English

• The current global economic development has accelerated carbon emission, and the amount of CO₂ generated by human activity is estimated to be 35 billion tons in 2015.^[1] The relentless rise of atmospheric CO₂ is thought to be a main cause of global warming and abnormal climate changes. However, CO₂ is an attractive C₁ building block in synthetic chemistry and chemical industry because of its abundance, non-toxicity, and low cost. Therefore, conversion of CO₂ into valuable products will meet the demand of sustainable chemistry, although it earns a small proportion from carbon emission at the present stage. In this respect, a lot of research has been conducted on the development of new transformations using CO₂ as a C₁ source, including transition metal catalysis, photocatalysis, electrocatalysis, ionic liquids catalysis, etc.^[2-18] Industrial synthetic applications of CO₂ have also been realized in the synthesis of salicylic acid, urea, methanol, and polycarbonates.^[18-21]

  In general, reactive reagents or harsh conditions are needed in the transformation of CO₂ because of its thermodynamic stability and/or kinetic inertness. First, the thermodynamic stability of CO₂ imposes an input of energy to convert it into fuels or chemicals. The standard reduction potential (E_R⁰) is a measure of the spontaneity of a given reaction relative to the hydrogen evolution reaction (HER) as equation ΔG°=-nFE_rxn⁰. Due to the relative stability of gaseous CO₂ (ΔG_f⁰ =-394.4 kJ•mol^-1), enough energy must be introduced into the reaction to drive its transformation to the products. Second, catalysts are usually required to ensure that the activation barriers remain as low as possible in the presence of chemical transformation processes. Thus, the overall carbon balance for CO₂ utilization is not hampered by thermal loading needed to overcome high energy transition states.^[22-23] Traditional methods for the transformation of CO₂ generally need high-energy substrates and reagents, such as strong nucleophilic organometallic reagents (the Grignard reagents or organolithiums), small-ring heterocycles with high ring strain (epoxides or aziridines), and highly reactive reductants, etc.^[24-26] Physical energy such as light or electricity was also employed to solve the issues brought by thermodynamic stability of CO₂.^[27-28] However, these traditional methods usually require multi-steps, resulting in waste or undesirable by-products.

  Over the past decades, catalysts have been widely used to activate CO₂ and facilitate its transformations. A variety of transition-metal-catalyzed carboxylations of alkenes, alkynes, dienes, allenes, organic halides, organometallic reagents, or C—H compounds have been reported for the synthesis of carboxylic acids and their derivatives.^[29-37] On the other hand, chemists have also utilized organocatalysts, such as N-heterocyclic carbenes, phosphines, amines, and frustrated Lewis pairs, to activate CO₂ in its conversion to small molecules.^[38-40] The efficiency and applicability of CO₂ transformations were thus improved greatly in organic synthesis. CO₂ fixation and subsequent catalytic conversion to fine chemicals, particularly to chiral molecules, represents a highly value-added process. Enantiopure carbonates, carbamates and carboxylic acid derivatives are versatile synthons and widespread in natural products, drug molecules, and advanced materials. Catalytic enantioselective methods for the synthesis chiral carbonates and carbamates with CO₂ via kinetic resolution of racemic epoxides and aziridines have been well explored. These achievements have been summarized in previous publications ^[41-43] and will not be discussed here. In 2017, a general review on the enantioselective synthesis of carboxylic acid derivatives and carbamates with CO₂ was reported in broad content, including asymmetric carboxylation, chiral transfer, and ring-opening of epoxides, etc.^[44] Herein, we would like to focus on the advances of CO₂ fixation by catalytic asymmetric via carbon-carbon (C—C) and carbon–oxygen (C—O) bonds formation (Scheme 1). The interaction between catalyst, CO₂ and substrate will be emphasized in this review to inspire the design of new catalytic systems for asymmetric CO₂ transformations.

  Scheme 1

  

  Scheme 1.  Enantioselective synthesis of small molecules via CO₂ fixation by asymmetric catalysis

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  1.   CO₂ Fixation by asymmetric catalysis via C—C bond formation

  The development of new C—C bond forming reactions using sustainable chemical feedstock is rather attractive in synthetic chemistry. In this respect, conversions of CO₂ into useful chemicals have gained considerable attention in recent years. One of the major challenges to synthesize chemicals with CO₂ as a raw material is to construct C—C bonds with high efficiency and selectivity, including chemo-, regio-, and stereo-selectivity, expecially enantioselectivity. Because of the high thermodynamic and kinetic stability of CO₂, most of the C—C bond-forming reactions with CO₂ were carried out under harsh conditions, such as high pressure of CO₂ and high reaction temperature. It is difficult to control the enantioselectivity in these reactions. Recently, transition-metal-catalyzed CO₂ fixation to form C—C bond has emerged to be a mild and powerful tool to convert CO₂ into useful chemicals. However, highly enantioselective C—C bond-forming reactions with CO₂ as C₁ source by asymmetric catalysis is still challenging and rare.

  2.   Enantioselective carboxylation

  In 1981, an asymmetric CO₂ fixation reaction was reported by Sato and co-workers^[45] but with low enantioselectivity control. Asymmetric carboxylation of π-allylti- tanium complex tethered with chiral cyclopentadienyl ligand was achieved and 2-methylbut-3-enoic acid was obtained in 70% yield with 19% ee.

  In 2010, Gawley and co-workers^[46] reported a catalytic dynamic resolution of rac-2-lithio-N-Boc-piperidine (2), which was obtained by deprotonation of N-Boc-piperidine (1) with s-BuLi and N, N, N', N'-tetramethylethylenediamine (TMEDA) at a low temperature (Scheme 2). Catalytic dilithiodiaminoalkoxide (L1) was found to be an efficient chiral catalyst in this reaction. The authors investigated the plot of ΔG versus temperature for racemization of 2 and dynamic thermodynamic resolution of 2•L1 in the presence of TMEDA. The result revealed that the barrier for dynamic thermodynamic resolution of 2•L1 is lower than racemization of it below -27 ℃. Lithiation of 1 at -78 ℃, followed by ligand exchange at -45 ℃ and quenching with various electrophiles at -78 ℃ afforded 2-substituted N-Boc-piperidines with excellent enantioselectivities. When CO₂ was used as an electrophile, (R)-N-Boc-pipe- colic acid (3) was obtained in 78% yield with 96% ee.

  Scheme 2

  

  Scheme 2.  Asymmetric dynamic resolution of 2-lithio-N-Boc- piperidine 2 with CO₂

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  Later, Alexakis and co-workers^[47] reported an asymmetric desymmetrization of 2, 2'-dibromobiphenyls to synthesize biphenyl molecules with axial chirality via an enantio- selective bromine-lithium exchange process. They investigated the influence factors on the backbone of the diether ligand. The increase of enantioselectivity was observed by decreasing the hindrance of the backbone of chiral ligand. With methyl-protected diol derivative L2 as a chiral ligand, 2, 2', 6, 6'-tetrabromobiphenyl (4) was converted to atropoisomeric 1, 1'-biphenyl-2, 2'-dicarboxylic acid (5) in 85% yield with 80% ee via bromine-lithium exchange followed by quenching with CO₂ (Scheme 3). This was the first highly selective (up to 82% ee) bromine-lithium exchange with a catalytic amount of ligand (0.5 equiv., 25% per BrLi exchange). Recently, Hamashima and co-workers^[48] disclosed that (S)-1, 1'-binaphthyl-2, 2'-dicarboxylic acid could be easily accessed by carboxylation of 2, 2'-dilithium-1, 1'- binaphthalene generated from lithiation of (S)-BINOL bis(diethylphosphate) with lithium/naphthalene. Notably, the reaction could be carried out on gram-scale and the corresponding di-acid was obtained with 99% ee.

  Scheme 3

  

  Scheme 3.  Asymmetric bromine-lithium exchange to synthesize axially chiral di-acid with CO₂

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  In 2002, a highly regio- and stereo-selective ring-closing carboxylation of bis-1, 3-dienes with CO₂ was reported by Mori and co-workers.^[49] After that, a breakthrough in asymmetric transition-metal-catalyzed C—C bond-forming reaction with CO₂ was reported by the same group in 2004 (Scheme 4).^[50] In the presence of 10 mol% of Ni(acac)₂ and 20 mol% of (S)-MeO-MOP (L3), a highly regio-, diastereo-, and enantio-selective ring-closing carboxylation of bis-1, 3-diene (6) was realized under atmospheric pressure of CO₂. Trans-disubstituted pyrrolidines or cyclopentanes 7 were obtained in high yields with 90%~96% ee. Chiral allylic carboxylic acid esters with three continuous stereocenters were synthesized efficiently in one-step. Single diastereoisomer was obtained with dimethylzinc or diphenylzinc as coupling reagents. When diethylzinc was used as a coupling reagent, cyclized products 7e and 7e' were obtained simultaneously with the same enantiomeric excess, indicating that they were generated from the same intermediate.

  Scheme 4

  

  Scheme 4.  Ni-catalyzed enantioselective carboxylative cyclization of bis-1, 3-dienes with CO₂

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  The authors proposed a plausible mechanism in Scheme 5. The reaction is initiated by oxidative cyclometalation of bis-1, 3-diene 6 with Ni⁰ complex generating cyclized bis-π-allylnickel species 8. Then insertion of CO₂ into the Ni—C bond produces carboxylates 9. After transmetallation with an organozinc reagent, π-allylnickel intermediate 10 is generated. Reductive elimination from 10 provides the corresponding carboxylates and regenerates the Ni⁰ catalyst. The final product 7 is obtained after quenching with diazomethane. When diethylzinc is used as coupling reagent, complex 10 can undergo β-hydride elimination to afford π-allylnickel hydride complex. Then the byproduct 7e' is generated after reductive elimination.

  Scheme 5

  

  Scheme 5.  Mechanism of Ni-catalyzed carboxylative cyclization of bis-1, 3-dienes with CO₂

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  In 2014, Tanaka and co-workers^[51] reported an enantioselective cyclization reaction of prochiral 1, 6-diyne 11 with CO₂ via desymmetrization (Scheme 6). The [2+2+2] cycloaddition product 12 was obtained in good yield with a promising enantioselectivity (20% ee) using catalytic cationic [Rh(cod)₂]BF₄ and (S)-H₈-BINAP.

  Scheme 6

  

  Scheme 6.  Rh-catalyzed enantioselective desymmetrization of 1, 6-diyne with CO₂

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  Recently, catalytic hydrocarboxylation of alkenes with CO₂ has been reported using transition-metal-catalyst, such as Ni, ^[52] Fe, ^[53] or Co, ^{[54, 55]} with diethylzinc or ethylmagnesium halides as a hydride source. In 2016, Mikami and co-workers^[56] reported a Rh-catalyzed hydrocarboxylation of alkenes with CO₂. Using [RhCl(cod)]₂ as catalyst and diethylzinc as hydride source, the corresponding carboxylic acids were obtained in moderate to excellent yields. They also investigated the asymmetric version of this reaction. Enantioselective hydrocarboxylation of α-aryl acrylates 13 afforded the corresponding acid 14 bearing a quaternary carbon center with moderate ee using cationic (S)-Segphos-Rh complex as catalyst and catalytic AgSbF₆ as cocatalyst (Scheme 7). In this reaction, the steric and electronic effect on substituent of the ester (R group) has moderate effect on the enantioselectivity of the product. The authors didn't show the influence of aryl group on the enantioselectivity of the product.

  Scheme 7

  

  Scheme 7.  Rh-catalyzed asymmetric hydrocarboxylation of α-aryl acrylates

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  The plausible reaction mechanism is displalyed in Scheme 8. First, Rh^I—Et complex 15 is generated by transmetalation of the ethyl group between diethylzinc and Rh^I precursor. Second, β-hydride elimination from 15 generates the Rh^I—H species 16 and releases ethylene. Then insertion of styrene derivatives into 16 affords benzyl rhodium intermediate 17, which undergoes insertion of CO₂, followed by transmetalation with diethylzinc producing 19 and regenerates Rh^I—Et complex 15. Finally, acids 14 are obtained after hydrolysis of 19.

  Scheme 8

  

  Scheme 8.  Mechanism of Rh-catalyzed hydrocarboxylation of styrene derivatives with CO₂

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  An enantioselective induction model has been proposed by the authors based on the absolute configuration of product 14 (Scheme 9). The corresponding (Z)-Rh^I-enolate is generated by insertion of α, β-unsaturated esters with Rh^I-H species in an s-trans fashion. Attack of CO₂ with the enolate by the Si face is prevented by the equatorial phenyl group on the phosphorus atom in the chiral ligand. Thus, attack of CO₂ from the Re face of the rhodium side would be favored to afford the corresponding product (S)-14.

  Scheme 9

  

  Scheme 9.  Plausible reaction model for asymmetric induction of rhodium-catalyzed hydrocarboxylation

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  In 2017, Marek and co-workers^[57] reported a highly diastereo- and enantio-selective Cu-catalyzed carboxylation of poly-substituted cyclopropenes with CO₂ and Grignard reagents. Cyclopropylmetal species was first obtained through a Cu-catalyzed carbomagnesiation reaction, followed by quenching with CO₂ as an electrophile affording the cyclopropyl acid product 21 in good yields with 88%~96% ee (Scheme 10). It was shown that the cyclopropylmagnesium species are configurationally stable. The addition of this cyclopropylmagnesium species to electrophile, such as CO₂, leads to the product with clean retention of configuration.

  Scheme 10

  

  Scheme 10.  Cu-catalyzed tandem carboxylation of cyclopropenes with CO₂

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  3.   Enantioselective hydroxymethylation

  In 2017, Yu and co-workers^[58] recently reported the first enantioselective Cu-catalyzed reductive hydroxymethylation of alkenes with CO₂, which provides an efficient method for the synthesis of chiral homobenzylic/allylic alcohols (Scheme 11). Using Cu(OAc)₂ as a catalyst, bulky (R)-DTBM-Segphos as a chiral ligand, and hydrosilane as a hydride source, styrene derivatives 22 were converted to a variety of chiral homobenzylic alcohols 23 in high yields with excellent enantioselectivities. Moreover, a series of chiral homoallylic alcohols 25 were also generated from aryl substituted 1, 3-dienes 24 with high regioselectivity and enantioselectivity under similar conditions. Notably, the homoallylic alcohols were obtained with high Z-selectivity for the alkene. The chiral benzylic alcohol 23c was easily transformed into the anti-inflammatory drug (S)-(+)-ibu- profen.

  Scheme 11

  

  Scheme 11.  Cu-catalyzed enantioselective hydroxymethylation of styrenes and 1, 3-dienes with CO₂

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  The proposed reaction mechanism is described in Scheme 12. At the beginning, active catalyst L*CuH 26 was generated by reaction of ligand coordinated pre-catalyst with silanes. Then, insertion of alkenes into the Cu—H bond of 26 affords alkyl copper species 27 with excellent regio- and enantio-selectivity. Subsequent carboxylation of 27 with CO₂ generates copper carboxylate 28. Next, reduction of 28 by hydrosilanes leads to copper alkoxide 29. Further transmetalation of 29 with hydrosilane generates 30 and releases catalyst 26. Finally, desilylation of 30 affords the corresponding alcohol 23 by the treatment with NH₄F.

  Scheme 12

  

  Scheme 12.  Mechanism of Cu-catalyzed reductive hydroxy- methylation of styrenes

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  Continuing the previous research, Yu and co-workers^[59] recently reported a Cu-catalyzed highly enantioselective hydroxymethylation of 1, 3-dienes with CO₂ to construct chiral all-carbon quaternary stereocenter in acyclic molecules. In this studies, homoallylic alcohols 32 or 34 were obtained in moderate to good yields with excellent ees using (S, S)-Ph-BPE as chiral phosphine ligand and Me- (MeO)₂SiH as hydride source (Scheme 13).

  Scheme 13

  

  Scheme 13.  Cu-catalyzed enantioselective reductive hydroxy- methylation of 1, 3-dienes to construct all-carbon quaternary stereocenter with CO₂

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  The mechanism of this transformation was shown in Scheme 14. First, a complex between the copper precatalyst and the chiral phosphine ligand reacts with silane generating a catalytically active chiral L*Cu—H species 35. Then a chiral allylic copper intermediate 36 is formed via a highly regio- and stereo-selective 1, 2-syn-addition of Cu—H species 35 into 1, 3-diene. Steric repulsion might prevent the energy-disfavored 1, 3-migration process to generate an isomeric allylcuprate. Through a possible six-membered ring chair like transition state in the nucleophilic addition of 36 to CO₂, the copper carboxylate intermediate 37 is formed. Further reduction of 37 by silane leads to copper alkoxide 38. Then, σ-bond metathesis of 38 with silane affords silyl ether 39 and regenerates the active catalyst 35. Finally, the desired alcohol 40 is formed after treatment of 39 with ammonium fluoride.

  Scheme 14

  

  Scheme 14.  Mechanism of Cu-catalyzed enantioselective reductive hydroxymethylation of 1, 3-dienes with CO₂

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  At the same time, Ding and co-workers^[60] reported a Cu-catalyzed highly enantioselective hydroxymethylation of 1, 1-disubstituted allenes with CO₂ to construct all- carbon chiral quaternary centers. Under the optimized reaction conditions of Cu/Mandyphos (L4) catalyst, the corresponding homoallylic alcohol product 42 was obtained in good yields with excellent enantioselectivities from 1, 1- disubstituted allenes 41 (Scheme 15).

  Scheme 15

  

  Scheme 15.  Cu^I-catalyzed enantioselective reductive hydroxy- methylation of 1, 1-disubstituted allenes with CO₂

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  4.   Enantioselective electrochemical carboxylation

  Another noteworthy achievement in CO₂ fixation via asymmetric catalysis is enantioselective electrochemical carboxylation. In 2004, the diastereoselective electrochemical CO₂ fixation using chiral auxiliaries was reported by Feroci, Inesi and co-workers.^[61-62] Moderate diastereoselectivity was obtained in electrochemical carboxylation of alkyl halide and cinnamic acid derivatives with CO₂. In 2009, Lu and co-workers^[63] reported an inspiring catalytic asymmetric electrochemical reductive carboxylation of acetophenone (43) with CO₂ giving 2-hydroxy-2-phenyl- propionic acid (44) (Scheme 16). The product was obtained in 25% yield with 30% ee using cinchonidine as chiral catalyst, butanol as co-catalyst, tetrabutylammonium iodide (TBAI) as supporting electrolyte on stainless steel cathodes. Recently, the efficiency of this reaction was improved by using phenol as co-catalyst and tetrahexyl-am- monium iodide as supporting electrolyte (41% yield, 49% ee) under similar conditions.^[64]

  Scheme 16

  

  Scheme 16.  Asymmetric electrochemical reductive carboxylation of acetophenone

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  The authors proposed a possible reaction pathway as shown in Scheme 17. First, a ketyl radical anion 45 is generated from 43 after receiving an electron from the cathode. Next, nucleophilic addition of 45 on CO₂ affords the radical carbonate anion intermediate 46. A possible chiral proton-donating complex is then generated after 46 bonding to protonated cinchonidine. Further single electron reduction and subsequent carboxylation produces 47. At last, the final product 44 is formed by decarboxylation and protonation.

  Scheme 17

  

  Scheme 17.  Reaction pathway of electrochemical carboxylation of acetophenone

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  Electrochemical reduction of organic halides has been investigated for several decades. Electrochemical carboxylation of organic halides with CO₂ could produce corresponding carboxylic acids.^[65-66] In 2014, Wang, Lu and co-workers^[67] reported an asymmetric CO₂ fixation by electrochemical carboxylation of 1-phenylethyl chloride (48) (Scheme 18). In the presence of 5 mol% (R, R)-salen- Co complex (Co cat. I) and tetrabutylammonium iodide, racemic 1-phenylethyl chloride 48 was electrocarboxylated affording optically active 2-phenylpropionic acid 49 in 27% yield with 83% ee on a glassy carbon (GC) cathode. The electrogenerated [(R, R)-salen-Co^Ⅰ)] species is considered to be the key catalytic species and alkyl Co^Ⅱ complex is thought to be reaction intermediate. Recently, Wang, Lu and co-workers further expanded this reaction into a heterogeneous system.^[68] They synthesized a [Co]@Ag composite by entrapment of (R, R)-salen-Co^Ⅱ complex within silver nanoparticles. Using the chiral [Co]@Ag composite as cathode, electrocarboxylation of racemic 1-phenylethyl bromide with CO₂ afforded 49 in 58% yield with 73% ee, without adding additional chiral source. Moreover, the catalyst [Co]@Ag showed remarkable stability and efficiency. The product could be obtained without significant loss of enantioselecitivity even after recycle the catalyst for 7 times.

  Scheme 18

  

  Scheme 18.  Asymmetric electrocarboxylation of 1-phenylethyl chloride

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  Recently, Mei and co-worker^[69] reported an enantioselective Pd-catalyzed electrochemical carboxylation of allylic acetates with CO₂ (Scheme 19). They found that moderate to good enantioselectivities could be obtained using Pd(OAc)₂ as catalyst and chiral bidentate phosphine as ligands. The carboxylation reaction was performed with Pd(OAc)₂ as catalyst, Et₄NOTs as electrolyte, EtOH as additive, and DMF as solvent at 30 ℃ under constant current electrolysis conditions of 8.0 mA (J=8.0 mA• cm^-2) and 3 F/mol. After screening several chiral bidentate phosphine ligands, carboxylation of 50 with CO₂ gave allylic acid 51 in 66% yield with 67% ee using (R)-MeO- BIPHMe as ligand.

  Scheme 19

  

  Scheme 19.  Pd-catalyzed reductive electrocarboxylation of allyl esters with CO₂

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  5.   CO₂ Fixation by asymmetric catalysis via C—O bond formation

  The central carbon atom of CO₂ bears strong electrophilicity and the oxygen atoms act as a Lewis base with weak nucleophilicity. The structure property of CO₂ makes it both an electrophile at carbon atom and a nucleophile at oxygen atom. Accordingly, cyclic carbonates and carbamates have been synthesized by reaction of CO₂ with epoxides and aziridines.^[70-72] In the other hand, nucleophiles such as alcohols and amines can attack the central carbon atom of CO₂ generating carbonate and carbamate anion. Subsequent anion trapping with electrophiles affords carbonate and carbamate. By design of proper substrates and choosing of suitable catalysts, chemists have developed several efficient catalytic systems for the enantioselectvie synthesis of carbonate and carbamate with CO₂. This type of asymmetric CO₂ fixation reaction is summarized as below.

  In 1987, Takeichi and co-workers^[73] reported a Co-cat- alyzed kinetic resolution of propylene bromohydrin (Scheme 20). Chiral propylene carbonate 53 was obtained in promising ee via cyclization under mild conditions. Based on the configuration of the products, this reaction was considered via an attack of alcoholate anion on CO₂ followed by an intromolecular nucleophilic substitution reaction.

  Scheme 20

  

  Scheme 20.  Co-catalyzed enantioselective cyclization of propylene bromohydrin

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  In 2003, Yoshida, Ihara and co-workers^[74] reported a Pd-catalyzed CO₂ recycling reaction for the preparation of chiral cyclic carbonates based on novel CO₂ elimination-fixation strategy (Scheme 21). With Pd₂(dba)₃•CHCl₃ as catalyst and (S)-BINAP as ligand, the asymmetric CO₂ elimination-fixation reaction of propargylic carbonates 57 with phenols afforded the corresponding cyclic carbonate 58 in 48%~94% yields with 23%~93% ee. The authors found that the reactions with longer chain substrates leaded to cyclic carbonates in higher enantiomeric purities. Thus, bulky substituents at the β-position of alkyl side chain resulted in increased enantioselectivities.

  Scheme 21

  

  Scheme 21.  Pd-catalyzed enantioselective CO₂ elimination- fixation of propargylic carbonates

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  A plausible mechanism for this CO₂ elimination-fixation is shown in Scheme 22. The key of this transformation is the π-allyl-Pd intermediate 60 generated by nucleophilic attack of allenylpalladium methoxide 59 by phenols. When this reaction is conducted under a CO₂ atmosphere, the released and external CO₂ promotes the reaction efficiently to generate carbonate. The direct cyclization of 60 leads to the dihydrofuran and epoxide byproducts. A new chiral center is formed by intramolecular nucleophilic attack by the carbonate anion 61 in the presence of (S)-BINAP-Pd complex. To examine whether CO₂ dissociates from the substrate in this reaction, the reaction was conducted in both the presence and the absence of added CO₂. While the reaction under argon atmosphere gave cyclic carbonate 58 in 85% yield, the process carried out under 101 kPa of CO₂ leaded to 58 in 96% yield. In addition, when the reaction is run under bubbling argon to remove the resulting CO₂, 58 was formed in only 21% yield together with by-product dihydrofuran (32%) and epoxide (11%). The results suggested that the process proceeded via a pathway involving decarboxylation-followed fixation of the liberated CO₂.

  Scheme 22

  

  Scheme 22.  Plausible mechanism of Pd-catalyzed elimination-fixation of propargylic carbonates

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  In 2010, another asymmetric CO₂ fixation with propargyl alcohols towards chiral cyclic carbonates is reported by Yamada and co-workers.^[75] Asymmetric desymmetrization of bispropargylic alcohols 62 was realized efficiently with chiral silver complex as a catalyst (Scheme 23). The alkyne moiety of bispropargylic alcohols 62 was activated by the π-Lewis acidic Ag-catalyst. Thus, the intramolecular nucleophilic addition of alkyne with the carbonate anion was facilitated from the opposite side of alkyne.^[76] By the combination of AgOAc and chiral Schiff base ligand L5, a range of α-alkylidene cyclic carbonates 63 were obtained in good yield with moderate to high enantioselectivities (47%~93% ee). Furthermore, stereospecific hydrolyzation of such products was readily performed to afford the α-hydroxyketone bearing a chiral quaternary carbon center without loss of optical purity.

  Scheme 23

  

  Scheme 23.  Ag-catalyzed enantioselectivede symmetrization of bispropargylic alcohols with CO₂

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  In 2015, Johnston and co-workers^[77] described a organocatalytic iodocarbonation of homoallylic alcohols 64 with N-iodosuccinimide and CO₂ to produce chiral cyclic carbonates 65 based on dual Brønsted acid/base activation (Scheme 24). They used StilbPBAM•HNTf₂ salt as a bifunctional hydrogen-bond donor and acceptor catalyst. The acid/base combination lowered the barrier of CO₂ incorporation and assisted the stabilization of the resulting carbonic acid. A variety of styrene derivatives were tested under the optimal conditions affording the corresponding carbonates in high yields with good to excellent enantioselectivities. However, no reaction occurred when using styrene derivatives with the bulky aryl group, such as 2-methylphenyl and 1-naphthyl (65c). High yield and moderate ee were obtained with homoallylic alcohol bearing aliphatic substituents (65e). Spirocyclic carbonate could be synthesized with trisubstituted alkene with promising enantioselectivity (65f).

  Scheme 24

  

  Scheme 24.  Asymmetric organocatalytic iodo carbonation of homoallylic alcohols

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  In the course of condition optimization, both efficiency and enantioselectivity were found to be sensitive to a variety of parameters, including concentration, temperature, stirring rate, counterion, and ratio of acid/base. The latter two factors were thought to influence the selectivity-determining hydrogen-bonding network. Moreover, 4Å MS was also crucial to the chemical yield, which attributed to maintaining the active complex formed by catalyst salt and CO₂ (Scheme 25). In the presence of water, carbonic acid salt 66 was formed which consumed the active catalyst. This offers an insight into the interaction between hydrogen-bond donor/acceptor catalyst and CO₂ along with water. Very recently, This group prepared six-membered cyclic carbamates through an high enantioselective amine CO₂-capture/cyclization reaction with MeO-StilbPBAM•HNTf₂ salt as catalyst and analogous amine as substrate.^[78] And mechanism-guided analysis of the reaction provides insight to various dominant substrate- reagent combinations in the solution.

  Scheme 25

  

  Scheme 25.  Equilibrium of catalyst poisoning and recovery

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  In 2017, Zhou and co-workers^[79] reported a Cu/Ag cocatalyzed tandem asymmetric aldehyde-alkyne-amine coupling-carboxylative cyclization with CO₂ for the highly enantioselective synthesis of chiral N-aryl 2-oxazolidinones 70 (Scheme 26). In this reaction, the chiral propargylamine was formed first with Cu(OTf)₂ as catalyst and PYBOX L6 as ligand. Then Ag-catalyzed cyclization of propargylamine with CO₂ afforded N-aryl 2-oxazolidinones in excellent yields with 90%~96% ee.^[80] This process is a rare example of multicatalyst-promoted asymmetric tandem reaction using CO₂ as a C₁ synthon.

  Scheme 26

  

  Scheme 26.  Cu and Ag cocatalyzed tandem asymmetric coupling-carboxylative cyclization

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  Very recently, Zhou and co-workers^[81] reported an Ag- catalyzed enantioselective carboxylative cyclization of propargylic alcohols and CO₂ via kinetic resolution under mild conditions. The reaction enabled the synthesis of chiral propargylic alcohols 72 and cyclic carbonates 73 with promising yield and enantioselectivity simultaneously (Scheme 27). They found that the substituents on the pyridyl ring had great influence on this reaction when they used the 2-pyridinecarboxyaldehyde derived Schiff base ligand. Chiral ligand L7 derived from ortho-chloro-substituted pyridine was demonstrated to be the best one.

  Scheme 27

  

  Scheme 27.  Enantioselective carboxylative cyclization of propargylic alcohol with CO₂ under mild conditions

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  Ir-catalyzed asymmetric allylic amination with alkyl amine has been widely reported for the synthesis of chiral allyl amines.^[82-92] Interestingly, Zhao and co-workers^[93-94] reported an Ir-catalyzed asymmetric allylation reaction to synthesize optically active carbamates with CO₂ and amine that involving both C—O and C—N bonds formation (Scheme 28). With linear allylic chlorides 74, amines 75, and CO₂ as substrates, Ir-catalyzed three-component domino reaction was developed. By addition of DABCO as base and using slight excess of allyl chlorides, the chiral carbamates were obtained as major products comparing with chiral allyl amines. A variety of alkyl amines and cinnamyl chlorides were employed affording carbamates 76 in moderate yield with high regioseletivity (branched/linear > 9/1) and moderate to good enantioselectivities (48%~94% ee). Notably, this reaction featured with excellent ortho substituent tolerance of aryl substituent in allylic chlorides, which is difficult in the preparation of branched allyl alcohols.^[95] They found that aryl substituted allyl chlorides with the electron-donating groups on the phenyl ring gave the branched allyl carbamates in fair to good yields with a high level of both regio- and enantio-selectivities. Aryl substituted allyl chlorides with the electron-withdrawing groups on the phenyl ring led to a good yields and excellent regioselectivities but with lower enantioselectivities. For substrate bearing an aliphatic group, it provided the corresponding allyl carbamate with lower ee value.

  Scheme 28

  

  Scheme 28.  Ir-catalyzed asymmetric allylation with CO₂ and amines

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  6.   Conclusions and perspectives

  We conclude the recent advances in the CO₂ fixation via asymmetric catalytic reactions for the synthesis of chiral small molecules. Although transformations of CO₂ into valuable chemicals have been widely reported over the last decade, the CO₂ fixation via asymmetric catalysis is still less developed. In general, transition metal catalysis is broadly applied in the CO₂ fixation via asymmetric cataly- sis. In comparison, there are just several successful examples of organocatalytic asymmetric CO₂ fixation. Notably, electrocatalysis could also be used in the asymmetric CO₂ fixation, albeit with moderate enantioselectivities.

  Carboxylic acids and their derivatives with an alpha chirality center are widely found in biological active natural products and medicinal molecules. In order to synthesize these compounds, asymmetric carboxylation of carbon nucleophiles with CO₂ as a building block is an attractive method. Although several efficient methodologies have been developed for the enantioselective synthesis of carboxylic acid derivatives via catalytic C—C bond formation with CO₂, special substrates such as styrenes, 1, 3-diene, allenes are needed. Using readily available substrates such as simple alkenes and alkanes will be highly desirable in the asymmetric carboxylation with CO₂ in the future.

  Chiral carbonates and carbamates are important chemicals and intermediates in synthetic chemistry. Asymmetric synthesis of these compounds by ring opening of epoxides and aziridines with CO₂ has been well developed. Beyond these achievements, several new asymmetric CO₂ fixation reactions for the synthesis of carbonates and carbamates have been developed with metal-catalysis or organocatalysis. However, the successful reactions are still limited and new types of asymmetric reactions are needed to be developed in this area.

  In summary, significant progress has been made on the asymmetric CO₂ fixation in recent years. Investigation of the mechanism of CO₂ activation is important for the development of CO₂ fixation reaction via asymmetric catalysis. It is anticipated that more researches will emerge in this area. The fixation of CO₂ with high efficiency and excellent stereoselectivities is a longstanding goal under with mild conditions, such as low catalyst loading, room temperature, and atmospheric pressure of CO₂. In this regard, converting CO₂ and H₂O to chiral carbohydrate with light by photosynthesis in nature is a perfect process. Thus, enzyme-catalyzed CO₂ fixation might inspire the development of new catalytic systems on asymmetric catalysis with CO₂ as a sustainable C1 synthon.

  
  
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• 

  Scheme 1  Enantioselective synthesis of small molecules via CO₂ fixation by asymmetric catalysis

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  Scheme 2  Asymmetric dynamic resolution of 2-lithio-N-Boc- piperidine 2 with CO₂

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  Scheme 3  Asymmetric bromine-lithium exchange to synthesize axially chiral di-acid with CO₂

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  Scheme 4  Ni-catalyzed enantioselective carboxylative cyclization of bis-1, 3-dienes with CO₂

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  Scheme 5  Mechanism of Ni-catalyzed carboxylative cyclization of bis-1, 3-dienes with CO₂

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  Scheme 6  Rh-catalyzed enantioselective desymmetrization of 1, 6-diyne with CO₂

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  Scheme 7  Rh-catalyzed asymmetric hydrocarboxylation of α-aryl acrylates

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  Scheme 8  Mechanism of Rh-catalyzed hydrocarboxylation of styrene derivatives with CO₂

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  Scheme 9  Plausible reaction model for asymmetric induction of rhodium-catalyzed hydrocarboxylation

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  Scheme 10  Cu-catalyzed tandem carboxylation of cyclopropenes with CO₂

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  Scheme 11  Cu-catalyzed enantioselective hydroxymethylation of styrenes and 1, 3-dienes with CO₂

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  Scheme 12  Mechanism of Cu-catalyzed reductive hydroxy- methylation of styrenes

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  Scheme 13  Cu-catalyzed enantioselective reductive hydroxy- methylation of 1, 3-dienes to construct all-carbon quaternary stereocenter with CO₂

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  Scheme 14  Mechanism of Cu-catalyzed enantioselective reductive hydroxymethylation of 1, 3-dienes with CO₂

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  Scheme 15  Cu^I-catalyzed enantioselective reductive hydroxy- methylation of 1, 1-disubstituted allenes with CO₂

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  Scheme 16  Asymmetric electrochemical reductive carboxylation of acetophenone

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  Scheme 17  Reaction pathway of electrochemical carboxylation of acetophenone

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  Scheme 18  Asymmetric electrocarboxylation of 1-phenylethyl chloride

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  Scheme 19  Pd-catalyzed reductive electrocarboxylation of allyl esters with CO₂

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  Scheme 20  Co-catalyzed enantioselective cyclization of propylene bromohydrin

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  Scheme 21  Pd-catalyzed enantioselective CO₂ elimination- fixation of propargylic carbonates

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  Scheme 22  Plausible mechanism of Pd-catalyzed elimination-fixation of propargylic carbonates

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  Scheme 23  Ag-catalyzed enantioselectivede symmetrization of bispropargylic alcohols with CO₂

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  Scheme 24  Asymmetric organocatalytic iodo carbonation of homoallylic alcohols

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  Scheme 25  Equilibrium of catalyst poisoning and recovery

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  Scheme 26  Cu and Ag cocatalyzed tandem asymmetric coupling-carboxylative cyclization

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  Scheme 27  Enantioselective carboxylative cyclization of propargylic alcohol with CO₂ under mild conditions

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  Scheme 28  Ir-catalyzed asymmetric allylation with CO₂ and amines

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•