Cobalt- and rhodium-catalyzed carboxylation using carbon dioxide as the C1 source

Carbon dioxide (CO2) is one of the most important materials as renewable chemical feedstock. In this review, the Co- and Rh-catalyzed transformation of CO2 via carbon–carbon bond-forming reactions is summarized. Combinations of metals (cobalt or rhodium), substrates, and reducing agents realize efficient carboxylation reactions using CO2. The carboxylation of propargyl acetates and alkenyl triflates using cobalt complexes as well as the cobalt-catalyzed reductive carboxylation of α,β-unsaturated nitriles and carboxyamides in the presence of Et2Zn proceed. A Co complex has been demonstrated to act as an efficient catalyst in the carboxylation of allylic C(sp3)–H bonds. Employing zinc as the reductant, carboxyzincation and the four-component coupling reaction between alkyne, acrylates, CO2, and zinc occur efficiently. Rh complexes also catalyze the carboxylation of arylboronic esters, C(sp2)–H carboxylation of aromatic compounds, and hydrocarboxylation of styrene derivatives. The Rh-catalyzed [2 + 2 + 2] cycloaddition of diynes and CO2 proceeds to afford pyrones.


Introduction
Carbon dioxide (CO 2 ) is one of the most important materials as renewable feedstock [1][2][3][4]. However, the thermodynamic and kinetic stability of CO 2 sometimes limits its utility. Classically, harsh reaction conditions such as high temperature and high pressure of CO 2 were required. To overcome these problems, the use of transition-metal catalysts has been considered as a fundamental and reliable method. In the last decade, considerable attention has been focused on the development of the catalytic fixation of CO 2 via carbon-carbon (C-C) bond formation using a variety of organic compounds as starting materials [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20]. A key factor for the successful catalytic fixation of CO 2 is the carbon-metal bond formation when transition metals are used as the catalyst. In addition, the choice of suitable reducing agents is also crucial for realizing effective carboxylation reactions.
In this review, the Co-and Rh-catalyzed transformations of CO 2 via C-C bond-forming reactions are summarized. First, we Scheme 1: Optimization of the Co-catalyzed carboxylation of 1a.
describe Co-catalyzed carboxylation reactions, including the carboxylation of propargyl acetates and alkenyl triflates. Then, the Co-catalyzed reductive carboxylation of α,β-unsaturated nitriles and carboxyamides is addressed. In addition, a Co catalyst can catalyze the allylic C(sp 3 )-H carboxylation of allylarenes when a suitable ligand is used. In the presence of zinc powder, the Co-catalyzed carboxyzincation of alkynes and the four-component coupling reaction between alkyne, acrylates, CO 2 , and zinc proceed in an efficient manner. Visible-lightdriven hydrocarboxylation reactions are shown. We also summarize carboxylation reactions catalyzed by rhodium that is a homologous element of cobalt. Carboxylations of arylboronic esters are described. Rh complexes are also effective catalysts in C(sp 2 )-H carboxylation reactions. Employing Et 2 Zn or visible light, the Rh-catalyzed hydrocarboxylation of styrene derivatives has been achieved. Furthermore, the formation of pyrones from diynes and CO 2 can be effectively catalyzed by Rh complexes.

Review Cobalt catalysts Carboxylation of propargyl acetates
Allyl and propargyl electrophiles, such as halides and esters, are well known as efficient reagents in transition-metal-catalyzed C-C bond-forming reactions [21][22][23]. In particular, the carboxylation of allyl esters with CO 2 has been catalyzed by Pd or Ni under electrochemical reaction conditions [24,25]. For catalytic reactions using reducing agents, Martin reported Ni-catalyzed regiodivergent carboxylation of allyl acetates in the presence of Mn as the reductant [26]. Mita and Sato found that Pd-catalyzed carboxylation of allylic alcohols proceeded using Et 2 Zn as the reducing agent [27]. The carboxylation of propargyl chlo-ride was reported as one of the examples concerning the Ni-catalyzed carboxylation of benzyl chlorides [28].
We have found that Co complexes can catalyze the carboxylation of propargyl acetates with CO 2 using Mn powder as the reducing agent [29]. Thus, the carboxylation of a propargyl acetate 1a was performed in the presence of CoI 2 (phen) (phen = 1,10-phenanthroline) and Mn powder (3.0 equiv) in N,Ndimethylacetamide (DMA) under an atmospheric pressure of CO 2 at room temperature (Scheme 1). Under optimized reaction conditions, the carboxylated product 2a-Me was obtained in 83% yield after derivatization to the corresponding methyl ester. In the absence of the Co catalyst, 2a-Me was not obtained. Moreover, Mn powder proved to be essential for the carboxylation to proceed. Using CoI 2 (bpy) (bpy = 2,2′-bipyridine) as the catalyst afforded 2a-Me in 76% yield, whereas CoI 2 (PPh 3 ) 2 and CoI 2 (dppe) (dppe = 1,2-bis(diphenylphosphino)ethane) suppressed the carboxylation.
The carboxylation of various propargyl acetates containing the trimethylsilyl (TMS) group as the R 1 group proceeded under the optimal reaction conditions, affording the corresponding carboxylic acids 2b-e in good-to-high yields (Scheme 2). Notably, the ester and chloro functionalities in 2b and 2c, respectively, were compatible with the reaction conditions. For the carboxylation of tertiary-alcohol-derived acetates to the corresponding carboxylic acids 2d,e, CoI 2 (bpy) was found to be an effective catalyst. The yields of product 2 decreased when less bulky substituents (R 1 ) were used. Thus, 1f (R 1 = tert-butyldimethylsilyl) afforded the corresponding product 2f in 88% yield, whereas 1g (R 1 = t-Bu) and 1h (R 1 = Cy) afforded 2g and 2h in 57% and 26% yields, respectively. Scheme 2: Co-catalyzed carboxylation of propargyl acetates 1.

Scheme 3:
Plausible reaction mechanism for the Co-catalyzed carboxylation of propargyl acetates 1. Scheme 3 presents a plausible reaction mechanism for this transformation. Accordingly, a Co(I) catalytic species A is first generated by the reduction of Co(II) with Mn. Secondly, the oxidative addition of the C-O bond in 1 occurs, affording Co(III) intermediate B (step a) [30]. Next, the propargyl Co(III) species B is reduced by Mn, producing the corresponding propargyl Co(II) intermediate C (step b). Subsequently, the nucleophilic Co species C reacts with CO 2 , which provides carboxylate

Carboxylation of alkenyl and aryl triflates
The catalytic carboxylation of aryl halides and pseudohalides using CO 2 is an important reaction to yield benzoic acid derivatives. In 2009, Martin reported the Pd-catalyzed carboxylation of aryl bromides using ZnEt 2 as the reductant [31]. In 2012, we first reported the Ni-catalyzed carboxylation of aryl chlorides and vinyl chlorides using Mn powder as the suitable reductant [32]. These reactions can be performed under mild conditions, i.e., an atmospheric pressure of CO 2 at room temperature.
We also reported the Co-catalyzed carboxylation of alkenyl and aryl trifluoromethanesulfonates (triflates) as substrates [33]. As a model reaction (Scheme 4), alkenyl triflate 3a was selected as the substrate, and the carboxylation of 3a was performed using Mn powder (1.5 equiv) as the reductant in DMA as the solvent under an atmospheric pressure of CO 2 at room temperature. Employing CoI 2 (Me 2 phen) (Me 2 phen = 2,9-dimethyl-1,10phenanthroline) as the catalyst, 4a-Me was obtained in 86% yield after esterification. Other bidentate ligands such as bpy, phen, and dppe were not suitable for this reaction. Control experiments revealed that both the Co catalyst and the Mn reductant were indispensable to the reaction.
The carboxylation of diverse alkenyl triflates was also examined. As a result, the desired carboxylic acids 4a-k were ob-tained in good-to-high yields, as shown in Scheme 5. Notably, the ester and p-toluenesulfonate functionalities in 4c and 4d, respectively, were tolerated. An indole-functionalized substrate 3f was converted into its corresponding carboxylic acid 4f. Conjugated alkenyl triflates 3g-i were also subjected to the reaction, and the desired carboxylic acids 4g-i were obtained in moderate-to-high yields. Furthermore, the seven-membered cyclic substrate 3j that was derived from cycloheptanone afforded its corresponding conjugated carboxylic acid 4j in 75% yield. The alkenyl triflate 3k prepared from the corresponding aldehyde was also carboxylated, and its corresponding product 4k was obtained in moderate yield.
The catalyst CoI 2 (Me 2 phen) was also effective with sterically hindered aryl triflates: the carboxylation of mesityl triflate (5) at 40 °C proceeded successfully, affording 6 in 77% yield (Scheme 6) Carboxylation of α,β-unsaturated nitriles and esters α,β-Unsaturated carbonyl compounds are good substrates for conjugate additions that use a catalytic amount of a metal complex and a stoichiometric amount of reductant, as exemplified by the reductive aldol reaction of α,β-unsaturated nitriles catalyzed by cobalt using phenylsilane as the reductant [34].
Yamada found that the reductive carboxylation of α,β-unsaturated compounds with CO 2 proceeded in the presence of Co catalysts and reductants (Scheme 7) [35,36]. When the reaction of 5-phenylpent-2-enenitrile (7a) was performed in the pres- ence of catalytic Co(acac) 2 and with Et 2 Zn as the reductant, the carboxylation proceeded to yield 8a-Me after its derivatization to the corresponding methyl ester. In these reactions, the selection of the reductant is crucial: other reductants such as Et 2 AlCl and Et 3 B yielded the corresponding product in low yields even after using a stoichiometric amount of Co(acac) 2 at high pressure.
Under the optimal reaction conditions with the Co(acac) 2 /Et 2 Zn system, various α,β-unsaturated nitriles were carboxylated to the corresponding products, which were isolated as methyl esters (Scheme 8). Thus, compound 7 bearing alkyl, ether, ester, and halide substituents exhibited good reactivity. Cinnamonitrile afforded 8f-Me in 81% yield using 10 mol % of catalyst. α-Phenyl-substituted α,β-unsaturated nitrile also reacted with Scheme 8: Scope of the reductive carboxylation of α,β-unsaturated nitriles 7. CO 2 , affording the carboxylated product 8g-Me in good yield. In addition, compound 8h-Me was obtained from the reaction of α-cyano-substituted dihydronaphthalene 7h with CO 2 in the presence of 15 mol % of catalyst and 8 equiv of Et 2 Zn.
The Co(acac) 2 /Et 2 Zn system can also be applied to carboxylate α,β-unsaturated carboxamides 9 (Scheme 9). By using 10 mol % of Co(acac) 2 and 4 equiv of Et 2 Zn, N-methylanilide derivatives 9a-f were smoothly converted into the corresponding products 10a-f in high yields. Trifluoromethyl and chloro substituents were tolerated in these reactions, judging by the formation of products 10c-Me and 10d-Me. With regard to other amide groups, morpholides 9g-i could be used and benzylmethylamide-and diethylamide-bearing substrates, which afforded the corresponding products 10j-Me and 10k-Me, albeit with moderate yields.

Allylic C(sp 3 )-H bond carboxylation
The development of methods for the catalytic carboxylation of less reactive C-H bonds with CO 2 is crucial regarding both C-H activation and CO 2 fixation processes. Mita and Sato reported a cobalt-catalyzed allylic C-H carboxylation of allylarenes (Scheme 10), in which 1-allyl-4-phenylbenzene (11a) was reacted with CO 2 (1 atm) in the presence of AlMe 3 (3 equiv) using catalytic amounts of a Co precursor and ligands [37]. The catalytic system comprising Co(acac) 2 and Xantphos (4,5-bis(diphenylphosphino)-9,9-dimethylxanthene) afforded the corresponding carboxylated product 12a-Me in an NMR yield of 71% after CH 2 N 2 treatment. In the reaction mixture, olefin isomers were also generated in 20% yield. The ligands were found to have a strong influence in yields and selectivity. Thus, the use of DPEphos (2,2′-bis(diphenylphosphino)diphenyl ether), dppf (1,1′-bis(diphenylphosphino)ferrocene), dppp (1,3-bis(diphenylphosphino)propane), and bpy as ligands afforded the olefin isomerization product as the major product. Further screening of the reaction conditions revealed that the amount of AlMe 3 was critical: the product yield increased with decreasing AlMe 3 to 1.5 equiv. The concentration of 11a also affected the efficiency of the reaction, and the isomerization of olefins could be suppressed at lower concentrations of 11a, affording the desired 12a-Me in 58% yield. With the addition of 1 equiv of CsF, the carboxylation was further accelerated to give 12a-Me in 71% yield.
Using optimized reaction conditions, the substrate scope was examined (Scheme 11). Allylbenzene was converted into its corresponding carboxylated product 12a in an isolated 68% yield, and various functionalized allylarenes bearing trifluoromethyl (11b) and alkoxy (11c,d) substituents were tolerated. The selectivity of the reaction was demonstrated with substrates 11g and 11h containing ester and ketone moieties, respectively, which are generally more reactive toward nucleophiles than CO 2 .
Notably, the Co-catalyst system was found to be applicable for the carboxylation of 1,3-diene derivatives 14 with CO 2 (Scheme 12), which afforded various hexa-3,5-dienoic acid derivatives. Diene 14a was converted into the corresponding carboxylic acid 15a in good yield. In addition, 1,4-dienes having cyclohexenyl and geminal diphenyl substituents (14b and 14c) produced their corresponding linear carboxylic acids 15b and 15c in 78% and 57% yields, respectively. Substrate 14e containing a bicyclo[2.2.2]octane framework with ketone and dimethyl ketal moieties was also converted and the corresponding product was isolated as the methyl ester 15e-Me after esterification.
For the Co-catalyzed C(sp 3 )-H carboxylation of allylarenes, a mechanism shown in Scheme 13 can be envisaged. The process starts with the generation of a low-valent methyl-Co(I) species A by the reaction of the Co(II) complex with AlMe 3 . The C-C double bond of the substrate then coordinates to the metal, and the subsequent cleavage of the adjacent allylic C-H bond affords η 3 -allyl-Co(III) species B (step a). Subsequently, the reductive elimination of methane from B yields the low-valent allyl-Co(I) species C (step b). Then, C-C bond formation at the γ-position occurs via a reaction with CO 2 , affording the carboxylate Co species D (step c). Finally, a linear carboxylated product is obtained by the transmetalation between D and AlMe 3 , with the concomitant regeneration of methyl-Co(I) A (step d).

Carboxyzincation of alkynes
The good reactivity and high functional-group compatibility of organozinc compounds render them as important reagents in organic synthesis [38,39]. For their preparation, direct and useful methods such as the transition-metal-catalyzed carbozincation of alkynes that affords stereodefined alkenylzinc compounds have been developed. To date, a variety of organozinc reagents (RZnX and R 2 Zn: R = aryl, alkyl, alkenyl, alkynyl, allyl, and benzyl groups) have been used in these reactions, and the corresponding alkenylzinc compounds can be prepared.
In this context, we reported the first carboxyzincation of alkynes using CO 2 and Zn metal powder in the presence of a cobalt complex as the catalyst (Scheme 14) [40]. 5-Decyne (16a) was treated with Zn powder (1.5 equiv) in the presence of CoI 2 (dppf) (10 mol %), Zn(OAc) 2 (10 mol %), and Et 4 NI (10 mol %) in a mixture of CH 3  though Zn(OAc) 2 and Et 4 NI were not indispensable for the reaction to proceed, these two additives caused an increased product yield. In contrast, the reaction did not occur in the absence of the catalyst. The use of the dppf ligand also proved to be essential, because other ligands such as dppe and bpy were not effective in the reaction.
After the reaction with 4-octyne (17b) under the aforementioned conditions, the reactions with I 2 and (PhSe) 2 produced 17b-I and 17b-Se in good yields (Scheme 15). Notably, 16b was successfully subjected to the Pd-catalyzed Negishi coupling with aryl bromide, affording the corresponding sterically congested alkene 17b-Ar in 56% yield after two steps. The Negishi coupling with benzyl chloride and the Cu-catalyzed allylation of allyl bromide also afforded the corresponding products 17b-Bn and 17b-Allyl, respectively, in good yields.
A possible reaction mechanism for the carboxyzincation reaction is displayed in Scheme 17. First, the Co(II) precursor is reduced to Co(I) (A) in the presence of metallic Zn. The oxidative cyclization of A with alkyne 16 and CO 2 affords cobaltacycle B (step a). Next, the transmetalation between B and the Zn(II) species occurs, which affords the alkenylzinc intermediate C (step b) [41], which is then reduced with Zn powder, thereby giving the carboxyzincated product and regenerating Co(I) species A (step c).
We also achieved the four-component coupling of alkynes 16, acrylates 18, CO 2 , and Zn metal, as depicted in Scheme 18 [40]. As a model reaction, the reaction using diphenylacetylene (16h), butyl acrylate (18a), CO 2 , and Zn was performed. After treatment with H 2 O and allyl bromide, the corresponding products 19a-H and 19a-Allyl were obtained in high yields. Chloro and trifluoromethyl functionalities were well tolerated under the reaction conditions, and 19b-Me and 19c-Me were obtained in 55% and 57% yields, respectively. The reaction of unsymmetrical 1-phenyl-1-hexyne with 18a afforded 19d-H and 19d-Et regioselectively. In addition, an alkyne with a thiophene ring regioselectively produced the desired product 19e-Me after methylation with MeI. It is noteworthy that an alkynoate was also converted into the corresponding product 19f-Me in good yield. Methyl, ethyl, and tert-butyl acrylates 18b, 18c, and 18d, respectively, also afforded the corresponding products. The product of the reaction with acrylamide 18e was also obtained. Scheme 19 shows a plausible reaction mechanism for this fourcomponent coupling reaction. In a similar manner to that described for the carboxyzincation, the reduction of the Co(II) precursor to Co(I) species A in the presence of Zn metal activates the catalytic cycle. Next, the oxidative cyclization of A, alkyne 16, and acrylate 18 proceeds regioselectively, and cobaltacycle B is formed (step a) [42]. Then, the insertion of CO 2 into the Co-C(sp 3 ) bond occurs, and the seven-membered Co intermediate C is obtained (step b). The transmetalation of C with the Zn(II) species proceeds then to afford the alkenylzinc species D (step c). The subsequent two-electron reduction of D with Zn metal occurs, and the alkenylzinc intermediate E is subsequently obtained, along with the regeneration of Co(I) species A (step d). After 1,4-migration of zinc in E, product 19 is obtained (step e).

Visible-light-driven hydrocarboxylation of alkynes
The Use of photoenergy to organic synthesis is of importance, since the highly reactive intermediate can be generated by photochemical reaction such as electron transfer and energy transfer [43][44][45]. Among them, light-energy-driven CO 2 fixation reactions via C-C bond formation are promising in terms of mimicking photosynthesis. In 2015, Murakami et al. found the direct carboxylation reaction with CO 2 under photo-irradiation reaction conditions [46]. Jamison et al. also reported the synthesis of α-amino acid derivatives using amine and CO 2 [47]. Iwasawa disclosed the Pd-catalyzed carboxylation of aryl halides using CO 2 in the presence of an Ir photo-redox catalyst under visiblelight irradiation conditions [48].
Zhao and Wu reported the visible-light-driven hydrocarboxylation of alkynes in the presence of a Co catalyst [49]. The reaction of alkynes was carried out using CoBr 2 /dcype (dcype = bis(dicyclohexylphosphino)ethane) as catalysts in the presence of [Ir(ppy)(dtbpy)](PF 6 ) and iPr 2 NEt as photoredox catalyst and a sacrificial reagent, respectively, in acetonitrile under an atmospheric pressure of CO 2 (Scheme 20). 1-Phenyl-1-propyne (16n) afforded hydrocarboxylated products as a mixture of regio-and stereoisomers. 4-Octyne (16b) afforded the product 20b in good yield. Other unsymmetrical internal alkynes were converted to the corresponding products regioselectively.
The same protocol could be expanded to the synthesis of γ-hydroxybutenolides by using arylalkynes bearing ortho-esters of the aromatic ring (Scheme 21) [49]. Various alkynes 21 were Scheme 22: One-pot synthesis of coumarines and 2-quinolones via hydrocarboxylation/alkyne isomerization/cyclization. converted to the corresponding products in moderate-to-good yields. Notably, ketone (22c) and ester (22d,g) functionalities were tolerated in the reaction. A bulky ester moiety took part in the reaction and the corresponding product 22h was obtained in good yield. Furthermore, the same group discovered the one-pot synthesis of coumarin derivatives via hydrocarboxylation/alkene isomerization/cyclization reactions (Scheme 22) [49]. A key of the sequential reactions is a use of aromatic alkynes bearing a momo-protected hydroxy group at the ortho position on the aromatic ring (23). The corresponding coumarin derivatives were obtained in moderate-to-good yields. Notably, ketone (24e), ester (24d) moieties were tolerated in the reaction. In addition, 2-quinolones (24f and 24g) were obtained using alkynes bearing a Boc protected carbamate in place of the MOM protected ether.
Scheme 23 shows a plausible reaction mechanism for these reactions. First, the Co(II) precursor is reduced to Co(I) A by the aid of an Ir photoredox catalyst and an amine under irradiation. The oxidative cyclization of A with 23 and CO 2 affords cobaltacycle B (step a). Next, the protonation of B affords an intermediate C (step b). Finally, two-electron reduction of Co(III) in C occurs and Co(I) species A regenerates (step c). ZnBr 2 may facilitate the step. Under the irradiation conditions, an E-isomer with aryl moiety can undergo a reversible isomerization to form the corresponding Z-isomer. Acid-mediated cyclization affords a coumarin derivative.

Rhodium catalysts Carboxylation of aryl and alkenylboronic esters
Aryl and alkenylboronic acids or their esters are of interest in organic synthesis because they are commonly used for C-C bond-forming reactions such as Pd-catalyzed Suzuki-Miyaura coupling reactions [50][51][52][53].
Iwasawa et al. reported the Rh-catalyzed carboxylation of arylboronic esters using CO 2 (Scheme 24) [54]. The reaction of 25a was performed using a catalytic amount of [Rh(OH)(cod)] 2 and 1,3-bis(diphenylphosphino)propane (dppp) in the presence of CsF as a base in 1,4-dioxane at 60 °C. Under these reaction conditions, the desired carboxylated product 26a was obtained in 75% yield. A variety of arylboronic esters (25b-i) were converted into the corresponding carboxylic acids 26b-i in good-to-high yields. It is noteworthy that ketone, ester, and nitrile functionalities in 26d, 26e, and 26f, respectively, were tolerated in the reaction. Sterically hindered substrates could be subjected to the reaction, and 26g was obtained from 25g. Moreover, a substrate having a heteroaromatic ring (25i) was converted into its corresponding carboxylic acid 26i.
Alkenylboronic esters 27 were also converted into the corresponding α,β-unsaturated carboxylic acids 28 using [RhCl(nbd)] 2 (nbd = norbornadiene) as a catalytic precursor (Scheme 25) [54]. When an alkyl-substituted substrate was examined, the p-methoxy-substituted dppp derivative was found to be the suitable ligand. After this contribution, the Cu-catalyzed carboxylation of aryl and alkenylboronic esters was independently reported by the groups of Iwasawa and How [55,56].

Direct C(sp 2 )-H bond carboxylation
As described above, C-H carboxylations with CO 2 , particularly C(sp 2 )-H carboxylation reactions, have attracted much research interest. As a consequence, Nolan [57] and Hou [58] independently reported Cu-catalyzed carboxylations using heteroarenes as substrates, which occur at the relatively acidic C-H bond.
Under the optimal reaction conditions using PCy 3 as the ligand, various 2-pyridylarenes 29 were converted into the corresponding products 30 (Scheme 28). Substrates bearing either electrondonating or electron-withdrawing substituents at the aryl ring afforded the corresponding products. Interestingly, a terminal alkenyl group remained intact after the reaction (30d). Furthermore, substrates bearing naphthyl or furyl rings were carboxylated, and the corresponding products, 30e and 30f, respectively, were obtained in good yields.
A plausible reaction mechanism for this Rh-catalyzed chelationassisted C(sp 2 )-H bond carboxylation is shown in Scheme 29. First, a low-valent methyl-Rh(I) species A is generated by transmetalation. Secondly, a pyridine ring in the substrate coordinates to the Rh center, which prompts the cleavage of the adjacent C-H bond, affording Rh(III) species B (step a). Subsequently, the reductive elimination of methane from B affords the low-valent Rh(I) species C. Then, C-C bond formation with CO 2 proceeds, and Rh carboxylate D is formed. Finally, the carboxylated product is obtained by the transmetalation between D and AlMe 2 (OMe), and methyl-Rh(I) A is regenerated.
A plausible reaction mechanism is shown in Scheme 32. First, a phenoxide 34' generated by the reaction of 2-arylphenol with t-BuOK reacts with a Rh complex A to generate a Rh complex B (step a). Then, chelation-assisted C-H bond activation proceeds to generate a rhodacycle C (step b). The reaction of C with CO 2 affords an eight-membered rhodacycle intermediate D (step c). Next, D is converted to the corresponding rhodium complex E by ligand exchange with KOAc (step d). Possibly another ligand exchange between E and KOAc regenerates the Rh complex A (step e). The desired product (35) is obtained after lactonization.
The same Rh-catalytic system proved to be applicable to the carboxylation of α,β-unsaturated esters 38 (Scheme 34). A variety of substrates 38a-f, including those containing an electron-donating substituent or benzyl-substituted esters, were converted into their corresponding products 39a-f in good-to-high yields.
Notably, the asymmetric hydrocarboxylation was archived by using a chiral bisphosphine as a ligand (Scheme 35). alkyl-Rh(I) species C (step b). Then, C-C bond formation with CO 2 proceeds to give Rh carboxylate D (step c) Finally, the carboxylated product is obtained by the transmetalation between D and Et 2 Zn, with the concomitant regeneration of ethyl-Rh(I) A (step d). Finally, an addition of Cs 2 CO 3 dramatically reduced the byproduct and 41a was obtained in 67% yield.

Visible light-driven hydrocarboxylation of alkenes
Under the optimal reaction conditions, several substrates were examined and the corresponding hydrocarboxylated products were obtained in moderate yields (Scheme 38).
[2 + 2 + 2] Cycloaddition of diynes with CO 2 The [2 + 2 + 2] cycloaddition of diynes with CO 2 is an important reaction in the field of CO 2 fixation. In these reactions, cyclic esters such as pyrones can be obtained, which are classically catalyzed by Ni complexes [76][77][78]. Tanaka et al. reported that a Rh complex with a suitable bidentate ligand was an efficient catalyst for the [2 + 2 + 2] cycloaddition reaction (Scheme 39) [79]. The reaction of 43a was performed using 20 mol % [Rh(cod) 2 ]BF 4 and a bidentate phosphine in 1,2dichloroethane at room temperature. Prior to the addition of 43a, the mixture of [Rh(cod) 2 ]BF 4 and phosphine was stirred for 30 min. Then, 43a was added dropwise to the mixture over 10 min, and the resulting reaction mixture was further stirred for 16 h. As ligands, SEGPHOS, BIPHEP, and DPPF were ineffective, but BINAP and H 8 -BINAP afforded the product 44a in moderate yields. Notably, the addition of 43a over 120 min improved the yield even at low catalyst loadings (5 mol %). A high (94%) yield was eventually obtained by reducing the prestirring time to 5 min.
A variety of diynes having different tether units (43a-g) were converted into the corresponding pyrones 44a-g in good-tohigh yields within 1 h (Scheme 40). Ester, ketone, and hydroxy groups were tolerated in the reaction. In the case of an unsymmetrical diyne bearing methyl and isopropyl groups (43g), a mixture of regioisomers 44g + 44g′ was obtained in high yield with high regioselectivity. For this transformation, the reaction pathways depicted in Scheme 41 can be envisaged. The Rh(I) species A reacts with a diyne to afford rhodacycle B (step a). Then, the reaction of B with CO 2 produces the seven-membered rhodium intermediate C (step b), from which reductive elimination occurs to yield its corresponding pyrone and the Rh(I) species A (step c). Alternatively, the oxidative cyclization of A proceeds as one of the C-C triple bonds of the diyne and CO 2 react regioselectively, and rhodacycle D is subsequently formed (step d). Then, the insertion of the alkyne into the Rh-C bond occurs to give rhodacycle C (step e).

Conclusion
In this review, the Co-and Rh-catalyzed transformation of CO 2 via carbon-carbon bond-forming reactions is summarized. Co complexes can catalyze the carboxylation of propargyl acetates and alkenyl triflates. The cobalt-catalyzed reductive carboxylation of α,β-unsaturated nitriles and carboxyamides proceeds using Et 2 Zn. In addition, a cobalt complex proved to be an efficient catalyst in the allylic C(sp 3 )-H carboxylation. In the presence of zinc as the reagent, carboxyzincation and the four-component coupling reaction between alkyne, acrylates, CO 2 , and zinc occur efficiently. Rh complexes also catalyze the carboxylation of aryl and vinylboronic esters, the C(sp 2 )-H carboxyla-tion of aromatic compounds, and the hydrocarboxylation of styrene derivatives. The Rh-catalyzed [2 + 2 + 2] cycloaddition of diynes and CO 2 proceeds to afford pyrenes. Combinations of metals (cobalt or rhodium), substrates, and reducing agents can realize efficient carboxylation reactions using CO 2 under mild reaction conditions. Furthermore, the development of novel carboxylation reactions using clean reducing agents such as non-metallic organic reductants such as amine, water, or hydrogen gas can be envisaged in the near future.