Catalytic Carboxylation of Terminal Alkynes with CO₂: An Overview

Abstract

A large amount of CO₂ is released into the atmosphere by energy and industrial plants resulting in significant environment impacts. A considerable effort went into decreasing CO₂ emissions. The carboxylation reaction of converting CO₂ with aromatic alkynes to important chemicals such as carboxylic acids is one of the promising CO₂ utilization methods, and it can be performed in the catalytic or non-catalytic pathway. Our review article provides an overview of recent publications on the use of catalytic systems with different compositions and structures for the carboxylation of terminal alkynes by involving CO₂, and the effect of a solvent and base. Relying on the research results, the use of heterogeneous catalysts is the most effective. The advantage of catalytic systems is a lower reaction temperature and pressure. Heterogeneous silver-containing catalysts exhibit good yields of products and high selectivity. Moreover, the catalysts may lose their efficiency when interacting with moisture. It has been found that the most effective catalysts for the carboxylation of phenylacetylene with carbon dioxide as a carboxylating agent are copper-based catalysts. These catalysts are characterized by high activity and stability. We highlight the challenges of developing novel catalyst systems tuning catalytic properties. The future outlook and perspectives are also discussed.

1. Introduction

One of the main issues associated with environmental pollution is the CO₂ emission produced mainly by the energetic, transportation, manufacturing, and constructing sectors [1]. The intensive use of fossil fuels leads to the release of greenhouse gases into the atmosphere, which can cause global warming. The CO₂ amount in the atmosphere has increased every year, and its concentration in atmospheric air was 409.8 ppm in 2019 [2,3,4,5,6,7,8], while, in 2024, it has grown to 422.81 ppm [6]. Effective CO₂ capture technologies are critical to mitigating the increasing CO₂ concentration in the atmosphere. Therefore, today, the efforts of scientific research are focused on the development of various methods for the reduction in CO₂ emissions into the atmosphere and the proper utilization of CO₂ [9]. The main approaches to reducing CO₂ emissions into the atmosphere are its capture and storage. The methods are subdivided into (i) high-concentration capture (5–50% CO₂) and (ii) low-concentration capture (under 1% CO₂) based on the CO₂ concentration in the gaseous mixture [1]. The former methods include the pre-combustion and oxy-fuel capture at a CO₂ concentration of 15–50%, as well as the post-combustion capture at 5–15%, and the latter one includes direct capture from air (around 400 ppm). The CO₂ capture technologies can be based on the different mechanisms of physical and chemical CO₂ interactions with liquids and solids. Absorption technologies have been developed for CO₂ capture with different chemical absorbents (amine-based and alkali solutions, and liquids; and nanofluids) and employed on the industrial scale. Unfortunately, they suffer from the high energy cost of regeneration and absorbent degradation. The adsorption technologies are also used in the large-scale application of various solid adsorbents (activated carbon; zeolites; and carbon nanotubes). However, it does not solve the problem of CO₂ utilization. The storing CO₂ in geological formations leads to problems such as high investment costs, CO₂ leakage, and the unsuitability of certain geological areas. Thus, CO₂ capture and the subsequent processing of CO₂ into valuable chemical products and fuels are one of the most promising ways to recycle CO₂ [10,11]. Both the absorption and adsorption can be used in combination with a chemical reaction, especially a catalytic one, producing a compound of considerable interest [1]. For example, the combined CO₂ adsorption and catalytic methanation process is affected both by the method of a separate adsorbent–catalyst system and over the novel dual-function materials (DFMs), exhibiting new pathways for CO₂ utilization [12]. The concept of DFMs has been realized for the production of CO and syngas as well. Considerable recent attention has been focused on the transformation of CO₂ to valuable chemicals [1,2,7,8,9,13,14]. CO₂ is a suitable source for C₁ chemistry due to its low cost, nontoxicity, renewability, and availability. Carbon dioxide has been used in many reactions with alkenes [4,15,16,17,18,19], alkynes [4,19,20], including terminal alkynes [4,18,19,20,21,22,23,24,25,26,27], and aromatic compounds [4,18,19,26,27,28,29,30,31,32,33]. The active substrates and catalysts are required because of the thermodynamic and kinetic stability of the CO₂ molecule. Most of the proposed processes are based on the catalytic reactions performed using the heterogeneous catalysts that are commonly the catalytically active solid materials consisted of transition or precious metals deposited on the supports of different nature. The catalytic behavior was shown to depend on the dispersity, particle configuration (single atom, clusters, and nanoparticles), and electronic state of metals. Based on an overview of the available literature, it was concluded [13] that the use of single-atom catalysts (SACs) in CO₂ hydrogenation results in CO formation, whereas CH₄ and CH₃OH are formed over nanoclusters and nanoparticles. Metallic SACs (M⁰) provide a higher activity (conversion, and reaction rate), while the SACs’ oxidized state (M^δ+) influences the selectivity of CO/CH₄. It was revealed also that some SACs exhibit a C-C coupling ability, which makes their use for the synthesis of C₂₊ chemicals promising.

The carboxylation reaction of terminal alkynes and CO₂ is an effective strategy for the chemical fixation of CO₂ wherein useful carboxylic acids are synthesized via the C-H bond activation of terminal alkyne molecules [34,35]. The reactions of the carboxylation of terminal alkynes by the direct involvement of CO₂ can occur both in the presence of catalysts and by a non-catalytic reaction in the presence of a base. However, non-catalytic methodologies suffer from several drawbacks such as the requirement of a stoichiometric metal reagent, harsh reaction conditions, and inferior functional group compatibility, thus greatly limiting its applicability [36]. It is necessary to note the excellent characteristics of Cu or Ag salts for the activation of the C-C triple bond [37,38], although stoichiometric amounts are still needed. This strategy has a profound effect on the activation of terminal alkynes, and, since then, the transition metal-catalyzed carboxylation of terminal alkynes using CO₂ has attracted much attention from researchers. At the same time, according to the results of numerous studies, these reactions can be carried out in the presence of catalysts promoted by metals such as Au, Ni, Co, Mo, and various rare earth metals (La, Ce, and Nd) [4,5,18,19,24,26,27,32].

However, it is not only a catalyst that determines the efficiency of the process. The success of the reaction mainly depends on the correct choice of the solvent and base. The solvents are often environmentally unfriendly [39,40]. He et al. [41] introduced ethylene carbonate (EC) as an ecologically friendly solvent for its low toxicity and high solubility, and the ability to reduce the energy barrier for CO₂ insertion, acting as a potential ligand [42,43]. Polar aprotic solvents like dimethylformamide (DMF), dimethylacetamide (DMA), and dimethyl sulfoxide (DMSO) are the most favorable for the carboxylation reaction of terminal alkynes [44,45,46,47,48,49,50,51]. However, Supercritical CO₂ is another green alternative [39], and the most suitable solvent for it is diazabicycloundecene (DBU), which not only promotes the reaction, but also facilitates the deprotonation of the alkyne and captures CO₂ [52].

The base is also necessary in the direct carboxylation reaction, since it plays the role of an “activator” of terminal alkynes, transforming it into a carbocation that is more sensitive to the interaction with CO₂. Cs₂CO₃ proved to be the most effective because of its ability to coordinate a terminal alkyne and silver catalyst [43,50]. Other inorganic bases such as K₂CO₃ or Na₂CO₃ resulted in significantly lower yields of the product [47].

This paper aims to provide a comprehensive overview of the current state and prospects of the catalytic carboxylation of terminal alkynes; we aim to highlight the potential of various catalysts mainly based on Cu and Ag and discuss their advantages and limitations. In addition, the subject of the influence of solvents and bases on the efficiency of the process is discussed here; the catalytic characteristics of reactions carried out in the presence of these components are compared.

2. Copper-Containing Catalysts for Carboxylation Reaction of Aromatic Alkynes with Carbon Dioxide

Copper-based catalysts have a number of positive characteristics, which make them the catalysts of choice in many catalytic carboxylation reactions: their availability, excellent activity, stability in hydrothermal conditions, and low cost.

The parameters of catalysts based on copper for the carboxylation reactions of aromatic alkynes with CO₂ are shown in

Table 1. Characteristics of copper-containing catalysts for terminal alkynes carboxylation ¹.

Catalyst	Solvent	Substrate		Reaction Conditions			Yield, %	Ref.
			Product	T (°C)	Pressure, MPa	Cat. Loading
5% CuBr@C Cs2CO3	EC (3 mL)	phenylacetylene	phenylpropiolic acid	80	-	196 mg, 0.6 mmol	90	[43]
		3-chlorophenylacetylene	3-chlorophenylpropiolic acid				77
Cu(IN)-MOF Cs2CO3	DMF (3 mL)	4-ethylphenylacetylene	4-ethylphenylpropiolic acid	80	0.1	30 mg 1.5 mmol	80	[44]
1% CZU-7 Cs2CO3	DMF (20 mL)	4-fluorophenylacetylene	4-fluorophenylpropiolic acid	100	0.3	4 mmol 6 mmol	78	[47]
30 Cu(IN)-MOF Cs2CO3	DMF (3 mL)	phenylacetylene	phenylpropiolic acid	80	0.1	30 mg 1.5 mmol	80	[48]
1% CuNO3/phenanthroline Cs2CO3	DMF (3 mL)	phenylacetylene	phenylpropiolic acid	50	0.5	-	98	[49]
5% CuNPs/Al2O3 Cs2CO3	DMF	2,4,6-trimethylphenylacetylene	butyl 2,4,6-trimethylphenylpropiolate	60	0.2	2 mmol	90	[50]
[Cu(Im12)2][CuBr2] Cs2CO3	DMF (2.5 mL)	cyclopropylacetylene	butyl cyclopropylpropiolate	25	-	0.019 g 0.6 mmol	97	[51]
5% CuCl Cs2CO3	-	phenylacetylene	phenylpropiolic acid	50	8	-	25	[52]
2% CuI DBU	-	phenylacetylene	-	50	8	-	92	[52]

¹ Catalysts: 1%CZU-7 (copper(II)-based metal–organic framework [Cu(Fbtx)₂Br₂]n with a mass metal content of 1%); and 5%CuNPs/Al₂O₃ (copper nanoparticles based on aluminum oxide with a mass metal content of 5%).

.

Most catalysts [44,45,46,47,48,49,50,51,52] are characterized by a high product yield (80–99%). Among the samples considered, the 1%CuNO₃/phenanthroline catalyst described in Brill’s work [49] should be noted. After the carboxylation at 50 °C at 0.5 MPa, the yield of phenylpropiolic acid was 98%. It was found that phenanthroline/copper complexes are highly active catalysts for both carboxylation/decarboxylation reactions.

The catalysts mentioned [44,45,46,47,48] are characterized by an approximately equal yield of the product (78–80%) at 80–100 °C. In the work of Shi [44], the optimal yield value was achieved when the catalyst loading was 30 mg at 80 °C. A further increase in the loading of the Cu(IN)-MOF catalyst (copper with isonicotinic acid (IN) based on a metal–organic framework (MOF)) and an increase in the reaction temperature led to a decrease in this catalytic characteristic. A possible reason is the occurrence of a side reaction (the formation of a by-product). In this work, similar experiments were conducted with phenylacetylene derivatives and it was revealed that the presence of substituents in the phenyl ring of the acetylenic molecule did not significantly affect the yield of the product. Thus, one of the main advantages of this catalyst is the ability to convert various substituted alkynes; this way, diffusion is prevented and easy access to the active centers of the catalyst is provided. The study of the solvent effect in this work (

Table 2. Phenylacetylene carboxylation on the copper-containing catalysts. Optimization of the reaction.

Entry	Time (h)	Catalyst Loading (mg (Cu % mmol) a)	Temperature (°C)	Solvent b (3 mL)	Yield c (%)
1	1	30 (0.47)	80	DMF	55
2	2	30 (0.47)	80	DMF	63
3	4	30 (0.47)	80	DMF	80
4	12	30 (0.47)	80	DMF	74
5	4	10 (0.16)	80	DMF	72
6	4	50 (0.78)	80	DMF	69
7	4	30 (0.47)	60	DMF	33
8	4	30 (0.47)	100	DMF	64
9	4	30 (0.47)	80	DMSO	62
10	4	30 (0.47)	80	CH3CN	52
11	4	30 (0.47)	80	DMI	45
12	4	30 (0.47)	80	MeOH	-
13	4	30 (0.47)	80	EC	19
14 d	4	30 (0.47)	80	DMF	12
15 e	4	30 (0.47)	80	DMF	31
16 f	4	28 (0.47)	80	DMF	45
17 g	4	17.5 (0)	80	DMF	-
18 h	4	30 (0.47)	80	DMF	-
19 i	4	30 (0.47)	80	DMF	83
20 j	4	30 (0.47)	80	DMF	84
21 k	4	30 (0.47)	80	DMF	84
22 l	4	30 (0.47)	80	DMF	83
23 m	4	30 (0.47)	80	DMF	79
24 n	4	30 (0.47)	80	DMF	68

Reaction conditions: Phenylacetylene (1 mmol), Cs₂CO₃: 1.5 equiv. (1.5 mmol), and CO₂: 1 atm, 99.999%. [a] Cu % mmol in catalyst were calculated based on the Cu content which was obtained from ICP analysis (30 wt% Cu in catalyst). [b] Solvent: DMF (Dimethylformamide), DMSO (Dimethyl sulfoxide), DMI (1,3-dimethyl-2-imidazolidinone), MeOH (Methanol), and EC (Ethylene carbonate). [c] Yields (isolated) were determined by 1H NMR and using 1,3,5,-trioxane as the internal standard. [d] Catalyst: Cu-BTC (HKUST-1), and [e] Catalyst: Cu-DABCO. [f] Catalyst: copper acetate and the weight based on the structure of MOF. [g] Catalyst: Isonicotinic acid catalyst and the weight based on the structure of MOF. [h] Blank test (no catalyst). [i–l] Recycle test 2nd, 3rd, 4th, and 5th, respectively. [m] The catalytic activity of Cu(IN)-MOF after being exposed in air for a week. [n] The catalytic activity of Cu(IN)-MOF after being immersed in water for 24 h. Reproduced with permission [44]. Copyright © 2020 with permission from Elsevier.

) showed that dimethylformamide (DMFA) is a more favorable solvent for the above-mentioned reaction conditions compared to acetonitrile (CH₃CN) or ethylene carbonate (EC). DMFA is a weak Lewis base and can increase the solubility of CO₂ and activate alkynes to release a terminal proton forming carbon–metal bonds.

Figure 1 shows the reaction mechanism involving the Cu(IN)-MOF catalyst. The first step is the deprotonation of an alkyne to form copper acetylenide (R-C≡C-Cu-MOF). At the same time, the dissociation of the Cu-N bond in the MOF structure occurs. The second step is the adsorption of CO₂ to form copper propiolate (R-C≡C−COOCu-MOF) [44,45,46]. Subsequently, copper propiolate reacts with cesium carbonate to form cesium propiolate (R-C≡C−COOCs), which was then dissolved in water and separated from the unreacted substrate and solvents by extraction. The target propiolic acid is obtained by acidification in a water bath with ice. It was found that, in the process of dissociation of the Cu-N bond, additional uncoordinated metal sites (catalytic sites) and base sites (N-based species for CO₂ adsorption) are formed, which promote the catalytic activity according to this mechanism.

The sample of 5% CuBr@C (copper bromide based on carbon with 5 wt% copper) [43] showed the highest yield of the product (90%) in the carboxylation reaction of phenylacetylene with carbon dioxide at 80 °C. With a decrease in the percentage of copper in the catalyst, a noticeable decrease in the yield of phenylpropiolic acid is observed (65% for 2% CuCl@C). It was revealed that the heterogeneous catalyst, 5% CuBr@C, could be easily recovered and reused without a significant loss in activity.

In the work of Xie [52], the catalyst [Cu(Im¹²)₂][CuBr₂] (Cu(I) complex (copper(I)-based ILs (ionic liquids)) (Im¹² = 1-dodecylimidazole)) was synthesized, which, during the carboxylation reaction, showed a high yield (97%) at 25 °C. It was found that the activity of the catalyst is significantly influenced by the central position of copper(I) in the cation, resulting from its interaction with 1-dodecylimidazole. The reason for the high efficiency of this sample may be the increased nucleophilicity of the C–Cu bond. Having a good solubility and stability, and a long alkyl chain of the Im¹² ligand, [Cu(Im¹²)₂][CuBr₂] exerts a strong effect on the reaction. Based on the results obtained, the authors of the article proposed a possible reaction mechanism (Figure 2). First, the alkyne is activated by copper, then, upon its interaction with cesium carbonate, copper acetylenide is formed. Next, CO₂ is adsorbed to form an intermediate product, copper propiolate. The product ester is esterified using iodoalkane and the copper catalyst is regenerated. It was suggested that the ligand (Im¹²) could increase the catalytic efficiency by allowing the reaction to proceed smoothly under mild conditions. The 2%CuI catalyst was characterized by the excellent product yield (92%) of the phenylacetylene carboxylation reaction with CO₂ at 50 °C at 8 MPa and a DBU base (diazabicycloundecene) [52]. However, another catalyst studied in this work, 2%CuI with a Cs₂CO₃ base, had a rather low yield at the same reaction conditions (only 25%). It was also noted that an increase in temperature to 100 °C leads to a decrease in the yield.

The reason for this effect is probably the decarboxylation of the copper(I) salt at high temperatures. It has been proven that DBU is the optimal base among the studied compounds and, by itself, can contribute to the direct carboxylation of terminal alkynes in the absence of a copper(I) catalyst (

Table 3. DBU-mediated direct carboxylation of phenylacetylene with CO₂ ^a.

Entry	Pressure (MPa)	T (°C)	Time (h)	Yield b (%)
1	8	60	24	64
2	8	70	24	78
3	8	80	24	84
4	8	90	24	71
5	6	80	24	73
6	12	80	24	86
7	12	80	16	90
8 c	12	80	16	72

[a] Reaction conditions [50]: phenylacetylene (2.0 mmol), DBU (200 mol%), and CO₂. [b] Isolated yield. [c] In the presence of 1 mol% CuI [52].

).

According to the data of the table, with an increase in the temperature from 60 °C to 80 °C and with a decrease in the reaction time from 24 h to 16 h, the yield of phenylpropiolic acid increases significantly from 64% to 90%.

Moreover, under similar conditions, the carboxylation reaction of various terminal alkynes with CO₂ was carried [52]. It was found that 4-fluorophenylacetylene showed the best product yield (96%). This effect can be explained by the good solubility of the fluoro-substituted molecule in scCO₂ (supercritical CO₂). It was revealed that terminal alkynes with an ester group cannot be carboxylated with CO₂ under the same conditions. It may be caused by the Michael-type addition, which occurs predominantly via the reaction of alkynyl esters and DBU.

Based on previous studies [53,54,55,56,57], it can be concluded that DBU functions both as a ligand of the copper(I) catalyst and as a base. The direct carboxylation reaction involves the formation of a DBU-CO₂ adduct followed by the nucleophilic addition of a terminal alkyne to produce a propiolate-DBU salt [58,59,60]. Thus, DBU acts as both a nucleophile and a base during the reaction.

Yu, in his work [61], carried out the carboxylation of terminal alkynes with CO₂ in the presence of CuI as a catalyst. Based on the results obtained, it was concluded that the optimal reaction conditions include the use of Cs₂CO₃ as a base, EC (ethylene carbonate) as a solvent, and 80 °C. DFT calculations [62] showed that the energy barrier for the CO₂ insertion into the sp-hybridized Cu–C bond could be reduced by using EC as a solvent. The authors studied the effect of different solvents (DMF, DEC, EC, and others) on the yield of the product in the carboxylation reaction (

Table 4. Influence of a solvent on the carboxylation reaction ^a.

Entry	Solvent	Yield b (%)
1	DMF	70
2	DMC	15
3	DEC	16
4	PC	73
5	EC	88

[a] Reaction conditions: phenylacetylene (0.0511 g, 0.5 mmol), CuI (0.0095 g, 0.05 mmol), Cs₂CO₃ (0.1955 g, 0.6 mmol), n-BuI (0.1104 g, 0.6 mmol), solvent (3 mL), CO₂ (99.999%, balloon), 80 °C, 12 h, DMC = dimethyl carbonate, DEC = diethyl carbonate, and PC = propylene carbonate. [b] The yields were determined by GC with biphenyl as the internal standard [61].

).

A fairly high rate (70%) was achieved using DMF (dimethylformamide) as a solvent, which corresponds to Kondo’s report [54]. As for other solvents, PC and EC showed higher yields than DMC and DEC. Moreover, the reaction with EC allowed the authors to obtain the highest yield of the product—88%. This may be due to the fact that ethylene carbonate helps to increase the stability of the intermediate and facilitates the interaction between the reagents.

In the work of Krisnandi [63], a copper-impregnated catalyst based on mesoporous carbon (Cu/MC) demonstrated the highest conversion of phenylacetylene (41%) at 75 °C and 1 atm using Cs₂CO₃ as a base. Two products were identified: phenylpropiolic acid as the main product [50] and cinnamic acid as a by-product (Figure 3). The carboxylation reaction with CO₂ was carried out with varying temperatures, the amount of the catalyst, and the type of base and carrier (Figure 4.)

Based on the data obtained, it can be concluded that the carboxylation reaction is a thermal-dependent reaction. As the temperature increases, the conversion of phenylacetylene and the yield of the main product increase significantly, with the highest conversion rate achieved at 75 °C. Cs₂CO₃ is a more suitable base for the reaction, as it is a more stable base and has the highest solubility in DMF among other carbonates [64]. The SEM, XRD, and SAA analyses showed that Cu/MC has a larger surface area, a larger mesopore volume, and a larger pore diameter, which ensures the better transportation of the reagent and promotes the reaction under mild conditions.

The catalytic characteristics of mesoporous carbon nitride (CN)-based Cu single-atom catalysts with a different mass content of copper in the phenylacetylene carboxylation reaction with CO₂ were investigated [65]. The results are shown in Figure 5.

It was found that all Cu−CN-x samples are effective in the carboxylation reaction at an atmospheric pressure. The highest yield of phenylpropiolic acid was observed for the Cu−CN-8.0 catalyst and amounted to 97% at 80 °C in the presence of Cs₂CO₃ as a base, which is in agreement with the reports [56,66]. Cu−CN-26.6 also showed excellent results, with a 95% yield under the same conditions. Thus, the catalyst with a low metal mass content proved to be more effective. The reason for this effect is probably that most of the copper atoms in the Cu−CN-8.0 catalyst are contained on the external surface of the catalyst, which greatly simplifies their contact with the reagent molecules. In Cu−CN-26.6, a large number of copper atoms are located inside the catalyst particles, which leads to a decrease in the efficiency of the process. Therefore, the Cu−CN-8.0 sample was chosen as the catalyst for the reaction. In this work, the Cu−CN-8.0 catalyst was also studied in the carboxylation reaction of various terminal alkynes. Acids with electron-donating (−CH₃, −Ph) substituents were obtained with the highest yield (95–97%).

Thus, heterogeneous catalysts based on copper have a sufficiently high activity and high product yield in the reactions of the carboxylation of phenylacetylene with carbon dioxide. Cu is also the element that could form C–C bonds directly from CO₂RR with good selectivity [43,50,67,68,69,70].

3. Silver-Containing Catalysts for Carboxylation Reaction of Aromatic Alkynes with Carbon Dioxide

One of the main advantages of silver-based catalysts is the higher activity compared to copper-based catalysts and lower operating temperatures. Having studied the literature on this topic, it can be suggested that catalysts containing silver are the most widely used for the reactions of carboxylation.

The parameters of catalysts based on silver for the reactions of the phenylacetylene carboxylation with CO₂ are shown in Table 5.

All catalysts [71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92] demonstrate a high yield of phenylpropiolic acid; however, in some cases [71,90,91,92], this parameter changed with varying temperatures, solvents, and the amount of the loaded metal.

Table 5. Characteristics of catalysts containing silver for terminal alkynes’ carboxylation.

Catalyst Base	Solvent	Substrate	Reaction Conditions			Yield of Product, %	Ref.
			T (°C)	P, atm	Cat. Loading
AgNO3 Cs2CO3	DMSO (20 mL)	phenylacetylene	50	60	0.4 mmol 6 mmol	phenylpropiolic acid—89.3	[71]
0.5% Ag/F-Al2O3 Cs2CO3	DMSO (20 mL)	phenylacetylene	50	60	0.47 mmol 13.7 mmol	phenylpropiolic acid—62.1	[71]
0.5 Ag@ZIF-8 Cs2CO3	DMF (20 mL)	tret-butylacetylene	40	1	50 mg 7.2 mmol	tret-butylpropiolic acid—91.0	[72]
4.16% Ag@MIL-101 Cs2CO3	DMF (5 mL)	phenylacetylene	50	1	70 mg 1.5 mmol	phenylpropiolic acid—96.5	[73]
0.10 Ag/IRFC Cs2CO3	DMSO (20 mL)	phenylacetylene	70	1	50 mg 10 mmol	phenylpropiolic acid—99.0	[83]
Au12Ag32(SR)30@ZIF-8 K2CO3	DMSO	phenylacetylene	50	1	-	phenylpropiolic acid—100.0	[84]
Ag/tert-GO Cs2CO3	DMF (4 mL)	4-chlorophenylacetylene	50	1	10 mg 0.75 mmol	4-chlorophenylpropiolic acid—96.5	[85]
AgNPs/rGO Na2CO3	DMF	phenylacetylene	50	-	-	phenylpropiolic acid—99.5	[89]
AgNPs/Co-MOF Cs2CO3	DMF (5 mL)	phenylacetylene	40	1	50 mg 1.5 mmol	28.0	[90]
AgNPs/Co-MOF Cs2CO3	DMF (5 mL)	phenylacetylene	80	1	50 mg 1.5 mmol	98.0	[90]
AgNPs/Co-MOF Cs2CO3	DMF (5 mL)	phenylacetylene	90	1	50 mg 1.5 mmol	81.0	[90]
1% Ag-NHC Cs2CO3	DMF (10 mL)	phenylacetylene	40	1	1.5 mmol	83	[91]
5% Ag-NHC Cs2CO3	DMF (10 mL)	phenylacetylene	40	1	1.5 mmol	68	[91]
2.5% Ag2WO4 Cs2CO3	DMF (3 mL)	phenylacetylene	25	-	0.0511 g 1.2 mmol	25	[92]
2.5% Ag2WO4 Cs2CO3	MeCN (3 mL)	phenylacetylene	25	-	0.0511 g 1.2 mmol	>99	[92]

Table 5. Characteristics of catalysts containing silver for terminal alkynes’ carboxylation.

Catalyst Base	Solvent	Substrate	Reaction Conditions			Yield of Product, %	Ref.
			T (°C)	P, atm	Cat. Loading
AgNO3 Cs2CO3	DMSO (20 mL)	phenylacetylene	50	60	0.4 mmol 6 mmol	phenylpropiolic acid—89.3	[71]
0.5% Ag/F-Al2O3 Cs2CO3	DMSO (20 mL)	phenylacetylene	50	60	0.47 mmol 13.7 mmol	phenylpropiolic acid—62.1	[71]
0.5 Ag@ZIF-8 Cs2CO3	DMF (20 mL)	tret-butylacetylene	40	1	50 mg 7.2 mmol	tret-butylpropiolic acid—91.0	[72]
4.16% Ag@MIL-101 Cs2CO3	DMF (5 mL)	phenylacetylene	50	1	70 mg 1.5 mmol	phenylpropiolic acid—96.5	[73]
0.10 Ag/IRFC Cs2CO3	DMSO (20 mL)	phenylacetylene	70	1	50 mg 10 mmol	phenylpropiolic acid—99.0	[83]
Au12Ag32(SR)30@ZIF-8 K2CO3	DMSO	phenylacetylene	50	1	-	phenylpropiolic acid—100.0	[84]
Ag/tert-GO Cs2CO3	DMF (4 mL)	4-chlorophenylacetylene	50	1	10 mg 0.75 mmol	4-chlorophenylpropiolic acid—96.5	[85]
AgNPs/rGO Na2CO3	DMF	phenylacetylene	50	-	-	phenylpropiolic acid—99.5	[89]
AgNPs/Co-MOF Cs2CO3	DMF (5 mL)	phenylacetylene	40	1	50 mg 1.5 mmol	28.0	[90]
AgNPs/Co-MOF Cs2CO3	DMF (5 mL)	phenylacetylene	80	1	50 mg 1.5 mmol	98.0	[90]
AgNPs/Co-MOF Cs2CO3	DMF (5 mL)	phenylacetylene	90	1	50 mg 1.5 mmol	81.0	[90]
1% Ag-NHC Cs2CO3	DMF (10 mL)	phenylacetylene	40	1	1.5 mmol	83	[91]
5% Ag-NHC Cs2CO3	DMF (10 mL)	phenylacetylene	40	1	1.5 mmol	68	[91]
2.5% Ag2WO4 Cs2CO3	DMF (3 mL)	phenylacetylene	25	-	0.0511 g 1.2 mmol	25	[92]
2.5% Ag2WO4 Cs2CO3	MeCN (3 mL)	phenylacetylene	25	-	0.0511 g 1.2 mmol	>99	[92]

The maximum output values were achieved in the studies [83,84,89,90]. In the Zhang’s work [83], the 0.10 Ag/IRFC (0.10% Ag based on a unique nitrogen-doped mesoporous single-crystal carbon (IRFC)) catalyst was synthesized and tested in the phenylacetylene carboxylation reaction with CO₂. The highest yield (99%) was obtained at 70 °C and 1 atm in the presence of Cs₂CO₃ and solvent dimethylsulfoxide (DMSO). It should be noted that an important role in achieving excellent catalytic characteristics was played by the carrier of this catalyst, namely, a new mesoporous monocrystalline carbon doped with N atoms (called IRFC in this article). The homogeneous morphology, small particle size, and large surface area greatly facilitate the adsorption of highly dispersed ultra-small Ag nanoparticles on this substrate, as well as provide smooth and short diffusion ways. Due to these unique structural features, the sample 0.10 Ag/IRFC exhibited excellent catalytic activity and stability in the carboxylation reaction of phenylacetylene by carbon dioxide under mild conditions. In addition to this, this catalyst is also highly effective for the conversion of dilute CO₂ gases.

Moreover, good results are demonstrated by a sample of 2.5% Ag₂WO₄ studied in the work of Guo [92]. It was found that, when the solvent MeCN (acetonitrile) was added to the catalyst–base catalytic system, a low yield of phenylpropiolic acid was observed (25%), and, when DMF (dimethylformamide) solvent was introduced into the system, the product yield increased significantly (>99%). The reason for this effect is probably the synergistic effect between the silver cation, capable of activating the C–C triple bond, and WO₄, capable of activating the CO₂ molecule. This way, the reaction proceeds smoothly at room temperature and atmospheric pressure.

In Finashina’s work [71], the properties of the following catalysts were studied: AgNO₃ for a homogeneous catalytic carboxylation process and 0.5% Ag/F-Al₂O₃ for a heterogeneous catalytic process. In the first case, the product yield was 89.3% at 50 °C and 60 atm, and, in the second case, under the same reaction conditions, it was 62.1%. However, despite the low value of this parameter, a heterogeneous catalyst is still considered more suitable for the carboxylation reaction. Phenylacetylene can form acetylenides not only with metal salts, but also with the metals themselves (in our case, Ag⁺ for the homogeneous catalyst, AgNO₃), which is unacceptable. Therefore, in this study, we decided to use a heterogeneous catalyst containing Ag⁰. In addition, according to the physico-chemical analysis, there are no Ag⁺ cations on the surface of the catalyst. Therefore, it can be suggested that silver on such a carrier acquires a partial positive charge and, in this form, contributes to the course of the carboxylation reaction.

High output values (94–97%) were also achieved in the work of Shi [72]. The XRD pattern and SEM image of the used catalyst 0.5 Ag@ZIF-8 (0.5% Ag based on a zeolitic imidazolate framework (ZIF)) (Figure 6) showed an insignificant variation in the crystal structure and morphology after the carboxylation reaction at 40 °C and 1 atm. These results confirmed the high stability of the sample.

The 4.16%Ag@MIL-101 (4.16% Ag based on framework MIL-101) catalyst synthesized in Liu’s work [73] showed a high yield of phenylpropiolic acid (96.5%) at a low temperature (50 °C) and atmospheric pressure. MIL-101 is a metal–organic framework (MOF) [72,74,75,76,77,78,79,80,81,82], and it was revealed that this porous material has exceptional bifunctionality; i.e., it is able to simultaneously capture and convert CO₂ with a low energy consumption, which contributes to the higher activity and stability of the catalyst.

In Zhang’s work [85], 4-chlorophenylacetylene carboxylation by carbon dioxide was performed in the presence of the catalyst Ag/tert-GO (p-tert-butylaniline–graphene oxide) [86,87,88]. The product yield was 96.5% at 50 °C and atmospheric pressure. Using the transmission electron microscopy (TEM) method, it was found that Ag nanoparticles are evenly distributed on the surface of the tert-GO carrier due to the fixing effect of oxygen-containing groups (Figure 7).

It should be noted that Cs₂CO₃ needs to be added into the reaction system, since it is beneficial for alkyne deprotonation. The high catalytic activity of Ag/tert-GO can be explained by a decrease in the size of Ag nanoparticles and an increased ability to adsorb CO₂ due to a large number of amide bonds.

The authors of the article also suggested a possible mechanism for the carboxylation of terminal alkynes with CO₂ on an Ag/tert-GO catalyst (Figure 8). At the beginning, when the alkyne interacts with a base in the presence of this catalyst, an intermediate product, Ag-acetylenide, is formed. The Ag–C bond fractures with the subsequent introduction of CO₂ to form a carboxylate intermediate (this stage is the rate-limiting one). This intermediate is then separated from the Ag surface as the final product under basic conditions. Thus, the formed amide group promotes the accumulation of CO₂ for the facile attack by the nucleophilic carbanion of the alkyne.

Examining the catalyst AgNPs/Co-MOF (nanosilver based on a porous Co(II)-salicylate metal–organic framework) reported in [81], it can be assumed that the yield of the product depends on the reaction temperature. At lower temperatures (40–60 °C), a small to medium yield is achieved (

Table 6. Effect of temperature and reaction time on carboxylation of phenylacetylene ^a.

Entry	Temperature (°C)	Time (h)	Conversion (%) b
1	40	14	28
2	50	14	51
3	60	14	69
4	70	14	73
5	80	14	98
6	90	14	81
7	80	8	56
8	80	10	68
9	80	12	81
10	80	16	98
11 c	80	14	23
12 d	80	14	46

[a] Reaction conditions: phenylacetylene (1.0 mmol), AgNPs/Co-MOF (50 mg), Cs₂CO₃ (1.5 mmol), CO₂ (1.0 atm), and DMF (5 mL). [b] GC yields. [c] Co-MOF (50 mg) used instead of AgNPs/Co-MOF. [d] AgNPs used instead of AgNPs/Co-MOF. Reproduced with permission [90]. Copyright © 2016 with permission from Elsevier.

, entries 1–3). It was found that the maximum yield is observed at 80 °C (Table 6, entry 5). It would seem that the pattern is that, the higher the temperature, the higher the yield. However, at 90 °C, the yield of the product decreases. It can be suggested that the intermediate silver propiolate is unstable at high temperatures and can decompose as a result of the decarboxylation reaction.

In Li’s work [91], the catalytic activity of the Ag-NHC (silver based on N-heterocyclic carbene) catalyst was investigated when the mass content of the metal in the sample was varied. In the presence of 1 mol% Ag-NHC, the reaction led to a good yield of the product (83%), and, with an increase in the catalyst loading to 5 mol%, a decrease in the yield (68%) was observed. The reason for this trend may be the decarboxylating activity of Ag-NHCs. Under the same conditions, the carboxylation reactions of other terminal alkynes were carried out. The best results were achieved for aromatic alkynes with electron-donating groups (the methyl, phenyl, propyl, and methoxy groups), the yield of the products varied from 78 to 85%. The yield of the corresponding acids for alkynes with strong electron-withdrawing substituents (CHO, CF₃, CN, and NO₂) was rather low (61–64%). This may be due to the low nucleophilicity of the α carbon of these alkynes.

Thus, it can be concluded that heterogeneous silver-based catalysts are more suitable for the phenylacetylene carboxylation with CO₂ due to their high catalytic activity and stability. However, it should be remembered that, as the process temperature increases, a decarboxylation reaction may occur in parallel with the main reaction, leading to a decrease in the product yield.

4. The Effect of Solvent and Base on the Carboxylation Reaction of Terminal Alkynes

The carboxylation of terminal alkynes is a crucial technological process that enables the production of carboxylic acids and their derivatives. The effectiveness of this process significantly depends not only on the availability of a catalyst, but also on the correct choice of the solvent and base.

A solvent plays a significant role in the solubility of the reactants, the stabilization of the intermediates, and the selectivity of the reaction [49,50,51,82,83,84]. In the context of carboxylating terminal alkynes, aprotic polar solvents (for instance, DMSO or DMF) are more preferable, as they dissolve both alkynes and bases effectively, and also contribute to the stabilization of carbocationic intermediates [71,72,88,89]. Some solvents (such as H₂O or CH₃CN) can interact with reactants, reducing the selectivity and efficiency of the process [44,61,70,92].

A base plays the role of an “activator” of terminal alkynes, transforming it into a carbocation, which is more reactive to the interaction with CO₂. Bases such as Cs₂CO₃ or K₂CO₃ are frequently used in the carboxylation of terminal alkynes [64,69,85]. However, strong bases like NaOH or KOH can result in hydrolysis or the decomposition of the product [93].

Thus, the choice of the solvent and base is one of the key factors in increasing the product yield and reaction selectivity.

4.1. Selecting an Optimal Base for Carboxylation of Terminal Alkynes

In the work of Chowdhury [94], various parameters (the types and amount of bases, and reaction time) were changed to increase in the efficiency of the process (

Table 7. Effect of type and amount of base on carboxylation of 1-ethynylbenzene ^a.

Entry	Base	Amount of Base (mmol)	Yield (%) b
1	Cs2CO3	0.8	45
2	Cs2CO3	1	67
3	Cs2CO3	1.2	80
4	Cs2CO3	1.5	98
5	Cs2CO3	1.8	94
6	tBuOK	1.5	67
7	Na2CO3	1.5	42
8	K2CO3	1.5	35
9 c	-	-	Trace
10 d	Cs2CO3	1.5	Trace
11 e	Cs2CO3	1.5	25
12 f	Cs2CO3	1.5	<5
13 g	Cs2CO3	1.5	<5

[a] Reactions carried out at atmospheric pressure (1 atm) of CO₂ using 1-ethynylbenzene (1.0 mmol), CuBr/ZnO (50 mg), base, reaction temp. (70 °C), propylene carbonate (3 mL), and 6 h. [b] GC yield. [c] Without base. [d] Without catalyst. [e] At room temperature (24 h). [f] Only ZnO. [g] Only CuBr. Reproduced with permission [94].

). The role of all these parameters is very important in the carboxylation of 1-ethynylbenzene. In the presence of different bases (K₂CO₃, Na₂CO₃, ^tBuOK, and Cs₂CO₃), the reaction was carried out for 6 h at 70 °C. Cs₂CO₃ was found to be the most optimal base [44,61,89,90,91] with the highest yield (Table 7, entry 4) and ^tBuOK gave a moderately good yield of phenylpropiolic acid (Table 7, entry 6). The lowest catalytic characteristics were obtained in the presence of K₂CO₃ and Na₂CO₃: the product yield was 35% and 42%, respectively. Varying the amount of Cs₂CO₃ from 0.8 to 1.8 mmol (Table 7, entries 1–5), the highest yield (98%) was achieved using 1.5 mmol of Cs₂CO₃ (Table 7, entry 4). Increasing the amount of Cs₂CO₃ in the reaction mixture results in a lower yield (Table 7, entry 5). When the process is carried out with a smaller amount of the base, the incomplete conversion of the reactant occurs, resulting in a relatively low yield of acid (Table 7, entries 1–3). Thus, the optimal base for the carboxylation of 1-ethynylbenzene is Cs₂CO₃, and its amount in the reaction mixture is 1.5 mmol.

Inorganic bases are frequently used for the carboxylation reactions of terminal alkynes. Li et al. [91] explained the significance of the inorganic base by examining the pKa level of anion CO₃²⁻, which is 10.33, while the basicity of other bases is considerably higher. The authors also suggested an identical reaction mechanism, as Molla et al. did in their work [90], which involved the insertion of CO₂ into the Ag-C bond of silver(I) acetylide to form a silver(I) propiolate intermediate. In this mechanism, the base is a crucial component. The bases K₂CO₃ and Cs₂CO₃ proved to be more effective than other ones (NaOH, NaO^tBu, KO^tBu, and DBU) and resulted in good yields of the product (20% and 82%, respectively). By reducing the amount of the base from 1.4 to 1.2 and 1.0 mmol, they achieved 78% and 41% of the acid, respectively, while the increasing amount had no impact on the yield.

Cs₂CO₃ can also function with dual roles: as a base that extracts hydrogen from a reactant and as a key component that forms catalytically active compounds [95,96]. Bu, in his work [97], noted that using Na₂CO₃, K₂CO₃, or triethylamine instead of Cs₂CO₃, all other factors remaining constant, leads to a dramatic decrease in the yield of the product (from 90 to 10%). Sun et al. [47] confirmed that Cs₂CO₃ is the most suitable base for the deprotonation of terminal alkynes [68,98]. It has been shown that an increase in the base concentration in the reaction mixture leads to an increase in conversion while maintaining a high selectivity (

Table 8. Optimization of the reaction conditions for the catalytic carboxylation of terminal alkynes with CO₂ ^a.

Entry	Cat. (mol%)	Base	Solvent b	Temp. (°C)	Conv. b (%)	Select. b (%)
1	CZU-7 (0.4)	Cs2CO3	DMF	100	41	100
2	CZU-7 (0.6)	Cs2CO3	DMF	100	53	100
3	CZU-7 (0.8)	Cs2CO3	DMF	100	72	100
4	CZU-7 (1.0)	Cs2CO3	DMF	100	87	100
5 c	CZU-7 (1.0)	Cs2CO3	DMF	100	82	100
6	CZU-7 (1.0)	Na2CO3	DMF	100	17	100
7	CZU-7 (1.0)	K2CO3	DMF	100	29	100
8	CZU-7 (1.0)	CsF	DMF	100	3	100
9	CZU-7 (1.0)	CsOAc	DMF	100	2	100

[a] Reaction conditions: CZU-7 was obtained from microwave method, 1-ethynylbenzene (4 mmol), solvent (20 mL), CO₂ (0.3 MPa), and 16 h. [b] Conversion and selectivity were determined by liquid chromatography (LC) analysis. [c] CZU-7 was obtained from hydrothermal method. Reproduced with permission [47].

, entries 1–5). The changing of Cs₂CO₃ with other bases, such as Na₂CO₃ and K₂CO₃, under identical conditions resulted in a decrease in the conversion to 17–29% (Table 8, entries 6 and 7). It was revealed that CsF and CsOAc were inactive to this process (Table 8, entries 8 and 9).

In Guo’s work [92], Cs₂CO₃ play a crucial role in the carboxylation of phenylacetylene [61]. Among other inorganic bases (K₂CO₃, KOH, CsF, ^tBuOLi, CsOAc, and NaNH₂), Cs₂CO₃ demonstrated the highest product yield (76%) in the absence of a catalyst (

Table 9. Base effect on carboxylation of phenylacetylene ^a.

Entry	Base	Yield b (%)
1 c	Cs2CO3	76
2	K2CO3	5
3	KOH	<1
4	CsF	<1
5	tBuOLi	2
6	CsOAc	<1
7	TBD	21.2
8	NaNH2	<1

[a] Reaction conditions: Phenylacetylene (0.0511 g, 0.5 mmol), Ag₂WO₄ (0.0056 g, 0.0013 mmol), base (0.6 mmol), n-BuI (0.1104 g, 0.6 mmol), DMF (3 mL), CO₂ (99.999%, balloon), 25 °C, 12 h. [b] The yields were determined by GC with biphenyl as internal standard. [c] Without Ag₂WO₄. Reproduced with permission [92].

, entry 1). Other bases were ineffective in this reaction, with the exception of TBD (Table 9, Entry 7). It is suggested that the reaction occurs via the TBD-CO₂ intermediate pathway [58].

Bhatt, in his work [23], found that only KO^tBu remained effective for the phenylacetylene carboxylation with CO₂. A similar conclusion was made by Yuan et al. [99] during the experiment. In the presence of base KO^tBu, the reaction occurs with a moderate yield of phenylpropiolic acid (47%). However, its high pH level suggested it was not a suitable choice.

4.2. Selecting an Optimal Solvent for Carboxylation of Terminal Alkynes

Generally, the polar solvent is more favorable for dissolving inorganic bases in alkyne activation and stabilizing a polar intermediate complex in a carboxylation reaction. It has previously been reported that N,N-dimethyformamide (DMF), dimethylsulfoxide (DMSO), or propylene carbonate (PC) are the most suitable solvents, depending on the catalyst used [39]. In the work of Bu [97], it was found that DMSO is the best solvent for the carboxylation reaction on the TpBpy-Cu catalyst, where TpBpy is a stable imine-type porous COF furnished with rich N,N- and N,O-chelating sites for Cu(I) immobilization. The conversion of phenylacetylene in the presence of these solvents, other things being equal, changes in the order of DMSO >> DMF >> PC (

Table 10. Optimization for the catalytic carboxylation of ethynylbenzene ^a.

Entry	Catalyst	Base	Solvent	Time (h)	Yield(%) b
1	TpBpy-Cu-14	Cs2CO3	DMSO	6	62
2	TpBpy-Cu-14	Cs2CO3	DMF	6	56
3	TpBpy-Cu-14	Cs2CO3	PC	6	11

[a] Reaction condition: alkynes (1 mmol), catalyst (10 mg), base (1.5 mmol), CO₂ (balloon), 60 °C, and solvent (3 mL). [b] Yield of isolated product. TpBpy-Cu-Cu(I)-modified COFs with the Cu contents being 14, 11, and 7 wt%, N, N-dimethyformamide (DMF), dimethylsulfoxide (DMSO), or propylene carbonate (PC). Reproduced with permission [97].

), and the last two solvents were the least active (the yield was 56 and 11%, respectively).

Jover and Maseras et al. [37] reported similar theoretical calculations and revealed a crucial role of DMSO as a ligand during the Ag(I)-catalyzed carboxylation of terminal alkynes [96]. Yuan, in his work [99], proved that polar aprotic solvents (DMF and DMSO) behaved excellently in the catalytic carboxylation of phenylacetylene with CO₂ by Ag catalysts containing imidazole N-Heterocyclic Carbene (L3/Ag) (

Table 11. Optimization of the reaction conditions for the catalytic carboxylation of phenylacetylene with CO₂ ^a.

Entry	Base	Solvent	Catalyst Loading (mol%) b	Yield
1	Cs2CO3	DMF	0.25	98
2	Cs2CO3	DMSO	0.25	92
3	Cs2CO3	CH3CN	1.00	11
4	Cs2CO3	H2O	0.50	-
5	Cs2CO3	MeOH	0.50	-
6	K2CO3 c	DMF	0.25	41
7	KOtBu	DMF	0.25	47
8 d	Cs2CO3	DMF	0.25	48
9 e	Cs2CO3	DMF	0.25	80
10	Cs2CO3	DMF	0.10	95
11	Cs2CO3	DMF	0.075	87

[a] Reaction conditions: Phenylacetylene (10 mmol), L3/Ag in situ system (0.0125 mmol of Ag₂O, 0.025 mmol of L3, and 0.025 mmol of KI), Cs₂CO₃ (1.5 equiv.), solvent (40 mL), 35 °C, 1 bar, and 24 h; and isolated yield. [b] Calculated based on Ag. [c] 3 equiv. [d] 12 h. [e] 18 h. Reproduced with permission [99].

, entries 1 and 2). The highest yield was obtained by using DMF as a solvent (98%). The reason for this is probably its weak alkalinity, which leads to the release of reactant protons and the formation of Ag−C bonds. The lowest yield of phenylpropiolic acid was observed in the presence of acetonitrile (CH₃CN) in the reaction mixture (Table 11, entry 3). The protonic solvents MeOH and H₂O were not suitable for this reaction at all (Table 11, entries 4 and 5).

DMF, being a weak base, is the best solvent that significantly increases the solubility of CO₂ [68,100,101]. In the work of Guo et al. [92], among the solvents studied (PC, DMC, DMF, MeCN, DEC, and others), DMF showed the most optimal results with a product yield of 76%. Bhatt et al. [23] investigated the effect of different solvents on the carboxylation of terminal alkynes to obtain α- and β-unsaturated carboxylic acids. In

Table 12. Optimization of reaction conditions ^a.

S. No.	Catalyst	Solvent	Light Irradiation	Conv. b	Yield		Ratio (A:B)
					A	B
1	CZU-7 (0.4)	DMF	Yes	30	18	12	1.5:1
2	CZU-7 (0.6)	DMF	Yes	20	11	9	1.1:0.9
3	ZBr-10	DMF	Yes	84	44	40	1.1:1
4	ZBr-10	DMF	No	-	-	-	-
5	-	DMF	Yes	-	-	-	-
6	ZBr-10	DMA	Yes	65	36	29	1.2:1
7	ZBr-10	THF	Yes	22	12	10	1.2:1
8	ZBr-10	DMSO	Yes	30	17	13	1.3:1
9	ZBr-10	Water	Yes	25	12	13	0.92:1
10	ZBr-10	ACN	Yes	45	24	21	1.14:1
11	ZBr-5	DMF	Yes	65	34	31	1.1:1
12	ZBr-15	DMF	Yes	83	42	40	1.1:1

[a] Reaction conditions: Phenylacetylene (1 mmol), KO^tBu (2 mmol), DMF (5 mL), photocatalyst ZBr-10: 25 mg, at ambient temperature under 1 atm pressure of CO₂ in the presence of photo-irradiation using 20 W light, and 24 h. [b] Conversion was determined by GCMS. Reproduced with permission [23].

, α-unsaturated carboxylic acids are designated as isomer A and β-unsaturated carboxylic acids are designated as isomer B. It was found that, in the presence of DMF, the reaction occurs with a high degree of phenylacetylene conversion (Table 12, entries 3 and 12). Other solvents proved to be ineffective (Table 12, entries 7–10). It should be noted that, in all cases, the output of isomer A was obtained in larger quantities than isomer B. Although water demonstrated low activity in this reaction (Table 12, entry 9), it provided almost equal amounts of both isomers (A and B). When washing a catalyst with other solvents, except DMF, the catalyst was recycled in Bondarenko’s work [50]. The reason was probably the destruction of a catalyst surface by traces of alcohols and oxygen. On the SEM-image of the catalyst surface (Figure 9), you can see the remaining amounts of adsorbed salts after washing with an aqueous-alcohol solution.

5. Conclusions and Future Perspectives

The carboxylation of aromatic alkynes with CO₂ is very important for scientific research. This type of reaction has many advantages: one of the main ones is the use of carbon dioxide as a source of C1, which will avoid the emission of exhaust gases into the atmosphere. A large number of catalysts necessary for the process have been investigated. However, it cannot be claimed that this problem has been completely solved. There are a number of problems, for example, in many cases, severe reaction conditions, insufficiently effective catalysts, and the cost of catalysts and their utilization. At the moment, the most suitable catalysts for the carboxylation process are the samples based on copper due to their high stability and activity. Non-catalytic systems can also be considered as a possible method for carrying out the carboxylation reaction. However, increased temperatures and pressures are crucial for the process efficiency.

For the successful carboxylation of terminal alkynes, it is necessary to choose the optimal base, which could promote a high product yield [100,101,102,103,104,105,106,107,108]. The most preferable base for the process is Cs₂CO₃. Although it is not the source of CO₂, this base is an important component for the coordination and deprotonation of alkyne, and the formation of a silver-acetylenide intermediate [94,106,107]. The catalytic parameters obtained by using other bases (K₂CO₃, Na₂CO₃, KOH, and so on) are much lower than those of Cs₂CO₃ [48,90,100,106]. That is why cesium carbonate plays a crucial role in the process [105].

The right choice of solvent is also one of the key factors which influences the effectiveness of the carboxylation reaction [61,71,79,104]. Screening different solvents proved that DMF is the most effective for the process; the conversion of the reactant is more than 80% [47]. The reason for that is likely its weak basicity and ability to dissolve CO₂ and a base. Among other solvents like THF or DMSO, DMF’s higher oxygen vacancy amount also increases the CO₂ adsorption [102,103]. CH₃CN or DMSO, in comparison with DMF, considerably decreases the conversion to 58% [85]. Overall, choosing a solvent and an inorganic base significantly influences the catalytic characteristics.

The methods of carboxylation with CO₂ that exist nowadays have some important disadvantages: the thermodynamic stability of CO₂, and a decrease in the product yield with increasing temperature, as well as the necessity of adding reducing agents, which limit the use of these methods from an economic and environmental point of view. Therefore, the development of highly efficient, available, and non-toxic catalysts for the carboxylation process based on the use of carbon dioxide is a promising scientific issue in the field of heterogeneous catalysis.