Recent advances in organic synthesis with CO2 as C1 synthon

Current Opinion in Green and Sustainable Chemistry 3 (2017) 22e27

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Review article

Recent advances in organic synthesis with CO2 as C1 synthon Gaoqing Yuan, Caorong Qi, Wanqing Wu, Huanfeng Jiang* Key Laboratory of Functional Molecular Engineering of Guangdong Province, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 19 October 2016 Received in revised form 2 November 2016 Accepted 24 November 2016 Available online 1 December 2016

Carbon dioxide as a green, nontoxic and low-cost C1 synthon holds great potential in organic synthesis. In this paper, the recent studies of conversion of CO2 with organic compounds (including alkenes, alkynes, amines, allylamines, propenyl ketones, propargylic alcohols/amines, arenes, and heteroarenes as well as their derivatives) to value-added chemicals are briefly reviewed. © 2016 Elsevier B.V. All rights reserved.

1. Introduction

2. Carbonylation of CO2 with alkenes

In the last century, the concentration of carbon dioxide (CO2) has obviously increased in the atmosphere. The continuous accumulation of CO2 as a greenhouse gas will lead to the global environmental problem. From the viewpoint of chemistry, carbon dioxide practically is nontoxic and low-cost C1 source, and holds great potential value as a C1 building block for carbon carbon bond formation and carbon heteroatom functionalization reactions in chemical synthesis. Using carbon dioxide as C1 raw material to synthesize useful chemicals has become an active research field. Nowadays, the industrial examples by the use of CO2 mainly include the synthesis of urea, cyclic carbonates and salicylic acid, etc. But only depending on these industrial processes, the utilization of CO2 is still very limited. Therefore, it is highly necessary to develop new and efficient routes for the conversion of CO2. Despite of CO2 thermodynamically being inert and stable, CO2 with organic compounds could be converted to value-added chemicals, such as carboxylic acids, esters, cyclic carbonates, formamides, carbamates, oxazolidinedione derivatives and so on. Significant progress has been made in the organic synthesis based on CO2 activation in the past decades. This review will exclusively focus on the recent advances in the conversion of CO2 with organic compounds in the past two years. The synthesis of cyclic carbonates from epoxides and CO2 has been extensively reviewed recently [1] and so will not be covered in this review.

The carbonylation of alkenes with CO2 represents a major technology for the production of bulk chemicals and fine chemical products. Nowadays, all industrial carbonylation processes make use of highly toxic carbon monoxide CO. Recently, Beller's group successfully achieved the methoxycarbonylation of alkenes 1 with CO2 and methanol (MeOH) using ruthenium as the catalyst (Scheme 1a) [2]. Notably, in this system CO2 works much better than the traditional combination of CO and alcohols. With the same catalyst, the methoxycarbonylation of cyclohexene 3 with CO2 and MeOH was realized in a micro flow system under supercritical conditions (Scheme 1b) [3]. The yield of the ester product 4 reached 77% at 180  C, 12 MPa and with a 90 min residence time. Xi et al. reported that zirconocene could catalyze ethylcarboxylation of alkenes with ethylmagnesium chloride and carbon dioxide. a-Aryl carboxylic acids 5 were obtained with styrene and its derivatives as the substrate, while the reaction with aliphatic alkenes afforded alkanoic acids 6 (Scheme 1c) [4]. Differing from Xi's work, Tanaka's investigation indicated that the reaction of a-arylalkenes 7 or trialkyl-substituted alkenes 8 with CO2 in the presence of EtAlCl2 and 2,6-dibromopyridine (base) could afford the corresponding a,b-and/or b,g-unsaturated carboxylic acids (9 and 10, Scheme 1d) [5]. This reaction may undergo the electrophilic substitution of EtAlCl2 with the aid of the base, followed by the carbonation of the resulting ate complex. In addition, our group successfully achieved the direct conversion of CO2 with olefins and water into cyclic carbonates 11 via a synergistic action of halogen I2 and base NH3 electrochemically generated in situ (Scheme 1e) [6].

* Corresponding author. E-mail address: [email protected] (H. Jiang). http://dx.doi.org/10.1016/j.cogsc.2016.11.006 2452-2236/© 2016 Elsevier B.V. All rights reserved.

G. Yuan et al. / Current Opinion in Green and Sustainable Chemistry 3 (2017) 22e27

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4. Carboxylation of CO2 with alkynes The carboxylation of alkyne with CO2 is one of efficient routes to utilize CO2. In the past two years, some new catalytic systems were reported. For example, Nd-based catalyst was proved to be efficient for this reaction [10] (Scheme 3a). He's group also verified that silver tungstate was a bifunctional catalyst which could activate terminal alkynes and CO2 simultaneously so that the carboxylation could be easily carried out to afford the propiolic acids or esters 20 in good to excellent yields under atmospheric pressure of CO2 at room temperature [11] (Scheme 3b). Fujihara and Tsuj et al. have developed a cobalt-based catalyst for the conversion of interminal alkynes with CO2 in the presence of Zn powder (1.5 equiv.) [12]. In this reaction process, regio- and stereodefined (Z)-b-zincated acrylates are first formed, and subsequent reaction with electrophiles, and finally acidified by HCl aqueous solution to afford the corresponding multi-substituted acrylic aids 22 (Scheme 3c). In addition, they found that cobalt could also catalyze the carboxylation of propargyl acetates 23 with CO2 [13] (Scheme 3d). The reaction proceeds at room temperature in the presence of Mn powder (3 equiv.) as a reductant, to give various carboxylic acids 24 in good to high yields. However, the use of a stoichiometric amount of metal powders may restrict their industrial application further. 5. N-formylation of CO2 with amines Scheme 1. Carbonylation of CO2 with alkenes.

3. Conversion of CO2 with allylamines or propenyl ketones Yu's group has developed a novel copper-catalyzed difunctionalization of allylamines 12 with CO2 and Togni's reagent II to generate important CF3-containing 2-oxazolidones 13 with excellent chemoselectivity, regioselectivity, and diastereoselectivity under mild conditions (room temperature and closed 0.1 MPa) [7] (Scheme 2a). Without transition-metal catalysts, Yu and coworkers found that the lactamization of 2-alkenylanilines 14 with CO2 could afford 2-quinolinones 15 in moderate to excellent yields in the presence of t-BuONa (4.5 equiv.) [8] (Scheme 2b). In addition, Zhang et al. reported that substituted 1-propenyl ketones 16 with CO2 (3.0 MPa) could be smoothly converted to useful a-pyrone compounds 17 in the presence of CsF (4.0 equiv.) at 100  C but a high pressure of carbon dioxide was required [9] (Scheme 2c).

Scheme 2. Conversion of CO2 with allylamines or propenyl ketones.

Formamides are important solvents and intermediates, which are currently produced by using CO in industry. Using CO2 to replace CO as C1 materials, it is an interesting topic for the formylation of amines to produce formamides in the presence of reductants. In the recent two years, various catalytic systems have been developed for this reaction (Table 1), including Rh [14], Pd [15], Cu [16], Fe [17,18], Zn [19], N-heterocyclic carbenes (NHC) [20] and thiazolium carbenes [21], ionic liquids (ILs) [22], biomassderived g-valerolactone (GVL) [23], amine modified mesoporous Al2O3@MCM-41 [24], and catalyst-free system [25]. For Rh complex catalyst, the turnover frequency (TOF) reached 237.5 h 1 (Table 1). This TOF value is much higher than those of the other catalysts. In addition, in the presence of a simple CuAlOx catalyst, amines with

Scheme 3. Carboxylation of CO2 with alkynes.

24

G. Yuan et al. / Current Opinion in Green and Sustainable Chemistry 3 (2017) 22e27

Table 1 Various catalysts for the N-formylation of CO2 with amines.

Ref.

Catalyst

R1R2NH

Reductant

P (MPa)

T ( C)

t (h)

Yield (%)

TOFa (h

[14]

Rh complex

Ph2SiH2

2.5

25

4e24

30e99

237.5

[15] [16] [17]

Pd/Al2O3 Cu complex Fe(acac)2/PP3

H2 PMHS PhSiH3

1 1 0.1

130 80 rt

24 3 18

10e96 41 24e95

0.074 1.421 1.06

[18] [19] [20] [21]

Fe(H)2(dmpe)2 F-PNHC-Znc NHC Thiazolium carbenes

9-BBNb PhSiH3 PhSiH3 PMHSd

0.1 1 0.1 0.1

rt 80 rt 50

1 24 24 15e24

83 61e89 99 53e90

83 0.592 0.825 0.844

[22]

[BMIM]Cl

PhSiH3

1

30

5

60e99

0.186

[23]

GVL

PhSiH3

3

80

2e24

31e98

e

[24]

Al2O3@MCM-41

DMABe

2

100

6

45e99

1.1

[25]

Catalyst-free

R1: H/alkyl/phenyl R2: alkyl/phenyl R1/R2: alkyl/phenyl piperidine R1: H/alkyl/phenyl R2: alkyl/phenyl ArNH2 R1/R2: phenyl Et2NH R1: H R2: alkyl/phenyl R1: H/alkyl/phenyl R2: alkyl/phenyl R1: H/alkyl/phenyl R2: alkyl/phenyl R1: H/alkyl/phenyl R2: alkyl/phenyl R1: H/alkyl/phenyl R2: alkyl/phenyl

PhSiH3

0.1

rt

24

51e99

e

a b c d e

1

)

Bsed on the model reaction. 9-BBN: 9-borabicyclo[3.3.1]-nonane. F-PNHC-Zn: fluoro-functionalized polymeric N-heterocyclic carbene (NHC)-Zn complex. PMHS: Polymethylhydrosiloxane. DMAB: dimethylamine-borane.

CO2 and H2 can be further transformed into the corresponding Nmethyl or N,N-dimethyl products with good to excellent yields [26]. 6. Reaction of CO2 with propargylic alcohols/propargylic amines Reaction of CO2 with propargylic alcohols can efficiently yield important a-alkylidene cyclic carbonates 31, which has been extensively studied. To date, different catalytic systems have been developed. For Ag-based catalysts, the related investigations were reviewed by Yamada et al. [27]. Recently, Our group's work demonstrated that in the presence of N,N-diisopropylethylamine

Scheme 4. Reaction of CO2 with propargylic alcohols.

(DIPEA), copper iodide is a very efficient catalyst for this reaction (Scheme 4a) [28]. Interestingly, the chemo- and stereoselective (E)a-iodoalkylidene cyclic carbonates 32 could be obtained in the presence of potassium iodide KI with Cu(OTf)2 as the promoter (Scheme 4b). The process is proposed to proceed through the trapping of the vinyl copper intermediate by in situ generated triiodide ion as electrophile. In addition, a ZnI2/NEt3 catalyst system developed by Han's group could efficiently promote the reaction at room temperature under solvent-free conditions, to afford aalkylidene cyclic carbonates 31 in excellent yields (Scheme 4c) [29]. The ZnI2 and NEt3 play synergistic roles in activating both CO2 and propargylic alcohols. Alkoxide-functionalized imidazolium betaines (AFIBs) are also efficient catalysts for this reaction [30] (Scheme 4d). Han and co-workers reported an effective protic ionic liquid 1,8diazabicyclo-[5.4.0]-7-undecenium 2-methylimidazolide [DBUH]

Scheme 5. Reaction of CO2 with propargylic amines.

G. Yuan et al. / Current Opinion in Green and Sustainable Chemistry 3 (2017) 22e27

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Scheme 8. Other reactions.

Scheme 6. Three-component coupling reaction related with CO2.

[MIm] as both a non-metal catalyst and solvent for the carboxylative cyclization of propargylic amines 33 with CO2 (Scheme 5a) [31]. They demonstrated that the activation of the amino group is crucial and the intramolecular cyclization is the rate-determining step. Their investigation further indicated that the ionic liquid (IL) 1-butyl-3-methylimidazolium acetate ([Bmim][OAc]) and 1-butyl3-methylimidazolium bis((trifluoromethyl)sulfonyl)imide ([Bmim] [Tf2N]) could synergisticly catalyze the reaction of CO2 with propargylic amines to produce 3,4,5-trisubstituted oxazolones 35 (Scheme 5b) [32]. He and coworkers have developed copper(II)

substituted polyoxometalate-based ionic liquid [(nC7H15)4N]6[aSiW11O39Cu] as a halogen-free bifunctional catalyst for the cyclization of propargylic amines with CO2 [33]. The reaction proceeded smoothly at atmospheric pressure under solvent-free conditions (Scheme 5c). 7. Three-component coupling reaction related with CO2 Three-component coupling reaction of CO2, amines with some organic compounds is interesting and will further extend the application scope of CO2. Recently, our group has developed a novel K2CO3 base-promoted the coupling reaction of CO2 (4 MPa), amines, and N-tosylhydrazones for the synthesis of organic carbamates 38 [34] (Scheme 6a). Further investigations made by Chung et al. demonstrated that this method could provide the desired carbamates under just 0.1 MPa of carbon dioxide without an additional base promoter [35] (Scheme 6b). In addition, the synthesis of O-aryl carbamates 40 could be realized through a threecomponent coupling of carbon dioxide, amines with diaryliodonium salts in the presence of 1,8-diazabicyclo[5.4.0]-undec-7ene (DBU) promoter [36] (Scheme 6c). Important oxazolidine-2,4diones 43 could be obtained via a phosphorus-mediated carboxylative condensation of primary amines and a-ketoesters with CO2, and a NaOMe-catalyzed cyclization sequence [37] (Scheme 6d). With Cu2O as the catalyst and Cs2CO3 as the base, various 2-bromo3-phenylacrylic acid derivatives with CO2 and amines could be transformed to the corresponding oxazolidinedione derivatives 45 in high yields [38] (Scheme 6e). The aluminium-based catalyst could effectively catalyze the coupling of (substituted) oxetanes, amines with CO2 to afford a variety of functionalized carbamates 47 with excellent chemoselectivity and good yields [39] (Scheme 6f). With Au/Fe2O3 as the catalyst, the reaction of epoxides with amines and CO2 could also be achieved [40] (Scheme 6g). 8. CeH bond activation: reaction of CO2 with heteroarenes or simple arenes

Scheme 7. Reaction of CO2 with heteroarenes and simple arenes.

The direct carboxylation reaction of heteroarenes or simple

26

G. Yuan et al. / Current Opinion in Green and Sustainable Chemistry 3 (2017) 22e27

Fig. 1. Summary of CO2 as C1 synthon in organic synthesis.

arenes with CO2 is a challenge topic due to low reactivity of CO2. Yoshino and coworkers have developed Cs2CO3 salt promoted CeH carboxylation followed by protonation to convert 2-furoic acid with CO2 into furan-2,5-dicarboxylic acid (FDCA)da highly desirable bio-based feedstock with numerous applications [41] (Scheme 7a). In addition, t-BuOK mediated CeH carboxylation of heteroarenes could be achieved at atmospheric pressure of CO2, to provide various heteroaromatic carboxylic acid derivatives in moderate to high yields [42] (Scheme 7b). Iwasawa et al. reported a rhodiumcatalyzed CeH bond activation method for direct carboxylation of simple arenes with CO2 (0.1 MPa) without the assistance of a directing group [43] (Scheme 7c). However, a stoichiometric amount of aluminum reagent is required in this process. 9. Other reactions With Pd/C [44] or the palladium nanoparticle embedded porous nitrogen doped carbon material (Pd@N-GMC) [45] as the catalyst, aryl iodides with CO2 could be transformed to aryl aldehydes or benzyl alcohols in the presence of reducing agent hydrosilanes (Scheme 8). Allenes could react with CO2 and hydrosilanes to selectively form homoallylic alcohols [46] (Scheme 8c). In addition, organoboronates or sodium sulfonates with CO2 could be converted to the corresponding carboxylic acids [47,48] (Scheme 8d and e). The recent progress made in the conversion of CO2 with organic compounds is briefly outlined in Fig. 1. 10. Conclusions In the past two years, some novel catalytic systems and methods have been developed for the effective conversion and utilization of CO2. The recent developments described in this review indicate

that CO2 as C1 synthon has appealing applications in organic synthesis. These investigations greatly extend the application scope of CO2. In some cases, the conversion of CO2 could be smoothly achieved under mild conditions (room temperature and atmospheric pressure of CO2), showing its industrial application prospects. Nevertheless, most of the reactions related with CO2 usually require high temperature, high pressure and a stoichiometric amount of additives. Thus, it is still challenge to develop sustainable catalytic systems and methods for the conversion of CO2. Acknowledgements This work was supported by the National Natural Science Foundation of China (21572070), and the National Key Research and Development Program of China (2016YFA0602900). References [1] J.W. Comerford, I.D.V. Ingram, M. North, X. Wu, Sustainable metal-based catalysts for the synthesis of cyclic carbonates containing five-membered rings, Green Chem. 17 (2015) 1966e1987. *[2] L. Wu, Q. Liu, I. Fleischer, R. Jackstell, M. Beller, Ruthenium-catalysed alkoxycarbonylation of alkenes with carbon dioxide, Nat. Commun. 5 (2014) 3091e3096. €l, Q. Wang, M. Beller, V. Hessel, Continuous ruthenium[3] S.C. Stouten, T. Noe catalyzed methoxycarbonylation with supercritical carbon dioxide, Catal. Sci. Technol. 6 (2016) 4712e4717. [4] P. Shao, S. Wang, C. Chen, C.J. Xi, Zirconocene-catalyzed sequential ethylcarboxylation of alkenes using ethylmagnesium chloride and carbon dioxide, Chem. Commun. 51 (2015) 6640e6642. [5] S. Tanaka, K. Watanabe, Y. Tanaka, T. Hattori, EtAlCl2/2,6-disubstituted pyridine-mediated carboxylation of alkenes with carbon dioxide, Org. Lett. 18 (2016) 2576e2579. *[6] X.F. Gao, G.Q. Yuan, H.J. Chen, H.F. Jiang, Y.W. Li, C.R. Qi, Efficient conversion of CO2 with olefins into cyclic carbonates via a synergistic action of I2 and base electrochemically generated in situ, Electrochem Commun. 34 (2013) 242e245.

G. Yuan et al. / Current Opinion in Green and Sustainable Chemistry 3 (2017) 22e27 *[7] J.H. Ye, L. Song, W.J. Zhou, T. Ju, Z.B. Yin, S.S. Yan, Z. Zhang, J. Li, D.G. Yu, Selective oxytrifluoromethylation of allylamines with CO2, Angew. Chem. Int. Ed. 55 (2016) 1e6. [8] Z. Zhang, L.L. Liao, S.S. Yan, L. Wang, Y.Q. He, J.H. Ye, J. Li, Y.G. Zhi, D.G. Yu, Lactamization of sp2 C-H bonds with CO2: transition metal-free and redoxneutral, Angew. Chem. Int. Ed. 55 (2016) 7068e7072. [9] W.Z. Zhang, M.W. Yang, X.B. Lu, Carboxylative cyclization of substituted propenyl ketones using CO2: transition-metal- free synthesis of a-pyrones, Green Chem. 18 (2016) 4181e4184. [10] H. Cheng, B. Zhao, Y.M. Yao, C.R. Lu, Carboxylation of terminal alkynes with CO2 catalyzed by bis(amidate) rare-earth metal amides, Green Chem. 17 (2015) 1675e1682. *[11] C.X. Guo, B. Yu, J.N. Xie, L.N. He, Silver tungstate: a single-component bifunctional catalyst for carboxylation of terminal alkynes with CO2 in ambient conditions, Green Chem. 17 (2015) 474e479. [12] K. Nogi, T. Fujihara, J. Terao, Y. Tsuji, Carboxyzincation employing carbon dioxide and zinc powder: cobalt-catalyzed multicomponent coupling reactions with alkynes, J. Am. Chem. Soc. 138 (2016) 5547e5550. [13] K. Nogi, T. Fujihara, J. Terao, Y. Tsuji, Cobalt-catalyzed carboxylation of propargyl acetates with carbon dioxide, Chem. Commun. 50 (2014) 13052e13055. [14] T.V.Q. Nguyen, W.J. Yoo, S. Kobayashi, Effective formylation of amines with carbon dioxide and diphenylsilane catalyzed by chelating bis(tz NHC) rhodium complexes, Angew. Chem. Int. Ed. 54 (2015) 9209e9212. [15] X.J. Cui, Y. Zhang, Y.Q. Deng, F. Shi, Amine formylation via carbon dioxide recycling catalyzed by a simple and efficient heterogeneous palladium catalyst, Chem. Commun. 50 (2014) 189e191. [16] K. Motokura, N. Takahashi, A. Miyaji, Y. Sakamoto, S. Yamaguchi, T. Baba, Mechanistic studies on the N-formylation of amines with CO2 and hydrosilane catalyzed by a Cu-diphosphine complex, Tetrahedron 70 (2014) 6951e6956. [17] X. Frogneux, O. Jacquet, T. Cantat, Iron-catalyzed hydrosilylation of CO2: CO2 conversion to formamides and methylamines, Catal. Sci. Technol. 4 (2014) 1529e1533. [18] G.H. Jin, C.G. Werncke, Y. Escudie, S. Sabo-Etienne, S. Bontemps, Iron-catalyzed reduction of CO2 into methylene: formation of C-N, C-O, and C-C bonds, J. Am. Chem. Soc. 137 (2015) 9563e9566. [19] Z.Z. Yang, B. Yu, H.Y. Zhang, Y.F. Zhao, G.P. Ji, Z.M. Liu, Fluoro-functionalized polymeric N-heterocyclic carbene-zinc complexes: efficient catalyst for formylation and methylation of amines with CO2 as a C1-building block, RSC Adv. 5 (2015) 19613e19619. [20] Q.H. Zhou, Y.X. Li, The real role of N-heterocyclic carbene in reductive functionalization of CO2: an alternative understanding from density functional theory study, J. Am. Chem. Soc. 137 (2015) 10182e10189. [21] S. Das, F.D. Bobbink, S. Bulut, M. Soudania, P.J. Dyson, Thiazolium carbene catalysts for the fixation of CO2 onto amines, Chem. Commun. 52 (2016) 2497e2500. [22] L.D. Hao, Y.F. Zhao, B. Yu, Z.Z. Yang, H.Y. Zhang, B.X. Han, X. Gao, Z.M. Liu, Imidazolium-based ionic liquids catalyzed formylation of amines using carbon dioxide and phenylsilane at room temperature, ACS Catal. 5 (2015) 4989e4993. [23] J.L. Song, B.W. Zhou, H.Z. Liu, C. Xie, Q.L. Meng, Z.R. Zhang, B.X. Han, Biomassderived g-valerolactone as an efficient solvent and catalyst for the transformation of CO2 to formamides, Green Chem. 18 (2016) 3956e3961. [24] D.B. Nale, D. Rath, K.M. Parida, A. Gajengi, B.M. Bhanage, Amine modified mesoporous Al2O3@MCM-41: an efficient, synergetic and recyclable catalyst for the formylation of amines using carbon dioxide and DMAB under mild reaction conditions, Catal. Sci. Technol. 6 (2016) 4872e4881. *[25] H. Lv, Q. Xing, C.T. Yue, Z.Q. Lei, F.W. Li, Solvent-promoted catalyst-free Nformylation of amines using carbon dioxide under ambient conditions, Chem. Commun. 52 (2016) 6545e6548. [26] X.J. Cui, X.C. Dai, Y. Zhang, Y.Q. Deng, F. Shi, Methylation of amines, nitrobenzenes and aromatic nitriles with carbon dioxide and molecular hydrogen, Chem. Sci. 5 (2014) 649e655. [27] K. Sekine, T. Yamada, Silver-catalyzed carboxylation, Chem. Soc. Rev. 45 (2016) 4524e4532. [28] L. Ouyang, X.D. Tang, H.T. He, C.R. Qi, W.F. Xiong, Y.W. Ren, H.F. Jiang, Copper-

[29]

[30]

[31]

[32]

*[33]

[34]

[35]

[36]

[37]

[38]

[39]

[40]

[41] [42] *[43]

[44]

[45]

[46]

[47] [48]

27

promoted coupling of carbon dioxide and propargylic alcohols: expansion of substrate scope and trapping of vinyl copper intermediate, Adv. Synth. Catal. 357 (2015) 2556e2565. J.Y. Hu, J. Ma, Q.G. Zhu, Q.L. Qian, H.L. Han, Q.Q. Mei, B.X. Han, Zinc(II)catalyzed reactions of carbon dioxide and propargylic alcohols to carbonates at room temperature, Green Chem. 18 (2016) 382e385. Y.B. Wang, D.S. Sun, H. Zhou, W.Z. Zhang, X.B. Lu, Alkoxide-functionalized imidazolium betaines for CO2 activation and catalytic transformation, Green Chem. 16 (2014) 2266e2272. J.Y. Hu, J. Ma, Q.G. Zhu, Z.F. Zhang, C.Y. Wu, B.X. Han, Transformation of atmospheric CO2 catalyzed by protic ionic liquids: efficient synthesis of 2oxazolidinones, Angew. Chem. Int. Ed. 54 (2015) 5399e5403. J.Y. Hu, J. Ma, Z.F. Zhang, Q.G. Zhu, H.C. Zhou, W.J. Lu, B.X. Han, A route to convert CO2: synthesis of 3,4,5-trisubstituted oxazolones, Green Chem. 17 (2015) 1219e1225. M.Y. Wang, Q.W. Song, R. Ma, J.N. Xie, L.N. He, Efficient conversion of carbon dioxide at atmospheric pressure to 2-oxazolidinones promoted by bifunctional Cu(II)-substituted polyoxometalate-based ionic liquids, Green Chem. 18 (2016) 282e287. W.F. Xiong, C.R. Qi, H.T. He, L. Ouyang, M. Zhang, H.F. Jiang, Base-promoted coupling of carbon dioxide, amines, and N-tosylhydrazones: a novel and versatile approach to carbamates, Angew. Chem. Int. Ed. 54 (2015) 3084e3087. J.Y. Hong, U.R. Seo, Y.K. Chung, Synthesis of carbamates from amines and Ntosylhydrazones under atmospheric pressure of carbon dioxide without an external base, Org. Chem. Front. 3 (2016) 764e767. W.F. Xiong, C.R. Qi, Y.B. Peng, T.Z. Guo, M. Zhang, H.F. Jiang, Base-promoted coupling of carbon dioxide, amines, and diaryliodonium salts: a phosgeneand metal-free route to O-aryl carbamates, Chem. Eur. J. 21 (2015) 14314e14318. W.Z. Zhang, T. Xia, X.T. Yang, X.B. Lu, Synthesis of oxazolidine-2,4-diones by a tandem phosphorus-mediated carboxylative condensationecyclization reaction using atmospheric carbon dioxide, Chem. Commun. 51 (2015) 6175e6178. S. Sharma, A.K. Singh, D. Singh, D.P. Kim, Chemical fixation of carbon dioxide by copper catalyzed multicomponent reactions for oxazolidinedione syntheses, Green Chem. 17 (2015) 1404e1407. W.S. Guo, V. Laserna, J. Rintjema, A.W. Kleij, Catalytic one-pot oxetane to carbamate conversions: formal synthesis of drug relevant molecules, Adv. Synth. Catal. 358 (2016) 1602e1607. J.P. Shang, X.G. Guo, Z.P. Lia, Y.Q. Den, CO2 activation and fixation: highly efficient syntheses of hydroxy carbamates over Au/Fe2O3, Green Chem. 18 (2016) 3082e3088. A. ** Banerjee, G.R. Dick, T. Yoshino, M.W. Kanan, Carbon dioxide utilization via carbonate-promoted CeH carboxylation, Nature 531 (2016) 215e219. S. Fenner, L. Ackermann, CeH carboxylation of heteroarenes with ambient CO2, Green Chem. 18 (2016) 3804e3807. T. Suga, H. Mizuno, J. Takaya, N. Iwasawa, Direct carboxylation of simple arenes with CO2 through a rhodium-catalyzed CeH bond activation, Chem. Commun. 50 (2014) 14360e14363. B. Yu, Y.F. Zhao, H.Y. Zhang, J.L. Xu, L.D. Hao, X. Gao, Z.M. Liu, Pd/C-catalyzed direct formylation of aromatic iodides to aryl aldehydes using carbon dioxide as a C1 resource, Chem. Commun. 50 (2014) 2330e2333. R.A. Molla, M.A. Iqubal, K. Ghosh, S.M. Islam, A route for direct transformation of aryl halides to benzyl alcohols via carbon dioxide fixation reaction catalyzed by a (Pd@N-GMC) palladium nanoparticle encapsulated nitrogen doped mesoporous carbon material, Green Chem. 18 (2016) 4649e4656. Y. Tani, K. Kuga, T. Fujihara, J. Terao, Y. Tsuji, Copper-catalyzed CeC bondforming transformation of CO2 to alcohol oxidation level: selective synthesis of homoallylic alcohols from allenes, CO2, and hydrosilanes, Chem. Commun. 51 (2015) 13020e13023. Y. Makida, E. Marelli, A.M.Z. Slawin, S.P. Nolan, Nickel-catalysed carboxylation of organoboronates, Chem. Commun. 50 (2014) 8010e8013. S. Sun, J.T. Yu, Y. Jiang, J. Cheng, Copper(I)-catalyzed desulfinative carboxylation of sodium sulfinates using carbon dioxide, Adv. Synth. Catal. 357 (2015) 2022e2026.