CO2 as a Building Block for the Catalytic Synthesis of Carboxylic Acids

C H A P T E R

6 CO2 as a Building Block for the Catalytic Synthesis of Carboxylic Acids Matilde V. Solmi*,†,1, Marc Schmitz*,1, Walter Leitner*,‡ *Institut f€ ur Technische und Makromolekulare Chemie, RWTH Aachen University, Aachen, Germany † Laboratory of Chemistry, Catalysis, Polymers and Processes (C2P2), University of Lyon, Villeurbanne, ulheim an der Ruhr, Germany France ‡Max Planck Institute for Chemical Energy Conversion, M€

O U T L I N E 1 Introduction 105 105 1.1 Scenario Context and Motivation 1.2 Applications of Carboxylic Acids 106 1.3 Industrial Production of Carboxylic Acids 107 2 Synthesis of Carboxylic Acids Using 108 CO2 as a Building Block

1 INTRODUCTION 1.1 Scenario Context and Motivation Daily reports about global warming, its serious consequences, and its causes, call scientists and politicians for solutions and more engagement. The World Meteorological Organization (WMO)

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2.1 Organometallic Substrates 2.2 Organic Substrates 3 Conclusion and Future Challenges Acknowledgment References

110 113 120 120 121

officially reported the current atmospheric CO2 concentration was 403.3 ppm in October 2017 [1]. Based on a carefully analyzed body of scientific evidence, it is widely accepted that anthropology CO2 emissions induced the global warming effects from the beginning of the industrial revolution in the 19th century. This is mainly related to the use of fossil resources, starting historically with the use of coal, followed by crude oil, and natural gas. The development of a closed anthropogenic carbon cycle through stepwise reduction of the carbon footprint in all industrial sectors is a major societal goal which needs novel,

These authors contributed equally to this work.

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# 2019 Elsevier B.V. All rights reserved.

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integrated, and interdisciplinary scientific approaches for adequate technological solutions. In this context, Carbon Capture and Utilization (CCU or Carbon Capture and Recycling, CCR) offers the potential to utilize CO2 produced in industrial sectors such as energy, steel, or cement productions as a feedstock for chemical transformations [2–4]. In detail, this embeds the synthesis of fuels, bulk chemicals, commodities, and even fine chemicals and pharmaceuticals. [5,6]. While these applications individually or even in total are unable to “recycle” the enormous amount of 36 Gt of CO2 emitted from anthropogenic sources [7], the use of CO2 can significantly reduce the carbon footprint of chemical production, in particular, when coupled with the use of renewable energy input. [8–10]. Thus, even if CO2 conversion will not provide a prompt “Philosopher’s stone” to directly mitigate the CO2 level in the atmosphere, its utilization offers plenty of ways and advantages toward a more sustainable chemical value chain. From a “Green Chemistry” point of view, CO2 is a highly attractive raw material [11,12]: it is renewable, ubiquitous, nontoxic, nonflammable, and a highly versatile building block in terms of potential applications [6,9,10,13,14]. Its nonharmful properties—compared to fossilbased C1 building blocks like CO, phosgene, HCN or formaldehyde—embodies positive aspects for the industry. It also forms a potentially important feedstock for a future largely renewable energy-chemistry nexus [13,14]. Among the many conceivable target products, carboxylic acids appear highly attractive, but are at the same time particularly challenging. Their general formula RC(O)OH implies a close relationship to the CO2 molecule. However, while CO2 reacts readily with O- and N-nucleophiles to give carbonic and carbamic acids, corresponding reactions to form CdC bonds require typically stoichiometric use of carbanions, such as Grignard reagents or other

metal alkyl and aryl species. Alternative pathways involving a catalytic combination of suitable substrates with CO2 are of great interest to synthesize carboxylic acids, a class of highly important chemical products as outlined in the next section.

1.2 Applications of Carboxylic Acids In general, carboxylic acids and their derivatives are highly important for their synthetic utilization in the production of polymers, pharmaceuticals, solvents, food additives (commodities), etc. These chemicals are important also from an economic standpoint. The global market for carboxylic acids is predicted to grow yearly by 5% to 2023, reaching about $20 billion [15]. The most important application of carboxylic acids is in polymer industry, where they are used both as monomers and additives. Many examples are known, with a wide variety of uses, from fibers to packaging and coating. Polyvinyl acetate (glue) is obtained from vinyl acetate monomer, which is mainly produced from acetic acid [16,17]. Polyamides are of high importance, and generally produced from dicarboxylic acids and amines. In particular, adipic acid is used for the production of nylon fibers, which are well known and used to substitute natural polyamides (i.e., wool and silk) [18]. Acrylic and methacrylic polymers are obtained from acrylic and methacrylic acids. These polymers are used for solid detergents, dishwasher powders, cement additives, super absorbers, and other products of daily life. Moreover, the monomers could be mixed with other acids or derivatives (i.e., maleic anhydride and fumaric acid) to obtain polymers with different properties. Some of them are widely employed in paint formulation [19]. Polyesters are obtained from dicarboxylic acids and diols. The most important polymer in terms of application and commercial value is PET

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1 INTRODUCTION

(polyethylene terephthalate), obtained from terephthalic acid and ethylene glycol. PET is mainly employed in beverage packaging. Changing the type of acid used influences the properties of the final polymers leading to different applications [20]. Cellulose esters are commonly used as fibers. They are mainly obtained from acetic acid and cellulose, but fibers with different properties are obtained when other acids are used instead (up to C4) [21]. Polyimides obtained from aromatic acids are usually heat resistant materials [22]. Alkyd resins are obtained from diacids and alcohols and are applied in coatings. In addition to the reported applications as monomers, carboxylic acids are also employed as additives to modify the properties of the synthesized polymers. Long chain carboxylic acids (>C9) are used as additives for alkyd resins films [23], while trimellitic anhydride is added as a plasticizer [22]. Besides their applications in the polymer industry, carboxylic acids are employed for different treatments in the textile and leather industry. In particular, oxalic acid [24] and formic acid [25,26] are used for treatments such as pickling or dyeing [27]. Oxalic acid is also used in the metal industry for different purposes (i.e., rust removal) [27]. Some carboxylic acids (acetic acid and isobutyric acid), their esters (C5 acids esters), and other derivatives (i.e., acetamide), are important solvents [28]. For instance, acetic acid is used as a solvent in the production of terephthalic acid and mixed with water in the production of acetic acid itself. In agrochemical industry, carboxylic acids are used to obtain herbicides, fungicides, and rodenticides. In particular, propionic acid is employed in the production of herbicides, as well as malonate. Isovaleric acid is used for the production of fungicides and rodenticides [28]. Carboxylic acids are used in the pharmaceutical industry as a synthetic tool or as medicine. Many molecules including a carboxylic acid moiety have beneficial effects. The most

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important examples in this field are acetyl salicylic acid, commercialized as aspirin [29], 2-(4-isobutylphenyl)propionic acid, commonly known as ibuprofen [15], and (RS)-2-(3-benzoylphenyl)-propionic acid, usually sold as ketoprofen [30]. Carboxylic acids are employed in the food and feed industries as well. Acetic acid diluted with water is used in vinegar. Preservatives for food and feed are obtained from propionic acid [31], formic acid, and salts of isobutyric acid. Moreover, carboxylic acids are used to enhance aroma and flavors [28]. Apart from the above-mentioned applications, carboxylic acids could be used in dyes and pigments. Recently, new applications of carboxylic acids have been developed. Shell Oil Corporation reported the use of valeric acid in biofuels [32], while formic acid is discussed as a storage for both H2 and CO, since it can be either decomposed to CO2 and H2 or to CO and H2O [33–35].

1.3 Industrial Production of Carboxylic Acids The majority of carboxylic acids are produced via the oxidation processes. Aliphatic carboxylic acids (C4–C13) are produced through oxidation of the corresponding aldehydes, usually obtained by the Oxo synthesis (alkene hydroformylation). Metal salt catalysts are generally used with O2 as an oxidizing reagent. Both liquid and gas phase reactions are possible; however liquid phase processes are generally implemented. Oxidation processes are also common for the production of aromatic carboxylic and dicarboxylic acids, starting from alkyl substituted aromatic compounds. Hereby, the catalysts are usually Co or Mn salts, O2 is used as an oxidant, and both gas and liquid phase processes are feasible [22,28]. For example, terephthalic acid is obtained by a liquid phase oxidation of p-xylene [36]. Apart from

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O2, HNO3 and KMnO4 could be used as an oxidant, too. For instance, adipic acid is obtained from the oxidation of cyclohexanol, cyclohexanone, or a mixture of them (KA-Oil) with HNO3 [37]. Other types of carboxylic acids are produced by carbonylation of alcohols or alkenes. Formic acid is mainly produced by BASF with a twostep strategy: First, the base-catalyzed carbonylation of methanol leads to methyl formate, which, in second-step, gets hydrolyzed to formic acid and methanol [25]. Pivalic acid and other tertiary carboxylic acids can be obtained from alkenes with the Koch synthesis. CO and H2O are used in two separated steps to hydroxycarbonylate the alkene using H3PO4
BF3 or H2SO4 as a catalyst [28,38]. Propionic acid is produced by the Reppe synthesis. This process (catalyzed by Ni complexes) uses a high pressure of CO and H2O to hydoxycarbonylate ethylene [39,40]. Acetic acid is also mainly produced via carbonylation, using, in this case, methanol as substrate. With over 7 mio. Tons per year acetic acid is no doubt one of the biggest homogeneously catalyzed processes worldwide [41]. Implemented by BASF for the first time in 1960 (Co complex as catalyts), and further developed by Monsanto in 1966 ([Rh(CO)2I2] as a catalyst), the large-scale production of acetic acid from methanol was possible [41]. The Cativa process ([Ir(CO)2I2] complex as a catalyst) has been developed and implemented on a large scale as it provides a higher selectivity for the CO conversion. Further processes derived of these fundamental works comprise the Acetica- [42] Process, using an immobilized Rh complex as a catalyst. The SaaBre process is fundamentally different as it uses zeolites as a catalyst for a multistep heterogeneously catalyzed synthesis of acetic acid [17]. Obviously, CO2 would be an attractive renewable and nontoxic alternative to CO as a C1 building block for carboxylic acids. Today, salicylic acid is already produced industrially using CO2 via the Kolbe-Schmitt synthesis

converting phenol, with CO2 in a basic environment (NaOH). However, this stoichiometric reaction has limited synthetic scope being applicable to phenolic-type substrates only. In terms of catalytic processes, the synthesis of formic acid starting from CO2 and H2 has been widely reported in literature. These processes have been extensively reviewed and discussed in recent years and will therefore not be re-iterated here [10,34]. In this chapter, we want to give a comprehensive overview on the developed protocols to produce higher carboxylic acids (C2+) starting from a wide variety of substrates, catalytic systems, and approaches. Hopefully, providing this concise overview will help to identify ways forward for improving the existing systems (up to industrial scales) and encourage scientists to develop new strategies.

2 SYNTHESIS OF CARBOXYLIC ACIDS USING CO2 AS A BUILDING BLOCK The use of CO2 as a building block for synthesis can be classified according to the oxidation state of the central carbon: it ranges from the “complete incorporation” of the molecule while retaining its structure (e.g., in organic carbonates) to a “complete hydrogenation” (to produce methane and other hydrocarbons). In between, there is the “partial reduction” which opens the use as selective C1 building blocks providing further classes of compounds such as carboxylic acids, esters, aldehydes. or alcohols depending on the substrates/reagents [6,43,44]. In this section, current synthetic routes to (relevant) carboxylic acids using CO2 to construct the carboxylate function are presented. Two different approaches on how the CO2 molecule can take part in the synthesis of carboxylic acids will be discussed. We will focus on protocols where CO2 directly serves as the C1 building block in combination with a substrate and a

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FIG. 6.1 Schematic overview of the substrates and the systems illustrated in this book chapter. The substrates are depicted starting from the more reactive (Grignard reagents) ending to the less reactive (sp3 CdH bonds).

suitable catalytic system to yield the carboxylic acid products. Alternatively, CO2 could be converted into more activated C1 buildings blocks like carbon monoxide or syngas in a separate transformation step and unit operation via reverse water gas shift (rWGS) reaction or co-electrolysis [10]. These primary products can then be converted via currently implemented routes and technology strategies. Both ways offer opportunities and new perspectives by using CO2 to reduce the implemented fossilbased streams. In fact, some of the protocols applying CO2 directly may involve a rWGS

equilibrium in their molecular mechanism (vide infra). A schematic overview of the substrates and the systems illustrated in this chapter is depicted in Fig. 6.1. The substrates are represented following their reactivity: from the more reactive (Grignard reagents) to the less reactive (sp3 CdH bonds). Organometallic reagents such as Grignard reagents, organo lithium reagents, or other highenergetic reagents can be used to increase the energy level of the starting systems providing enough driving force for CdC bond formation at CO2 directly. The chemical coupling with

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FIG. 6.2 The synthesis of carboxylic acids using CO2 as building block can be reached reducing the energy barrier by different strategies: increasing the energy of the starting system using highly reactive substrates or additives (organometallic reagents, metals, H2, etc.) or decreasing the energy barrier acting on the transition state in the presence of a catalyst (homogeneous, heterogeneous or photocatalysts). The two approaches are often combined and both additives and catalysts are present in order to reduce the energy barrier.

higher energy compounds such as alkenes, alcohols, ketones, or epoxides can also provide sufficient thermodynamic driving force in particular when combined with (renewable) H2 as reductant (see Fig. 6.2). The energy of the starting system can also be improved by means of additives such as organometallic or metallic reagents as reducing agents or for derivatization of the acid product. In most cases, a catalytic system is required to decrease the energetic barrier influencing the energy of the transition state or, even more frequently, opening a new molecular trajectory with an overall lower energy profile (Fig. 6.2).

2.1 Organometallic Substrates In this, and in the following section, selected examples for carboxylations of organometallic substrates are reported. In general, the electrophilic carbon atom of CO2 tends to react with nucleophilic centers. Many organometallic

reagents and related substrates have a strongly nucleophilic carbanionic-type functionality which usually reacts more or less easily with CO2 to give the corresponding carboxylic acid salt (Fig. 6.3). Three types of substrates can be differentiated: very polarized C-MXn (C-metal-ligands) bonds can react directly with CO2 without the need of additional catalysts/ auxiliaries (Grignard reagents, organolithium and organoalanes), intermediate polarized C-EXn (C-element-ligands) bonds which need a transition metal catalyst to react with CO2 and often stoichiometric amounts of reducing agents (organozinc, alkenylzirconocenes, organostannanes, alkenylboronic esters), less polarized C-EXn (C-element-ligands) bonds, which need stoichiometric additives to create the intermediate that reacts with CO2 (organosilanes). We will first discuss briefly the reagents that do not require a catalyst and then in the next section give examples for those that need a catalyst for activation.

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R

R

MXn

EXn

CO2

R

COOH

R

COOH

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(acidic work up) CO2 1) Catalyst (+ additive) 2) (Acidic work up)

M = Mg, Al, Li. E = Zn, Zr, Sn, B, Si. X = Halogen, CxHy or O-CxHy

FIG. 6.3 General reaction scheme of organometallic reagents with CO2. Grignard reagents, organoalanes and organolithium do not need catalysts or additives to activate CO2 (top scheme). Other reagents require mostly a transition metal catalyst to activate CO2 (bottom scheme).

2.1.1 Grignard Reagents Grignard reagents (RMgX, X ¼ halogen, R ¼ alkyl or aryl) are highly active nucleophiles. They were reported to activate CO2 to carboxylic acids already in the 1900s, by Grignard [45]. Unfortunately, their high reactivity limits the possibility of using substrates with functional groups (electrophiles), which would react faster with the Grignard nucleophile leading to a low chemo-selectivity in the desired carboxylic acid. Nevertheless, their high activity allows their transformation in carboxylic acids under mild conditions such as 1 bar of CO2 and room temperature [46] and the development of a continuous flow processes [47]. Dowson reported an analysis from which it was concluded that the synthesis of acetic acid starting from CH3MgX and captured CO2 was estimated to be economically feasible, with costs comparable to those of well-established processes [48]. However, the Grignard reagents should be regenerated by electrolysis of MgX2 salts obtained as byproducts of the reaction, requiring extremely high amounts of energy to be obtained from renewable resources at low cost. Furthermore, the handling of Grignard reagents on the scale of bulk chemicals is prohibitive with current technologies for safety reasons. Therefore, such

a process would not follow the Green Chemistry principles, despite the use of CO2 as feedstock. 2.1.2 Organoalanes Organoalanes (R-AlR0 2, with R being alkyl or aryl, and R0 alkyl, alkoxy, or halogen) compounds are also reactive in presence of CO2 leading to the corresponding carboxylic acids. The first report of this reaction was made in 1888 by Friedel and Crafts. They found that PhAl2Cl5 reacts with CO2, giving benzoic acid [49]. Another way to exploit Al reagents is to use the Lewis acidity of aluminum salts, which can coordinate either the CO2 molecule leading to a higher electrophilic carbon or activate the substrate. For example, AlBr3 can coordinate to the oxygen atom of phenol, which is transformed into salicylic acid in the presence of supercritical CO2 [50]. 2.1.3 Organolithium Organolithium compounds are strong nucleophiles, which can be used as substrates for carboxylations. In their granted patent from 1998, Merck described the transformation of aryl halides via organolithium compounds and reaction with CO2 to carboxylic acids [51]. Like

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Grignard reagents, organolithiums have a limited tolerance toward electrophilic functional groups, but they are highly reactive and therefore suitable for developing continuous flow processes [52]. 2.1.4 Organometallic Substrates Coupled With Catalytic Systems The previous examples of reactive substrates have the common feature of being generally limited to substrates with few functional groups. In this sense, less polarized metalcarbon bonds have been used to implement the “chemical tools” to obtain carboxylic acids with a wider group tolerance [53]. Due to the fact that they are less reactive than the hard nucleophiles, a suitable catalyst ([M]) is often required to achieve practical reaction rates. Up to now, a number of efficient homogeneous catalysts have been reported to facilitate such carboxylation reactions. Generally, all examples reported follow a very similar reaction mechanism. By an initial transmetallation step, first a [M]dC bond is formed. CO2 is than inserted into that bond to give the corresponding [M]dOOCdC complex. If [M] is a late transition metal complex, the [M]dO bond will be easier to break compared to the [M]dC bond of the starting complex, leading to the production of the carboxylic acid [54]. An additive, or the organometallic reagent itself, may support the reductive liberation of the product, regenerating the active species and releasing the carboxylic acid or derivatives. In some cases, an acidic work up is required to observe the free carboxylic acid. 2.1.5 Organozinc and Organostannanes Organozinc compounds are sensitive and highly reactive compounds, but a broader substrate scope is accessible compared to the previous reported methods. Many protocols have been developed in the past years. In 2008, Dong et al. transformed arylzinc compounds into aryl carboxylic acids using Aresta’s complex or related Pd analogous as a catalyst. The

transformation is quite efficient and allows up to 95% yield of the desired acid product at mild conditions [55]. Another example is the production of α,β-unsaturated carboxylic acids starting from alkynes, via an in-situ formed vinylzinc compound [56]. In 2015, the synthesis of α,β-unsaturated tri-substituted carboxylic acids was performed starting from alkynes, after an initial coordination step on a Zr complex. [57] α,β and β,γ-unsaturated carboxylic acids can be obtained starting from organostannanes using a Pd complex as a catalyst [54,58]. 2.1.6 Boronic Esters Alkenylboronic esters have been widely studied as nucleophiles, which can react with CO2 in the presence of a transition metal catalyst. Alkenylboronic esters can be readily synthesized and show a broad functional group tolerance [59]. In general, all the protocols to produce carboxylic acids, starting from alkenylboronic esters, require the use of (over) stoichiometric amount of base, in addition to a transition metal catalyst. In 2006, Iwasawa et al. reported the production of carboxylic acids starting from arylboronic esters catalyzed by a Rh complex, in the presence of CsF (over stoichiometric amounts), in order to gain the desired products in good yields [59]. Interestingly, even dicarboxylic acids were obtained starting from alkynes in yields up to 76% by Skrydstrup et al. [60]. Other systems using tBuOK as a base were also developed [61,62]. 2.1.7 Organosilanes Organosilanes are attractive nucleophiles compared to other organometallic species: they are less toxic, easier to prepare, and easier to handle. The reported examples for organosilane carboxylations differ from the other mentioned activated substrates because no transition metal catalyst is used for the carboxylation step [63]. The CdSi bond features a low polarity, therefore, reaction with CO2 is rather difficult. In the reported examples, a fluoride source is used to generate a carbanion synthon ([R4FSi]),

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which can react with CO2 to form a carboxylic acid [63]. In 2016, the group of Cantat presented a system able to transform heteroaromatic silanes into their corresponding esters. The protocol requires a stoichiometric fluoride source (TBAT ¼ tetrabutylammonium difluorotriphenylsilicate) and an organohalide. Additionally, this example is very interesting because they managed to obtain polyesters, which could incorporate up to eight units of CO2. Although the apparent degree of polymerization is low, this is the first example of synthesis of polyesters incorporating CO2 [63]. Overall, all the above-mentioned substrates can be easily converted into carboxylic acids using relatively mild reaction conditions. Many of the reported examples can convert even low pressures of CO2 (1 bar) at room temperature. This is mainly due to the high reactivity of these substrates, which also has some drawbacks. The activated molecules must be prepared, and they are sometimes not easy to handle. Moreover, the presence of stoichiometric amounts of reducing agents ((organo)metallic), salts, or simply activating (metal) groups, will lead to the production of stoichiometric amount of wastes. Although no Life Cycle Assessment (LCA) has been reported for all the shown systems, the application of these methods will be best directed to quite specific synthetic challenges typically on a smaller scale to be consistent with principles of “Green Chemistry.” Among the principles, there are the recommendations of improving the atom efficiency, minimizing derivatives and using catalytic processes instead of stoichiometric [11]. Therefore, the development of new catalytic processes, which allows the incorporation of CO2 directly with organic substrates, is desirable.

2.2 Organic Substrates The sections above discusses examples which deal with the coupling of organometallic reagents which can be viewed as already highly activated substrates toward CO2 to generate carboxylic acids. These transformations proceed

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via stoichiometric use of the metal component of the substrate. However, the recent examples discussed for the CO2 utilization further demonstrate with no doubt that this field is growing and is of high interest. Considering fewer, or even nonactivated substrates, one can anticipate higher energy demands for the actual coupling step, both in terms of a thermodynamic driving force as well as a kinetic barrier. However, as these requirements are basically “outsourced” in the generation of the organometallic reagents, the direct conversion of the organic substrates seems highly attractive if it can be achieved with sufficient efficiency. The following part will thus focus on stateof-the-art protocols for catalytic coupling of CO2 with nonactivated organic substrates containing functional groups like halides, oxygen, or even carbon-hydrogen bonds solely. These include compounds based on implemented value chains, like cracking- or reforming-products, FischerTropsch-synthesis, or further common petrochemical techniques, but in particular, substrates which can be produced from renewable and biogenic feedstocks. 2.2.1 CdHalide Bonds (Organohalides) Organohalides are versatile substrates for CdC coupling reactions. The carbonylation of organic halides (or other substrates transformed in situ in organic halides) is well known and used in industrial processes/applications [64]. Generally, the catalytic cycle starts with an oxidative addition of the halide substrate. Therefore aryl, benzyl, or methyl halides are preferred substrates due to their higher tendency toward nucleophilic substitution [65]. Analogous sequences are possible with CO2 instead of CO. Because the last step of the catalytic cycles is a reductive elimination, as discussed earlier, the step has to be facilitated by an organometallic species (Et2Zn, Mn, MgCl2/Zn) [66,67]. The carboxylation of linear alkyl halides based on this principle was reported in 2014 by Martin et al. These simpler substrates are generally less favorable for the oxidative addition compared to the

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previously-mentioned aryl halides. Moreover, the presence of β-hydrogen in the alkyl chain may lead to alkenes through competing elimination, which can undergo additional side reactions (e.g., dimerization). Therefore, the development of a catalytic system (Ni as a catalyst, bidentate and bulky N-ligands, and Mn as a reducing agent) that allowed the conversion of these substrates into carboxylic acids with yields up to 85% represents an impressive achievement [68]. In 2016, Martin et al. published a similar catalytic system to activate the CdH bond in a secondary alkyl halide [15]. They exploited the β-hydrogen elimination to activate a position far away from the coordinated metal center. In this way, the CdH bond of the organohalide is activated and reacts with CO2. Remarkably, they were able to control the product regio-selectivity [15]. In general, the transformation of CdX bonds into CdCOOH bonds using CO2 requires slightly harsher conditions compared to those needed to activate CdEXn bonds (E ¼ Zn, Zr, Sn, B, Si). However, these substrates are versatile and easier to produce compared to the reported activated substrates. Therefore, they are of high interest as shown in the recent review by Martin et al. [66]. 2.2.2 CdO Bonds (Oxygenated Substrates) CdO units can be considered as good alternatives to organic halides as substrates for the carboxylation if they are part of similarly or even better-suited leaving groups. They can be derived from alcohols (which are naturally

abundant), and do not lead to the formation of halogenated wastes [66]. Among potentially suited CdO functionalities present in organic molecules, very few have been carboxylated with CO2 to produce carboxylic acids. In particular, more reactive but less available CdO bonds (Fig. 6.4) are reported to react with CO2 giving carboxylic acids [69]. Most of the reported examples deal with the carboxylation of esters or sulfonates. Catalytic systems similar to those employed for the conversion of halides and CO2 into carboxylic acids have been reported to transform aryl tosylates and triflates into the corresponding carboxylic acids. The Ni complex, Mn and Et2NI system reported by Tsuji et al. to catalyze the carboxylation of halides is reported to catalyze the carboxylation of sulfonates (with a higher temperature of 60°C). Moderate-to-good yields are obtained starting from the sulfonates and reach a maximum of 73% [67]. In general, and in most cases, the proposed mechanism follows the one suggested starting from organic halides based on a first oxidative addition step. One of the first examples for a catalytic carboxylation of esters was reported in 2014 by Correa et al. They reported the reductive carboxylation of aryl or benzyl esters with CO2 using a molecular Ni catalyst, employing Mn as stoichiometric reducing reagent [70]. The first example for a carboxylation of alcohols was reported before by Mita, Sato et al. in 2015 [71]. They reported the synthesis of allylic acid starting from allylic alcohol and CO2 with a Pd catalyst and ZnEt2 as

FIG. 6.4 Scheme of the CdO bonds which can be transformed in carboxylic groups. They are ordered based on their reactivity and their availability.

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a stoichiometric transmetallation and reducing agent [71]. The group of Leitner reported the production of carboxylic acids starting from a simple aliphatic alcohol, like cyclohexanol or hexanols, based on CO2 and H2 [72]. The catalytic system consists of a molecular Rh precursor with PPh3 as stabilizing ligand and iodide as promoting reagent. In a first step, the system undergoes a rWGSR from CO2 and H2 to obtain CO and H2O, followed by an hydroxycarbonylation. Under acidic conditions the free carboxylic products were formed directly with water being the only by-product. The actual substrate for the hydroxyarbonylation are most likely either the alkyl iodide or the corresponding alkene, which are constantly formed in small amounts and regenerated from the alcohol under the reaction conditions. Despite the complex catalytic mechanism, the system gives very good to excellent yields when simple alcohols were transformed directly into carboxylic acids using CO2 as carboxylation and H2 as a reducing agent with no need of (over)-stoichiometric organometallic or metallic reagents. One notable example toward the CO2-based formation of acetic acid was reported in 2016 by the group of Buxing Han [73]. They showed the synthesis of acetic acid (yield of 70%) from methanol, CO2, and H2 using a combination of two molecular catalysts based on rhodium and a ruthenium. With imidazole as ligand, the system shows high selectivity and almost

no formation of CO nor CH4. The authors concluded that their results were consistent with a mechanism where CO2 was incorporated directly to form the carboxyl group rather than from potentially formed CO. Protocols dealing with a direct carboxylation or hydrocarboxylation of further oxygenated substrates such as ketones or aldehydes directed at the CdO bond are currently not reported in literature. 2.2.3 C(sp1)dH Bonds (Alkynes) The triple bond of alkynes is another functional group that can be utilized for reaction with CO2 to yield carboxylic acids. Literature protocols comprise generally two principles for reacting an alkyne with CO2 to an acid moiety. On the one hand, the acidic sp1 CdHd is activated with a base to enable a carbanionictype attack at the electrophilic carbon of CO2 as in the synthesis of propiolic acid [74], where the initial triple-bond is maintained (see Fig. 6.5 [top]). On the other hand, in presence of a reductant like Mn, Zn, or silanes, reductive couplings to produce α,β-unsaturated carboxylic acids are feasible as well (see Fig. 6.5 [bottom]) [75]. The latter transformation involves a reductive step corresponding to a formal hydrocarboxylation of the carbon-carbon triple bond and is of high interest, especially toward the formation of acrylic acid and its derivates. These compound are commonly used as

(1) cat. (2) work-up

C}H carboxylation

R = e.g., H, alkyl, aryl

(1) cat., reductant (2) work-up

Reductive carboxylation (e.g., hydrocarboxylation)

FIG. 6.5

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Common routes from alkynes and CO2.

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monomers on a large scale for beneficial organic intermediates and for manufacturing polymeric materials [76,77]. One of the first incorporations of CO2 into alkynes (acetylene) to afford carboxylic acids (acrylic acid) was reported 1984 by Hoberg et al. [78]. The stoichiometric reaction of a ligand-assisted (TMEDA: N,N,N0 ,N0 -Tetramethylethane-1, 0 2-diamine) Ni precursor with acetylene and CO2 resulted in an oxidative coupling to the isolable intermediate 1-oxa-2-nickela-5cyclopentanone in a 60% yield. After hydrolysis of the complex with an aqueous HCl solution and work-up in methanol, the formation of acrylic acid could be indirectly detected by the methyl ester. For similar coupling of CO2 with alkynes, apart from nickel [79], further low-valent transition metal catalysts including silver [80], iron [81], and different molecular copper species [74,82] have been described. Because both pathways, shown in Fig. 6.5, are performed under basic conditions, the corresponding carboxylic acids salts are formed as primary products, which have to be converted consecutively by hydrolysis to the free acid moiety. While the reductive coupling reactions can be carried out under very mild conditions and sometimes even under ambient temperatures and/or low CO2 pressures, recent protocols involving the use of organometallic (super)stoichiometric-reducing auxiliaries help obtain the desired acrylic-acid derivatives. While this makes an application to

FIG. 6.6

a bulk chemical such as acrylic acid unfavorable in terms of industrial applications, the synthetic pathways may open novel pathways to biologically active products pharmaceuticals. 2.2.4 C(sp2)dH Bonds (Alkenes) Coupling of alkenes with CO2 in order to synthesize higher carboxylic acids is a very attractive transformation, as alkenes are quite abundant substrates, even accessible from biomass-derived alcohols [83], and can further be tailored by sophisticated metathesis chemistry to obtain a variety of α-alkenes. For the most direct conversion with CO2 toward carboxylic acids two principle possibilities for acid formation are possible (see Fig. 6.6). For the pathway corresponding to a formal insertion of CO2 into the sp2 CdH bond, one of the most interesting target molecules would be the production of acrylic acid. However, as shown in the following paragraphs, this reaction remains still quite challenging. As previously mentioned, the reductive coupling of acetylene with CO2 would be an alternative route to this target [78]. The most widely studied strategy to generate acrylates is based on the initial formation of metallalactones through oxidative coupling of olefin and CO2 in the coordination sphere of the transition metal catalyst. Especially metal complexes of Ni, Mo and W, and Pd offer this type of reactivity [84,85]. Vogt demonstrated the formation of acrylic acid from ethene and

Common routes to carboxylic acids from nonactivated alkenes and CO2.

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2 SYNTHESIS OF CARBOXYLIC ACIDS USING CO2 AS A BUILDING BLOCK

CO2 with turnover numbers (TONs) up to 21, using a catalytic system composed of a Nicomplex and a “hard” Lewis acid [86]. Most recently, the group of Schaub could enhance the corresponding TON with a Palladium system and the use of an amide solvent (CHP: N-cyclohexylpyrrolidone) and alcoholates as base in a single run toward sodium acrylate to 514 [87,88]. Additionally, for some further simple alkenes, such as propene or cyclopentene, their corresponding carboxylic acid salts were obtained at 145°C with TONs up to 92. By starting the implementation of a continuous reaction concept, they could achieve ethene conversion to acrylate with a total TON from two cycles (TTON2) of 235 and catalyst recycling in its active form (Fig. 6.7). The second pathway to combine olefins with CO2 to yield carboxylic acids involves an overall reductive process whereby saturated aliphatic acids are obtained as the final products (Fig. 6.6). Also, for this case, coupling CO2 with an alkene on a Nickel-complex to the corresponding nickelalactone, as pioneered in particular by

FIG. 6.7

117

Hoberg and the team of Walther and Dinjus, has been revisited. Aiming for a catalytic approach to regenerate an active catalyst species, Rovis et al. reported on the first Nickel-catalyzed reductive carboxylation of styrenes using CO2 under ambient conditions (23°C, 1 bar CO2). The transformation lead regio-selectively to saturated α-substituted carboxylic acids (phenylacetic acid derivatives) [89]. Transmetallation of nickel by Et2Zn was employed to reductively regenerate the active species. Under comparable reaction conditions (RT, 1 bar CO2, THF) an iron-catalyzed system with similar regioselectivities of the formed α-aryl carboxylic acids has been reported in 2012 using an excess of an Grignard reagent (EtMgBr) as a reductant [46]. One of the first known protocols for the metalcatalyzed synthesis of a saturated carboxylic acid by converting an alkene in combination with CO2 was published 1978 by Lapidus et al. They reported on the synthesis of propionic acid from ethene and CO2 [90]. By using the Wilkinson’s catalyst [RhCl(PPh3)3] they reacted ethene with CO2 in aqueous HBr under harsh reaction

Potential process concept for the Pd-catalyzed synthesis of sodium acrylate from ethene and CO2 [87].

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conditions (180°C, 700 bar) to obtain propionic acid in up to 38% yield after 12 h. However, an exact mechanism of this transformation has so far remained unclear as no reduction equivalents are mentioned in the protocol that could explain product formation. The formation of H2 from the aqueous HBr media itself would be conceivable. In 2013 Leitner et al. reported on the rhodiumcatalyzed formal hydrocarboxylation of alkenes and, as shown previously, even for some alcohols. Based on CO2 and H2 the catalytic system integrates two catalytic cycles in a direct synthesis of the corresponding by one carbon atom elongated carboxylic acid (Fig. 6.8) [91]. With a [RhCl(CO)2]2/PPh3 system and a promoting iodide reagent, very-high-to-excellent yields were obtained for a range of cyclic and linear olefins. Double bond migration occurs under reaction conditions, leading to the same regio-isomeric product mixtures if starting from different double bond of isomers. Mechanistically, CO2 and H2 get converted in situ through reverse Water-Gas-Shift-Reaction (rWGSR) to CO and H2O. In a directly coupled second catalytic cycle they get connected with the substrate in a Reppe-type hydroxycarbonylation [39] to yield the free carboxylic acid. According to the use of CO2 as a potential source for in-situ-formed CO (and H2O, respectively) in alkene transformations, further protocols were published about related carboxylation-, alkoxycarbonylation- or hydroformylation reactions and may open new strategies in CO2 utilization [84,92–95].

FIG. 6.8

2.2.5 C(sp2)dH Bonds (Aromatics) A highly desirable reaction is the directly catalyzed “green” carboxylation of aromatic compounds by insertion of CO2 directly into the sp2 CdH bond. For nonactivated simple aromatics such as benzene, this remains still a kind of “dream-reaction.” An effective and industrially applied coupling of aromatics with carbon dioxide is the synthesis of salicylic acid based on CO2 and phenol first published by Kolbe in 1860. After an improvement by Schmitt in 1885 this reaction is now well-known as the Kolbe-Schmitt synthesis [96,97]. The carboxylation at the pre-formed sodium phenoxide directs the carbon dioxide toward the adjacent ortho-position of an aryl compound. However, the essential presence of a directing phenol group, which can be stoichiometrically deprotonated to provide by tautomerism, a carbaniontype reagent, limits its applications in industrial chemistry to specific products such as salicylic acid or 3-hydroxy-2-naphthoic acid [98] synthesis. Thus, for converting aryl compounds like benzene and derivatives, more efforts are required to overcome both the thermodynamic as well as the kinetic boundary conditions [99]. An interesting system was published in 2002 by the group of Olah and Prakash which comprises analogies to the Friedel-Crafts acylation reaction [100]. Benzene could be carboxylated to yield up to 88% benzoic acid at 80°C after a consecutive work-up with HClaq. The active role of additional aluminum in the reaction course is believed to capture formed HCl to

Rh-catalyzed formal hydrocarboxylation of simple alkenes with CO2 and H2 [91].

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2 SYNTHESIS OF CARBOXYLIC ACIDS USING CO2 AS A BUILDING BLOCK

regenerate the active Lewis acid (AlCl3) which can again form dimeric Al2Cl6 [100]. In 2015 Zhenmi et al. reported another strategy for activating CO2 in carboxylation reactions of aryl moieties [101]. By the use of “Frustrated Lewis Pairs” (FLPs), they developed a feasible catalytical system for the carboxylation of different aryl compounds with Si/Al-derived FLPs. Starting from benzene they were able to gain 20% yield of benzoic acid. Apart from mono acid moieties, a growing market can be found for furan-2,5-dicarboxylic acid (FDCA) as biomass-derived diacids to produce more sustainable polymers (like polyethylene furandicarboxylate, PEF). As a highly attractive substitute for petroleum-derived polyethylene terephthalate (PET), this compound is mainly made from fructose, embracing unwanted side-reactions by unselective oxidation processes and moderate yields. In 2017, Kanan et al. [102] described an alternative route based on 2-furoic acid (furan-2-carboxylate), which is premised on dehydrogenated lignocellulose (¼>furfural). Heating a salt mixture containing 2-furoic acid and alkali carbonates (M2CO3) under CO2 pressure and in absence of solvents or metal catalysts they observed furan-2,5-dicarboxylate in high quantities (up to 1 mol). Via subsequent protonation with HClaq. the free diacid (FDCA) could be produced. Tackling the formed water removal and the product inhibition, they demonstrated an integrated carboxylation to pure FDCA on a molscale using a fixed-bed flow reactor with 89% isolated yield. For hetero-arenes, Boogaerts and Nolan published in 2010 the carboxylation of acidic CdH (of arenes) and NdH (of N-heterocycles) for a broad range of substrates in the presence of a gold catalyst, with N-heterocyclic carbene ligand (NHC) [103], or a copper catalyst, and a base additive [104]. The reaction could even be performed under ambient conditions (20°C and 1.5 bar CO2) with isolated yields from 69% to an excellent 96% for certain carboxylic acids (after acidic work-up). The group around

119

Ackermann showed a transition metal-free CdH carboxylation just mediated by tBuOK at atmospheric pressure of CO2 and comparatively low temperatures (100°C) considering the lack of a metal catalyst [105]. Their system leads toward the carboxylic acid ester which have to be cleaved to gain the free acid functionality. The system seems, by now, limited to oxazol compounds as substrate class. In 2014, Iwasawa et al. showed a catalytic system consisting of a rhodium complex and stoichiometric amounts of Al-reagents able to directly activate CdH bond to initiate the carboxylation of simple arenes [106]. 2.2.6 C(sp3)dH Bonds The carboxylation of nonactivated sp3 CdH bonds is even more challenging due to their very low reactivity. Some examples for the conversion of methane CH4 into acetic acid have been reported. They occur on highly reactive metal surfaces such as Cu/Co systems [107], Ag-doped Rh/SiO2 material [108], or on Pd/ Carbon or Pt/alumina [109]. While these examples demonstrate the principle feasibility of activating simple CdH bonds for insertion of CO2, they are typically noncatalytic facing the unfavorable thermodynamics of converting a mixture of CH4 and CO2 to acetic acid [110,111]. As with the aromatic sp2 CdH bonds, CdH acidic substrates can be used as substrates effectively. Pioneering works done in the 1970s deal with the insertion of CO2 into the acidic proton in the β-position of ketones to form a dCOOH group. Thus the reaction leads to the formation of β-ketocarboxylic acids, which are quite unstable and have to be readily transformed into a more stable compound (ester, salt, β-hydroxycarboxylic acids, etc.) [112]. In the first reports of this transformation, overstoichiometric amounts of reagents such as DBU were used [113]. Later, DBU received high attention as a promoter of the transformation into β-ketocarboxylic acids and

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derivatives in the absence and presence of transition metal complexes [112,114,115]. In 2013, Munshi and co-workers immobilized DBU on a methylhydrosiloxane support and they obtained very good yields for the corresponding β-ketocarboxylic acids (up to 99%) at mild conditions [115].

3 CONCLUSION AND FUTURE CHALLENGES The development of new processes to produce carboxylic acids by starting from a renewable, nontoxic, and abundant building blocks, like CO2, has been an active field of research, one which has seen a high dynamic in the past decades. Many new synthetic pathways have been reported by groups all around the world. This sets the stage for opening new perspectives in the development of processes which follow the “Green Chemistry” principles. Unfortunately, many of the developed protocols still suffer from important drawbacks. Among them, the need for activated substrates or organometallic stoichiometric reagents. The activated molecules require previous preparation steps and are sometimes not easy to handle. The presence of stoichiometric amounts of reducing agents, salts, or simply activating (metal) groups, will inevitably lead to the production of large amounts of waste. In other cases, especially, when nonactivated substrates are used, harsher conditions (high pressure and temperature) have to be applied in order to overcome kinetic barriers to obtain satisfying yields. In general, the reported catalytic processes usually exhibit only low-to-moderate turnover numbers (TON) and turnover frequency (TOF) values. Moreover, many protocols require an additional acid work up to obtain the free carboxylic acid. Many of the known synthetic procedures are mechanistically not fully understood and optimization remains very empirical. In order to develop

these protocols (hopefully up to industrial scale) deeper knowledge about the molecular transformation and advanced reaction engineering solutions are required. In this context, protocols relying on the use of CO2 and H2 (“hydrocarboxylation”) appear very attractive, as they can lead directly to the desired free carboxylic acids with 100% atom efficiency, producing only water as the by-product. Many published transformations and applications involving CO2 as reagent now face the challenge to get across the so-called “Valley of Death,” which demonstrates the difficulties along the way in transforming a new discovery into a final process/application. Using CO2 as feedstock or a building block to produce carboxylic acids, a lot of discoveries are at that (early) stage, and tools to assess their environmental and economic potential need to be developed and applied. The “Twelve Principles of Green Chemistry” [11] may serve as a rough first guideline, but detailed LCA studies including sensitivity analysis of changing economic and regulatory boundary conditions would be highly desirable [10]. These may help to identify promising applications for the use of CO2 in combination with catalysis that can lower the carbon footprint for the synthesis of carboxylic acids, and by that, provide more sustainable alternatives to conventional petrochemical production. The creativity of scientists in the field of catalysis is needed to generate innovative options toward this goal.

Acknowledgment This work was co-funded through a SINCHEM Grant. SINCHEM is a Joint Doctorate Programme selected under the Erasmus Mundus Action 1 Programme (FPA 20130037). Furthermore, the work was supported by the Cluster of Excellence “Tailor-Made Fuels from Biomass,” which is funded under contract EXC 236 by the Excellence Initiative by the German federal and state governments to promote science and research at German universities. Additionally, we gratefully appreciate the support received from the Federal Ministry of Education and Research (Bundesministerium f€ ur Bildung und Forschung, BmBF) within the joint project Carbon2Chem SynAlk (03EK3041).

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