has been investigated and compared with the corresponding NHC–CO₂ zwitterion: The protonation of 4 with HX led to the imidazolium salts [NHO–CO₂H][X], which underwent decarboxylation to [EMMIm][X] in the presence of nucleophilic catalysts. NHO–CO₂ zwitterion 4 can act as an efficient carboxylating agent towards CH acids such as acetonitrile. The [EMMIm] cyanoacetate and [EMMIm]₂ cyanomalonate salts formed exemplify the first C?C bond‐forming carboxylation reactions with NHO‐activated CO₂. The reaction of the free NHO with dimethyl carbonate selectively led to methoxycarbonylated NHO, which is a perfect precursor for the synthesis of functionalized ILs [NHO–CO₂Me][X]. The first NHO‐SO₂ adduct was synthesized and structurally characterized; it showed a similar reactivity pattern, which allowed the synthesis of imidazolium methyl sulfites upon reaction with methanol.     

二氧化碳与不饱和烃的还原羧化反应

过渡金属催化CO₂参与的不饱和烃还原羧化反应是合成羧酸及丙烯酸类化合物的重要途径, 具有重要的研究价值和工业应用潜力．过渡金属试剂与不饱和烃、CO₂生成稳定的金属杂环内酯或金属羧酸盐．还原剂能够与金属杂环内酯或金属羧酸盐发生转金属作用, 重新生成活泼催化剂, 从而实现催化剂的循环利用．本文总结了还原剂, 包括有机金属试剂、硅烷、硼烷、金属粉末、甲醇和氢气等在不饱和烃与CO₂的还原羧化反应中的应用, 并着重描述其反应特点和反应机理.     

Silver‐Catalyzed Three‐Component Coupling of Carbon Dioxide,Amines and α‐Diazoesters

A silver‐catalyzed three‐component coupling reaction of carbon dioxide, amines and α‐diazoesters has been developed for the first time. The novel reaction provides an efficient and practical methodology for the synthesis of a wide range of new α‐carbamoyloxy esters, which are difficult to prepare by existing methods. The advantages of the method include the use of readily available starting materials, simple catalytic system, good atom economy and high functional group tolerance.     

Bifunctional Catalysts Based on m‐Phenylene‐Bridged Porphyrin Dimer and Trimer Platforms: Synthesis of Cyclic Carbonates from Carbon Dioxide and Epoxides

Highly active bifunctional diporphyrin and triporphyrin catalysts were synthesized through Stille coupling reactions. As compared with a porphyrin monomer, both exhibited improved catalytic activities for the reaction of CO₂ with epoxides to form cyclic carbonates, because of the multiple catalytic sites which cooperatively activate the epoxide. Catalytic activities were carefully investigated by controlling temperature, reaction time, and catalyst loading, and very high turnover number and turnover frequency were obtained: 220 000 and 46 000 h^?1, respectively, for the magnesium catalyst, and 310 000 and 40 000 h^?1, respectively, for the zinc catalyst. Results obtained with a zinc/free‐base hybrid diporphyrin catalyst demonstrated that the Br^? ions on the adjacent porphyrin moiety also function as nucleophiles.     

Highly Efficient CO2 Electroreduction on ZnN4‐based Single‐Atom Catalyst

The electrochemical reduction reaction of carbon dioxide (CO2RR) to carbon monoxide (CO) is the basis for the further synthesis of more complex carbon‐based fuels or attractive feedstock. Single‐atom catalysts have unique electronic and geometric structures with respect to their bulk counterparts, thus exhibiting unexpected catalytic activities. A nitrogen‐anchored Zn single‐atom catalyst is presented for CO formation from CO2RR with high catalytic activity (onset overpotential down to 24 mV), high selectivity (Faradaic efficiency for CO (FE_CO) up to 95 % at ?0.43 V), remarkable durability (>75 h without decay of FE_CO), and large turnover frequency (TOF, up to 9969 h^?1). Further experimental and DFT results indicate that the four‐nitrogen‐anchored Zn single atom (Zn‐N₄) is the main active site for CO2RR with low free energy barrier for the formation of *COOH as the rate‐limiting step.     

Mechanism of the Cycloaddition of Carbon Dioxide and Epoxides Catalyzed by Cobalt‐Substituted 12‐Tungstenphosphate

Co^II‐substituted α‐Keggin‐type 12‐tungstenphosphate [(n‐ C₄H₉)₄N]₄H[PW₁₁Co(H₂O)O₃₉]‐ (PW₁₁Co) is synthesized and used as a single‐component, solvent‐free catalyst in the cycloaddition reaction of CO₂ and epoxides to form cyclic carbonates. The mechanism of the cycloaddition reaction is investigated using DFT calculations, which provides the first computational study of the catalytic cycle of polyoxometalate‐catalyzed CO₂ coupling reactions. The reaction occurs through a single‐electron transfer from the doublet Co^II catalyst to the epoxide and forms a doublet Co^III–carbon radical intermediate. Subsequent CO₂ addition forms the cyclic carbonate product. The existence of radical intermediates is supported by free‐radical termination experiments. Finally, it is exhilarating to observe that the calculated overall reaction barrier (30.5 kcal mol^?1) is in good agreement with the real reaction rate (83 h^?1) determined in the present experiments (at 150 °C).     

Temperature‐Responsive Microgel Films as Reversible Carbon Dioxide Absorbents in Wet Environment

Hydrogel films composed of temperature‐responsive microgel particles (GPs) containing amine groups work as stimuli‐responsive carbon dioxide absorbent with a high capacity of approximately 1.7 mmol g^?1. Although the dried films did not show significant absorption, the reversible absorption capacity dramatically increased by adding a small amount of water (1 mL g^?1). The absorption capacity was independent of the amount of added water beyond 1 mL g^?1, demonstrating that the GP films can readily be used under wet conditions. The amount of CO₂ absorbed by the GP films was proportional to their thickness up to 200–300 μm (maximum capacity of about 2 L m^?2). Furthermore, the films consisting of GPs showed faster and greater absorption and desorption of CO₂ than that of monolithic hydrogel films. These results indicated the importance of a fast stimulus response rate of the films that are composed of GPs in order to achieve long‐range and fast diffusion of bicarbonate ions. Our study revealed the potential of stimuli‐responsive GP films as energy‐efficient absorbents to sequester CO₂ from high‐humidity exhaust gases.