Transition-Metal-Catalyzed Carboxylation of Organic Halides and Their Surrogates with Carbon Dioxide

Key words

carbon dioxide - carboxylic acids - carboxylation - electrophiles - halides

Yue-Gang Chenwas born and raised in Zhejiang, China. He received his bachelor’s degree in chemistry at Tongji University of China in 2014. Then, he joined Prof. Tian-Sheng Mei’s group at the Shanghai Institute of Organic Chemistry, working on Ni-catalyzed reductive carboxylation. Kun Zhang was born in Guangdong (China) in 1962. He received his B.S. degree in analytical chemistry at Guangdong University of Technology in 1985, and his M.S. degree in fine chemistry industry at Guangdong University of Technology in 1990. Then he completed his Ph.D. degree in natural organic chemistry at Lanzhou University under the supervision of Prof. Yaozu Chen. Now, he is a professor at Wuyi University, and his research focuses on natural organic chemistry, organic methodology, and medicinal chemistry. Xue-Tao Xu was born in Shandong (China) in 1979. He received his B.S. degree in chemistry at Lanzhou University in 2001. Then he completed his Ph.D. degree in applied chemistry at Guangdong University of Technology under the supervision of Prof. Kun Zhang. Then, he joined Prof. Kun Zhang’s group as a research associate at Wuyi University. His research focused on natural organic chemistry, organic methodology, and medicinal chemistry. Yi-Qian Li was born in Jiangsu province, China, in 1992. She received her B.S. and M.S. from Changzhou University and China Pharmaceutical University in 2014 and 2017 respectively. Now she is a research assistant at Prof. Tian-Sheng Mei’s group and working on transition-metal-catalyzed C–H oxidation via electrochemistry. Li-Pu Zhang was born in Shandong (China) in 1989, he received his B.S. degree in chemical engineering and technology at Qilu University of Technology in 2014. Then he received his M.S. degree in chemistry at East China University­ of Science and Technology. Now he is a research assistant at Prof. Tian-Sheng Mei’s group and his research focuses on the reduction of CO₂ via electrochemistry. Ping Fang was born in Yunnan (China) in 1981. She received her bachelor’s degree in chemistry at Lanzhou University in 2004. Then, she completed doctoral studies under the supervision of Prof. Xue-Long Hou at the Shanghai Institute of Organic Chemistry. In 2010, she joined Professor Haibo Ge’s group at Indiana University-Purdue University Indianapolis, working on C–H functionalization with transition metals. In 2011, she joined Professor Zachary Aron’s group at Indiana University Bloomington to further her training as a postdoctoral fellow. In August 2012, she joined Prof. Xue-Long Hou’s group as a research associate at the Shanghai Institute of Organic Chemistry. In September 2015, she joined Prof. Tian-Sheng Mei’s group as a research associate. Her research focuses on organometallic chemistry and electrochemistry. Tian-Sheng Mei received his B.Sc. in chemistry in 2001 from Lanzhou University for research into the total synthesis of terpene natural products under the supervision of Prof. Yu-Lin Li. In 2005 he moved to the group of Prof. Jin-Quan Yu at Brandeis University, where he received his M.Sc. He subsequently relocated to the Scripps Research Institute, where he obtained his Ph.D. in 2012 for research into Pd-catalyzed C–H functionalization under Prof. Jin-Quan Yu. He joined the research group of Prof. Matt Sigman at University of Utah, where his postdoctoral research involves the development of intermolecular asymmetric Heck reactions. In 2014, he then began his independent career at State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry (SIOC). His group studies carbon dioxide chemistry and organometallic electrochemistry.

1

Introduction

Figure 1 The representative examples of carboxylation reactions in this review

The continuous increase of carbon dioxide concentration in the atmosphere has potentially caused the rise of atmospheric temperature and abnormal changes in the global climate.[1] Obviously, the control of CO₂ emissions and the development of efficient carbon capture systems are in high demand. Carbon dioxide is not only an essential component of ‘greenhouse gases’, but also an abundant and renewable C1 feedstock in organic synthesis.[2] Therefore, there is great interest in transforming carbon dioxide into useful chemicals, from an environment-protection point of view. Due to the thermodynamically stable and/or kinetically inert of CO₂, the activation and utilization of CO₂ is still problematic. Most transformations of CO₂ require highly reactive substrates and/or severe reaction conditions. In the case of C–C bond formation with CO₂, the use of carbon nucleophiles is limited to phenolates and strongly nucleophilic organolithium, as well as Grignard reagents.[3] On the other hand, highly reactive substrates such as epoxides and aziridines are used to couple with CO₂ to generate C–O or C–N bonds.[4] In this article, recent developments for the transition-metal-catalyzed carboxylation of electrophiles, such as aryl, vinyl, benzyl, allyl, propargyl, alkyl and halides and pseudohalides will be highlighted (Figure [1]). The carboxylation of alkenes and alkynes as well as organometallics, such as organotin, organozinc, and organoboron reagents, with CO₂ have been well documented and will not be discussed further in this review.[5]

2

Carboxylation of Aryl Halides and Pseudohalides

The carboxylation of arylzinc and arylboronic esters with CO₂ has been investigated intensively, due to the compatibility of various functionalities that were not tolerated with Grignard reagents. However as these compounds were derived from the corresponding aryl halides, the direct carboxylation of the aryl halides is a more straightforward transformation. The stoichiometric carboxylation of Ni–aryl complexes with CO₂ has been studied intensively. For example, the insertion of CO₂ into the Ni–aryl carbon bond of 1 and 2 delivers corresponding nickelalactones 1a and 2a, respectively (Scheme [1]).[6] The Ni(0)-mediated carboxylation of aryl halides 3 with CO₂ was demonstrated by Yamamoto, Osakada, and Sato. Oxidative addition of the aryl halide 3 to Ni(cod)₂ in the presence of 2,2′-bipyridine gives NiX(Ph)(bpy) 4, which reacts with CO₂ to deliver the carboxylation products 5 (Scheme [1]).[7]

Scheme 1 Ni-mediated carboxylation of aryl halides

Early examples of the Ni-catalyzed carboxylation of aryl halides were reported as electrochemical processes. For instance, Perichon, Fauvarque, and co-workers described the electroreductive carboxylation of aryl halides 3 with CO₂ in the presence of nickel catalyst.[8] The proposed mechanism is shown in Scheme [2]. Oxidative addition of aryl bromide 3 (X = Br) to Ni(0)(dppe) gives ArNi^IIBr(dppe), which undergoes one-electron reduction to deliver a Ni(I) complex. Carboxylation­ of the arylnickel(I) species and one-electron reduction closes the cycle by regeneration of Ni(0) and formation of arenecarboxylate (Scheme [2]). On the basis of the kinetic analysis of the CO₂ insertion step, a Ni(III) intermediate is also proposed prior to the formation of ArCO₂Ni(dppe)­.[9]

Scheme 2 Ni-catalyzed carboxylation of aryl bromide via electrochemical reduction

Similarly, Torii, Fauvarque, and co-workers disclosed an efficient Pd-catalyzed electroreductive carboxylation of aryl halides with CO₂ (Scheme [3]).[10] Both aryl iodides 3 (X = I) and aryl bromides 3 (X = Br) are effective, however aryl chlorides did not give the corresponding products. It is noteworthy that the formation of arenecarboxylate was not competitive with the formation of biaryl, which is the byproduct for the metal-free electroreductive carboxylation.[11]

Scheme 3 Pd-catalyzed carboxylation of aryl halides

The transition-metal-catalyzed electroreductive carboxylation of aryl halides with CO₂ is a promising tool for the preparation of arenecarboxylic acids.[12] However the reaction scope is quite restricted. In 2009, Correa and Martín reported a Pd-catalyzed carboxylation reaction of aryl bromides 6 using a substituted biphenyl monophosphine ligand L₁ (t-Bu-Xphos) under 10 atm of CO₂.[13] The insertion of CO₂ into the Pd–aryl σ-bond is suggested in the proposed mechanism. Diethylzinc behaves as the reducing agent to regenerate Pd(0) and zinc carboxylates are formed (Scheme [4]). In 2012, Tsuji and co-workers described the Ni-catalyzed carboxylation of aryl chlorides 3 (X = Cl) under 1 atm of CO₂ using Mn powder as a reducing agent.[14] The proposed mechanism involves an oxidative addition of the aryl chlorides to Ni(0), formed by reducing Ni(II) with Mn. The resulting Ni(II) intermediate is reduced to a Ni(I) intermediate, which reacts with CO₂ to give the product (Scheme [5]).

Scheme 4 Pd-catalyzed carboxylation of aryl halides

Scheme 5 Ni-catalyzed carboxylation of aryl halides

Copper catalysts have been used in the carboxylation of arylboronates with CO₂, and an aryl–copper species is suggested­ as a key intermediate for the reaction. In 2013, Tran-Vu and Daugulis reported the Cu-catalyzed carboxylation of aryl iodides 7 under 1 atm of CO₂.[15] The reaction is compatible with various functional groups such as ester, bromide, chloride, fluoride, ether, amino, hydroxy, and carbonyl. The carboxylation of aryl–Cu species is suggested to be rate-limiting step and the mechanism is likely to involve copper clusters (Scheme [6]).

Scheme 6 Cu-catalyzed carboxylation of aryl iodide

Besides aryl halides, aryl pseudohalides are suitable electrophiles for carboxylation reactions. For instance, Martín and co-workers demonstrated the Ni(II)-catalyzed carboxylation of C(sp²)–O bonds with CO₂, and it is necessary that the C–O bonds are pre-activated as esters, e.g. 8, derived from cheap alcohols.[16] The reaction mechanism is similar to C–halides carboxylation using Mn as reducing reagent (Scheme [7]). These carboxylation reactions provide alternative protocols for the preparation of arenecarboxylic acids with the advantage of functional group tolerance compared with the classic carboxylation of Grignard reagents. Obviously, the development of novel carboxylation reactions to avoid the use of stoichiometric amounts of metals (Mn or Zn) or zinc agents as reducing agents is in high demand.

Scheme 7 Ni-catalyzed carboxylation of aryl pivalates

3

Carboxylation of Vinyl Halides and Pseudohalides

The carboxylation of a vinyl bromide 10 was achieved through an electroreductive process in the presence of palladium catalyst (Scheme [8]).[10] Interestingly, this carboxylation gives dicarboxylic acid 11, which is formed via the primary product 10a. Electrolysis of carboxylate 10a gives the dicarboxylic acid 11 in 61% yield. In 1997, Juland and Négri reported the Pd-catalyzed carboxylation of vinyl trif­lates 12 with CO₂ (Scheme [9]).[17]

Scheme 8 Pd-catalyzed carboxylation of vinyl bromides

Scheme 9 Pd-catalyzed carboxylation of vinyl triflates

In 2015, Tsuji and co-workers[18] reported a cobalt-catalyzed reductive carboxylation reaction. In this work, various alkenyl triflates 14 were converted into α,β-unsaturated carboxylic acids 15 under mild conditions (Scheme [10]). In addition, a range of hindered aryl triflates substituted in the 2-aryl position were smoothly carboxylated.

Scheme 10 Co-catalyzed carboxylation of vinyl triflates

4

Carboxylation of Benzyl Halides and Pseudohalides

The adduct of Co(salen) and CO₂ was described by Fachinetti­ and Floriani in 1974.[19a] In 1985, Perichon and co-workers demonstrated the Co-catalyzed reductive carboxylation of benzyl chloride 16 with CO₂ via an electrochemical process, although the reaction mechanism was not explored (Scheme [11]).[19b]

Scheme 11 Co-catalyzed carboxylation of benzylic or allylic chlorides

In 2013, the Martin group reported the Ni-catalyzed carboxylation of benzyl chlorides 16 with CO₂ (Scheme [12]);[20] the reaction tolerated various functional groups. Interestingly, with slightly modified reaction conditions, the carboxylation of secondary alkyl bromides 19 was achieved (Scheme [13]).[20] The proposed mechanism suggests that a Ni^I intermediate may be crucial in the catalytic cycle and MgCl₂ promotes the carboxylation of benzyl chloride with CO₂ by stabilizing the Ni–CO₂ adduct and accelerating the CO₂ insertion (Scheme [14]).[20] [21]

Scheme 12 Ni-catalyzed carboxylation of benzyl chlorides (Cp = cyclopentyl)

Scheme 13 Ni-catalyzed carboxylation of benzyl bromides

Scheme 14 The proposed mechanism for the carboxylation of benzyl halides

In 2014, the Martin group also demonstrated the carboxylation of benzylic C(sp³)–O bonds via a similar pathway (Scheme [15]).[16] Notably, the carboxylation of secondary benzyl–type derivatives (e.g., 18f,h,i) was also effective. However, the protocol is limited to π-extended systems; simple phenyl-containing compounds are not effective. Taking advantage of hemilabile directing group,[22] the Martin group elegantly developed the efficient carboxylation protocol with simple benzyl esters 20 (Scheme [16]).[16] Furthermore, the Martin group demonstrated that ammonium salts 21 could be used as benzyl electrophiles for carboxylation (Scheme [17]).[23] Interestingly, this reaction was insensitive to electronic changes on the arene and suitable for non-extended π-systems. Most striking, secondary benzyl ammonium salts 22 possessing α-hydrogen atoms were successfully carboxylated. The dimerization and α-hydride elimination pathways were avoided in this protocol (Scheme [18]).[23]

Scheme 15 Ni-catalyzed carboxylation of benzyl C(sp³)–O bonds

Scheme 16 Ni-catalyzed carboxylation of benzyl esters with directing groups

Scheme 17 Ni-catalyzed carboxylation of primary ammonium salts

Scheme 18 Ni-catalyzed carboxylation of secondary ammonium salts

In 2015, He and co-workers reported the Pd-catalyzed carboxylation of benzyl chlorides 16 with CO₂ (Scheme [19]).[24] The reaction afforded the corresponding phenylacetic acids 17 in combination with Mn powder as a reducing reagent and MgCl₂ as an indispensable additive.

Scheme 19 Pd-catalyzed carboxylation of benzyl chlorides

Lu and co-workers reported the asymmetric electrocarboxylation of 1-phenylethyl chloride (16b) with CO₂ in the presence of Co catalyst (Scheme [20]).[25] Although the yield and the ee values are moderate, this elegant study provides an alternative method for the synthesis of optically active carboxylic acids.

Scheme 20 Co-catalyzed asymmetric electrocarboxylation of 1-phenylethyl chloride (GC = glassy carbon)

5

Carboxylation of Allyl Halides and Pseudohalides

In 1976, Inoue and co-workers described the carboxylation of allylic Pd species with CO₂.[26] In the 1980s, Perichon­[19b] and Fauvarque[10] described the electroreductive carboxylation of allylic substrates with CO₂ in the presence of cobalt and palladium catalysts, respectively (Scheme [21]). Unfortunately, regioselectivity is not achieved under these conditions.

Scheme 21  Electroreductive carboxylation of allylic substrates with CO2 in the presence of� cobalt and palladium catalysts

In 2014, the Martin group demonstrated the Ni-catalyzed carboxylation of allyl esters with CO₂ (Scheme [22]).[27] The regioselectivity is highly controlled by the ligand and both carboxylic acid products were achieved in a highly selective manner with 1,10-phenanthroline L₂ or quaterpyridine L₅ ligands.

Scheme 22 Ni-catalyzed regiodivergent reductive carboxylation of allyl esters with CO₂

Carboxylation with an allylic alcohol without pre-activation is a highly attractive transformation from the perspective of step-economy. Mita, Sato, and Higuchi elegantly demonstrated the Pd-catalyzed carboxylation of allylic alcohols 28 and 29 with CO₂ using ZnEt₂ as the reducing agent (Scheme [23]).[28] The allylic alcohol is likely activated by the Lewis acid ZnEt₂ or by carbonate formation with CO₂.[29] This carboxylation is highly regioselective and gives predominantly branched carboxylic acid. The proposed mechanism is shown in Scheme [23].

Scheme 23 Pd-catalyzed regioselective carboxylation of allylic alcohol with CO₂

In 2017, the Martin group elegantly demonstrated the switchable site-selective Ni catalyzed carboxylation of allylic alcohols with CO₂. The regiodivergency can be modulated by the type of ligand employed (Scheme [24]).[29]

Also in 2017, Mei and co-workers developed an efficient Ni-catalyzed reductive carboxylation of allylic alcohols with CO₂, providing linear β,γ-unsaturated carboxylic acids 24 as the sole regioisomer with generally high E/Z stereoselectivity (Scheme [25]).[30] In addition, the carboxylic acids were generated from propargylic alcohols via hydrogenation to give allylic alcohol intermediates, followed by carboxylation. A preliminary mechanistic investigation suggests that the hydrogenation step is made possible by a Ni–hydride intermediate produced by a hydrogen atom transfer from water.

6

Carboxylation of Propargyl Halides and Pseudohalides

Scheme 24 Ni-catalyzed regiodivergent carboxylation of allylic alcohol with CO₂

Scheme 25 Ni-catalyzed carboxylation of allylic and propargyl alcohols with CO₂

In 2014, Tsuji, Fujihara and co-workers reported the Co-catalyzed carboxylation of propargyl acetates 32 with CO₂ at room temperature using Mn powder as a reducing agent (Scheme [26]).[31] The proposed mechanism suggests a Co(II) C as a possible intermediate that reacts with CO₂ in the catalytic cycle (Scheme [27]).[31]

Scheme 26 Co-catalyzed carboxylation of propargyl acetates with CO₂

Scheme 27 The proposed mechanism for the Co-catalyzed carboxylation of propargyl acetates

7

Carboxylatioin of Alkyl Halides and Pseudohalides

In 2014, the Martin group developed the catalytic carboxylation of unactivated primary alkyl bromides 36 with CO₂ with various functional groups (Scheme [28]).[32] Notably, alkyl sulfonates and trifluoroacetates 38 are also reactive (Scheme [29]).[32] The reaction is proposed to proceed by a free-radical pathway.

Scheme 28 Ni-catalyzed carboxylation of alkyl bromides with CO₂

Scheme 29 Ni-catalyzed carboxylation of alkyl sulfonates and trifluoroacetates with CO₂

In 2016, the Martin group developed the catalytic carboxylation of unactivated primary, secondary, and tertiary alkyl chlorides 39 with CO₂ (Scheme [30]).[33] The Martin group also described the Ni-catalyzed reductive cyclization/carboxylation of unactivated alkyl halides 40 and 42 with CO₂ under mild conditions. This is an efficient way to synthesize elusive tetrasubstituted olefins 41 (R¹ ≠ H) (Scheme [31]).[33] [34]

Scheme 30 Ni-catalyzed carboxylation of alkyl chlorides with CO₂

Scheme 31 Ni-catalyzed reductive cyclization/carboxylation of unactivated alkyl halides with CO₂

In 2016, the Martin group employed the nickel-catalyzed reductive carboxylation protocol for the synthesis of cyclopropanecarboxylic acids 44 with carbon dioxide as a C1 synthon (Scheme [32]).[35] In 2017, the Martin group elegantly developed the nickel-catalyzed chain-walking carboxylation of alkyl halides at remote sp³ C–H sites, which was realized via iterative β-hydride elimination/migratory insertion sequences (Scheme [33]).[36] The use of ligand L₁₁ was crucial in achieving the site-selectivity. This discovery provides an efficient method to convert cheap starting materials into valuable fatty acids.

Scheme 32 Ni-catalyzed carboxylation of cyclopropyl bromides

Scheme 33 Ni-catalyzed carboxylation of discrete alkyl halides at remote sp³ C–H sites

8

Direct Carboxylation of C–H Bonds

Direct carboxylation of C–H bonds is one of the most straightforward ways to afford carboxylic compounds. The Nolan and Hou groups both independently succeeded in the C–H carboxylation of aromatic compounds using N-heterocyclic carbene (NHC) transition-metal complexes as catalysts. The reaction relies heavily on the acidity of the C–H bonds and uses a gold complex (Scheme [34])[37] or a copper complex (Scheme [35]).[38]

Scheme 34 Au-catalyzed aromatic C–H carboxylation with CO₂

Scheme 35 Cu-catalyzed carboxylation of benzoxazole with CO₂

In 2011, the Iwasawa group reported a catalytic nucleophilic carboxylation reaction using carbon dioxide as the carbon source through C–H bond activation (Scheme [36]).[39] The proposed catalytic cycle is shown in Scheme [37].[39] 2-Phenylpyridine undergoes C–H bond activation to form intermediate F, which releases methane to afford the key intermediate G that reacts with CO₂ via nucleophilic addition.

Scheme 36 Rh-catalyzed carboxylation of arylpyridines with CO₂

Scheme 37 Proposed catalytic cycle of Rh-catalyzed carboxylation of arylpyridines with CO₂

In 2017, Hou and co-workers developed a copper-catalyzed allylic C–H bond carboxylation of allyl aryl ethers 54 with CO₂, which was achieved through deprotonative alumination with an aluminum ate compound [i-Bu₃Al(TMP)Li] followed by NHC–copper-catalyzed carboxylation of the resulting aryloxy allylaluminum species (Scheme [38]).[40]

Scheme 38 Cu-catalyzed carboxylation of allylic C–H bond of allyl aryl ethers with CO₂

Also in 2017, Sato and co-workers developed a catalytic direct allylic C(sp³)–H carboxylation by using the Co(acac)₂/Xantphos/AlMe₃ system, which enabled highly regioselective transformation of allyl groups into linear but-3-enoic acid motifs with good functional group tolerance (Scheme [39]).[41] The proposed mechanism suggests a Co^I intermediate N might be crucial in the catalytic cycle and AlMe₃ plays a dual role in the reaction (Scheme [40]).[41]

Scheme 39 Co-catalyzed carboxylation of the allylic C(sp³)–H bond of terminal alkenes with CO₂

Scheme 40 Proposed mechanism of Co-catalyzed carboxylation of the allylic C(sp³)–H bond with CO₂

Besides these carboxylations of C–H bonds under transition-metal catalysis, some carboxylations of C–H bonds under transition-metal-free[42] or light-promoted[43] conditions were reported, which will not be discussed in detail here.

9

Conclusions and Perspectives

In the past few years, transition-metal-catalyzed carboxylation of electrophiles with carbon dioxide has become a promising new catalytic transformation. However, the development of this field is still in early stages. In particular, site-selective C–H carboxylation is still a challenge. Furthermore, enantioselective carboxylation is another clear frontier in this area. Finally, the development of large-scale catalytic carboxylation reactions with low catalyst loadings is needed.

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