Chemical fixation of carbon dioxide to cyclic carbonates under extremely mild conditions with highly active bifunctional catalysts

Chemical fixation of carbon dioxide to cyclic carbonates proceeds effectively under extremely mild temperature and pressure by using a bifunctional nucleophile–electrophile catalyst system of tetradentate Schiff-base aluminum complexes ((Salen)AlX) in conjunction with a quaternary ammonium salt (n-Bu₄NY) in the absence of any organic solvent. Electrophilicity of central Al³⁺ ion and the steric factor of substituent groups on the aromatic rings of (Salen)AlX (electrophile), and nucleophilicity and leaving ability of the anion Y⁻ of n-Bu₄NY (nucleophile) have a great effect on the catalytic activity of the bifunctional catalyst.     

Quaternary ammonium salt-catalyzed carboxylative cyclization of propargylic amines with CO2

By employing quaternary ammonium salts as catalysts, the carboxylative cyclization of the propargylic amines with CO₂ proceeded to afford the corresponding 2-oxazolidinones. In particular, tetra-n-butylammonium fluoride was the most effective catalyst for the reaction, providing a 2-oxazolidinone derivative in a maximum chemical yield of 99%. From a screening of the structure of a catalyst, it was found that both a quaternary ammonium cation and a basicity of the counter anion were essential to catalyze the carboxylative cyclization of the propargylic amines with CO₂.     

Palladium-catalyzed carboxylative cyclization of α-allenyl amines in dense carbon dioxide

Carboxylative transformation of 2,3-allenic amines into 5-vinyl-1,3-oxazolidin-2-ones was promoted by palladium catalysts under a pressurized CO₂ condition. The presence of a stereogenic center adjacent to the allene group resulted in the formation of the corresponding cyclic urethane as a single trans-diastereomer.     

Aqueous media carboxylative cyclization of propargylic amines with CO2 catalyzed by amphiphilic dendritic N-heterocyclic carbene–gold(I) complexes

We prepared amphiphilic dendritic N-heterocyclic carbene (NHC)–gold(I) complexes having poly(ethylene glycol) units at the peripheral layer. By employing 1 mol % of the dendritic NHC–gold(I) catalyst, the aqueous media carboxylative cyclization of propargylic amines proceeded smoothly to provide the corresponding 2-oxazolidinone under atmospheric pressure of CO₂ at room temperature.     

Nickel-mediated carboxylative cyclization of enynes

Nickel-mediated carboxylative cyclization of α,ω-enyne using carbon dioxide was investigated. Oxidative cycloaddition of enynes having an electron withdrawing group on alkene to a zero-valent nickel complex smoothly proceeded to provide nickelacyclopentene intermediates, which regioselectively reacted with CO₂ at the Csp³-nickel bond, giving cyclized carboxylation products in good yields.     

Heterogeneous catalysts for cyclic carbonate synthesis from carbon dioxide and epoxides

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Convenient synthesis of ethylene carbonates from carbon dioxide and 1,2-diols at atmospheric pressure of carbon dioxide

An efficient and convenient synthesis of ethylene carbonates was achieved by the reaction of carbon dioxide with 1,2-diols in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), followed by treatment with 1-bromobutane. This DBU-promoted transformation proceeded at an atmospheric pressure of carbon dioxide at 25 °C and gave ethylene carbonates in good yields.     

Titanocene-catalyzed metallation of propargylic acetates in homopropargyl alcohol synthesis

The titanium-catalyzed metallation and subsequent carbonyl addition of propargylic acetates enable the direct formation of homopropargylic alcohols in good yields. The corresponding products were obtained as single regioisomers without the corresponding allene adducts observed.     

Synthesis of ene-allenes via palladium-catalyzed hydride-transfer reaction of propargylic amines under mild conditions

The palladium-catalyzed allene transformation reaction from propargylic amines proceeded in the presence of Pd₂(dba)₃·CHCl₃ (5 mol %) and (C₆F₅)₂PC₂H₄P(C₆F₅)₂ (10 mol %) in CHCl₃ at room temperature to give the corresponding allenes in good to high yields. Dicyclohexyl groups substituted on the nitrogen of propargylic amines were found to be effective for the current transformation and the conjugated ene-allenes 4 were synthesized from the corresponding propargylic amines 3 under mild conditions.     

Cyclic amidine hydroiodide for the synthesis of cyclic carbonates and cyclic dithiocarbonates from carbon dioxide or carbon disulfide under mild conditions

Hydroiodides of amidines can catalyze the reaction of carbon dioxide and epoxides under mild conditions such as ordinary pressure and ambient temperature, and the corresponding five-membered cyclic carbonates were obtained in high yields. The reaction of epoxide with carbon disulfide was also examined under the same conditions. Detailed investigation showed that the catalytic activity was highly affected by the counter anions of the amidine salts; the iodides were effective catalysts for both of the reaction of epoxide with carbon dioxide and carbon disulfide, whereas the bromide, chloride and fluoride counterparts exhibited almost no catalysis.     

离子液体中CO_2与炔醇在温和条件下高效合成-亚甲基环状碳酸酯

本文研究了离子液体1-乙基-3-甲基咪唑二乙基磷酸酯([EMIM][(EtO)2PO2])中醋酸银(AgOAc)对CO2与炔醇反应生成-亚甲基环状碳酸酯的催化性能,发现该反应可以在温和的条件下进行.在AgOAc/[EMIM][(EtO)2PO2]催化体系中,30℃、4 MPa CO2压力下,反应6h时-亚甲基环状碳酸酯的产率为97%;当压力降到0.5 MPa时,反应24 h产物产率为65.8%.[EMIM][(EtO)2PO2]在反应中起到了溶剂和碱的双重作用.AgOAc/[EMIM][(EtO)2PO2]体系重复利用3次活性仅略有下降.另外,该体系也可用于催化CO2与其他炔醇(3-甲基-1-戊炔-3-醇、3,5-二甲基-1-己炔-3-醇和2-苯基-3-丁炔-2-醇)的反应.     

酞菁锌高效催化以二氧化碳、含氢硅烷和有机胺为原料合成甲酰胺类衍生物

目前为了有效地利用好CO2,主要策略有以下几种:(1)"水平途径"——无价态及能量变化,譬如生成尿素、环状碳酸酯、聚碳酸酯及噁唑烷酮类衍生物等;(2)"垂直途径"——有价态及能量变化,譬如直接加氢转化成碳一产品(甲酸、甲醛、甲醇、甲烷)等;(3)"对角线途径"——有价态及能量变化,即结合石油化工原料将CO2还原生成醇、醚、羧酸、亚胺、酰胺、酯等系列高附加值的精细有机化工产品.其中以二氧化碳和含氢硅烷为原料,通过有机胺的N-甲酰化反应合成甲酰胺类衍生物符合绿色化学和可持续发展的要求.基于仿生催化CO2分子活化的基本理论,我们借鉴强极性的有机溶剂可有效活化硅氢键的性质,创新性地将廉价易得的酞菁锌(ZnPc)作为类酶催化剂,并以化学计量的N,N'-二甲基甲酰胺(DMF)为添加剂,构成组分新颖并高效绿色的类酶协同催化体系,实现了在温和反应条件下高效高选择性地合成甲酰胺类衍生物.研究发现:以苯硅烷作为还原剂,当加入0.5 mol%ZnPc和2 mmol DMF,在25℃和0.5 MPa下仅需反应6 h,可得到收率为99%的N-甲基甲酰苯胺.更值得注意的是,当以更易得的聚甲基氢硅烷(PMHS)为还原剂时,加入5 mol%ZnPc和1 mL DMF,在80℃和1 MPa下反应8 h,N-甲基甲酰苯胺的收率也高达99%.实验结果表明:添加剂DMF可以通过溶剂化和强极性作用高效活化含氢硅烷中的Si?H键,然后具有亲电性的金属活性中心能够稳定氢负离子生成高活性锌氢中间体.即ZnPc/DMF之间的协同催化作用能够促进氢化物从含氢硅烷转移到CO2分子,进而有利于CO2分子的高效活化.综上所述,利用类酶催化剂反应专一性的特点,通过有机胺的N-甲酰化反应,实现了以CO2和含氢硅烷为原料在温和条件下甲酰胺类衍生物的绿色高效合成.这对于设计和开发更加高效的催化体系具有一定的指导作用和借鉴意义.     

Efficient synthesis of di- and trisubstituted 2-aryloxazoles via ytterbium(III) triflate catalyzed cyclization of tertiary propargylic alcohols with aryl amides

An efficient synthetic method to prepare di- and trisubstituted 2-aryloxazoles based on Yb(OTf)₃ catalyzed cyclization of trisubstituted propargylic alcohols with aryl amides is described. The reaction was accomplished in moderate to excellent product yields and with complete regioselective control. The mechanism is suggested to involve activation of the starting alcohol by the metal catalyst that results in its ionization. Subsequent cyclization of this newly generated carbocationic species with the aryl amide then affords the oxazole. In view of the mild conditions along with the low cost, commercially availability of Yb(OTf)₃ and its high tolerance to air and moisture, the present synthetic approach offers an operationally simplistic and convenient route to this important aromatic heterocycle.     

Electrochemical carboxylation with carbon dioxide

Studies carried out in the past two years on electrochemical fixation of carbon dioxide with carbon-carbon bond formation, so-called electrochemical carboxylation or electrocarboxylation, are reviewed. Among about twenty papers on electrochemical carboxylation published from 2014 to the present, recent advances in electrochemical carboxylation regarding asymmetric carboxylation, sacrificial anode-free carboxylation, and carboxylation following aryl radical cyclization are focused on and discussed.     

Cyclic and linear amidine catalysts for the efficient synthesis of cyclic trithiocarbonates from carbon disulfide and episulfides under mild conditions

Cyclic and linear amidines effectively catalyzed the reaction of carbon disulfide and episulfides under mild conditions, such as ordinary pressure and ambient temperature, to give the corresponding cyclic trithiocarbonates in high yields.     

Calcium phosphate-vanadate apatite (CPVAP)-catalyzed aerobic oxidation of propargylic alcohols with molecular oxygen

Calcium phosphate-vanadate apatite (CPVAP) works effectively as a catalyst for the aerobic oxidation of propargylic alcohols to the corresponding carbonyl compounds under an atmospheric pressure of molecular oxygen. Moreover, CPVAP can be readily separated by filtration and reused at least 10 times without appreciable loss of the catalytic activity.     

Enzymatic resolution of trans-2-hydroxycyclohexanecarbonitrile in supercritical carbon dioxide

A novel, highly enantioselective (E ? 100) and environmentally benign method is presented for the kinetic resolution of trans-2-hydroxycyclohexanecarbonitrile in supercritical carbon dioxide. Using Candida antarctica Lipase B as a biocatalyst and vinyl acetate as an acyl donor, enantiomerically pure (1S,2R)-acetoxycyclohexanecarbonitrile and (1R,2S)-hydroxycyclohexanecarbonitrile were obtained in quantitative yields and excellent ee (98%) values.     

N-Halo succinimide-promoted one-pot three-component electrophilic cyclization: Facile access to dihalogenated heterocycles under mild conditions

An effective and concise electrophilic cyclization of 1,4-butynediols, 4-aminobut-2-yn-1-ols, pent-2-yne-1,5-diols, 2-propynolphenols and 2-propynolanilines with N-halo succinimides and trimethylsilyl halides opens a one-pot synthetic access to five- and six-membered dihalogenated heterocycles in moderate to good yields. The optimized cyclization conditions are tolerated with many functional groups.     

One‐pot carboxylative cyclization of propargylic alcohols and CO2 catalysed by N‐heterocyclic carbene/Ag systems

A series of N ‐heterocyclic carbene (NHC)/Ag systems were developed for the carboxylative assembly of propargylic alcohols and carbon dioxide (CO₂). With the catalysis of these catalytic systems, a variety of target α‐alkylidene cyclic carbonates could be obtained smoothly under atmospheric CO₂ pressure in straightforward one‐pot processes. Particularly, these reactions could be performed without any stoichiometric addition of bases or additives. Further mechanistic investigation reveals that the excellent activities are attributed to the effective activations of CO₂ accomplished by the NHCs via the formation of the NHC‐CO₂ adducts.     

Development in the green synthesis of cyclic carbonate from carbon dioxide using ionic liquids

This brief review presents the recent development in the synthesis of cyclic carbonate from carbon dioxide (CO₂) using ionic liquids as catalyst and/or reaction medium. The synthesis of cyclic carbonate includes three aspects: catalytic reaction of CO₂ and epoxide, electrochemical reaction of CO₂ and epoxide, and oxidative carboxylation of olefin. Some ionic liquids are suitable catalysts and/or solvents to the CO₂ fixation to produce cyclic carbonate. The activity of ionic liquid is greatly enhanced by the addition of Lewis acidic compounds of metal halides or metal complexes that have no or low activity by themselves. Using ionic liquids for the electrochemical synthesis of the cyclic carbonate can avoid harmful organic solvents, supporting electrolytes and catalysts, which are necessary for conventional electrochemical reaction systems. Although the ionic liquid is better for the oxidative carboxylation of olefin than the ordinary catalysts reported previously, this reaction system is at a preliminary stage. Using the ionic liquids, the synthesis process will become greener and simpler because of easy product separation and catalyst recycling and unnecessary use of volatile and harmful organic solvents.