*3.4. Onium Salts as Catalysts*

Quaternary ammonium (most often tetrabutylammonium bromide, TBAB, and iodide (TBAI), phosphonium and sometimes even sulfonium salts [55]) are common catalysts for the formation of cyclic carbonates from epoxides and CO2 [5,6,12] (Figure 5).

**Figure 5.** Structures of cations of commonly used ILs.

The low-melting (below 100 ◦C) quaternary ammonium salts are called ionic liquids (ILs). In addition, ILs show good solvating ability including CO2, variable polarity and negligible vapor pressure [56]. ILs are considered to be sustainable ("green") solvents because of their properties such as relatively high thermal stability and negligible vapor pressure, high chemical stability and simple separability, and the modularity of their properties changing the structure of anions and cations.

The first published article that mentioned the formation of cyclic carbonates via the cycloaddition of CO2 in epoxides catalyzed solely by ILs was published by Deng and Peng [57]. The authors studied different 1-butyl-3-methylimidazolium (BMIM) and *N*-butylpyridinium (BPY) salts varying in anions (chloride and tetrafluoroborate (BF4 −, hexafluorophosphate (PF6 −)) and observed the highest activity in the case of BMIMBF4 salt. The catalytic activity increase was in the order: BMIMPF6 < BPYBF4 < BMIMCl < BMIMBF4. This observation is in agreement with the solubility of CO2 in these Ils; the highest solubility of CO2 was determined in BMIMBF4 [58].

Dyson et al. and Wang et al. studied the abilities of different ILs that differed in terms of the cations (alkylated imidazolium and tetraalkylammonium) and anions (halides) used [59,60]. Interestingly, cheap Bu4NCl was found to be a very active catalyst compared with the much more expensive alkylated imidazolium halides. Based on the experimental results obtained at ambient pressure and 50 ◦C, they hypothesized that the balance between the nucleophilicity of anions and the acidity of hydrogens bound in the imidazolium ring of used IL is most important.

Yang et al. published an even higher catalytic activity of corresponding bromides when comparing them with tetrafluoroborates (Table 9, Entries 10 and 11) [29].

An increase in catalytical activity was observed with the increasing of the lipophilic alkyl chain length in RMIMXs when comparing 1-butyl-3-methylimidazolium and 1-octyl-3-methylimidazolium bromide [29].

Based on this idea, Akhdar et al. successfully tested the carbonation of internal epoxide produced via the epoxidation of methyl oleate [61–63]. The catalysts were prepared by means of the alkylation of *N*-alkylimidazoles with oligoethylene iodide and modified by ion-exchange to the corresponding bromide. *N* -Oligoethylene-*N*-butylimidazolium bromide was recognized as the most active catalyst enabling the carbonation of epoxidized methyl oleate in 96% yield even at 100 ◦C and 2 MPa of CO2.

**Table 9.** Effect of the ionic liquid structure on the cycloaddition reaction of PO.



**Table 9.** *Cont.*

The catalytic activity of ILs cannot be known based on a comparison of the anion effects using one type of cation. In the case of *N*-benzyl-*N* -methylimidazolium salts, the most active one is *N*-(o-methyl)benzyl)-*N* -methylimidazolium chloride *o*-MeBzMIMCl; the catalytic activity of the corresponding *o*-MeBzMIMBF4 − and *o*-MeBzMIMPF6 − salts is lower (Table 9, Entries 14–16) [60].

Anthofer et al. studied the relationship between the structure, affinity to CO2 and catalytic activity of different low-melting *N*,*N* -dialkylimidazoles in detail [64] (Figure 6).

**Figure 6.** Structures of tested N,*N* -dialkyl IM-based ILs in the carbonation of PO [64]. R1 = Me; R3 = Bu; R2 = H = 1-methyl-3-butyl imidazolium bromide [BMIm]Br; R1 = Me; R3 = Bu; R2 = Me = 1,2-dimethyl-3-butylimidazolium bromide [BM2Im]Br; R1 = Me; R3 = Bu; R2 = Et = 1-methyl-2-ethyl-3-butylimidazolium bromide [BEMIm]Br; R1 = Me; R3 = Oct; R2 = H = 1-methyl-3-octylimidazolium bromide [OMIm]Br; R1 = Me; R3 = Oct; R2 = Me = 1,2-dimethyl-3-octylimidazolium bromide [OM2Im]Br; R1 = Me; R3 = Oct; R2 = Et = 1-methyl-2-ethyl-3-octylimidazolium bromide [OEMIm]Br; R1 = Bz; R2 = H; R3 = Bu = 1-benzyl-3-butylimidazolium bromide [BBzIm]Br; R1 = Bz; R2 = H; R3 = Oct = 1-benzyl-3-octylimidazolium bromide [OBzIm]Br; R1 = CH2C6F5; R2 = H; R3 = Bu = 1-(2,3,4,5,6-pentafluoro)benzyl-3-butylimidazolium bromide [BBzF5Im]Br; R1 = CH2C6F5; R2 = H; R3 = Oct = 1-(2,3,4,5,6-pentafluoro)benzyl-3-octylimidazolium bromide [OBzF5Im]Br.

As could be seen, the most active ILs were those that contained bulk lipophilic substituents (*N*-octyl-*N* -pentafluorophenyl-, *N*-butyl-*N* -pentafluorophenyl- or *N*-methyl-*N* octyl-imidazole). The observed catalytic activity correlates well with the measured sorption of CO2 in the studied ILs under the reaction conditions of this study. The substitution of the 2-position also significantly reduced the activity of the tested ILs [64].

The authors detected an interaction (hydrogen bond formation) between the acidic hydrogen of the used imidazole bromides (bound in position 2) and the oxygen atom of PO using FT-IR spectroscopy [64]. The mentioned catalyst was very effective even for the carbonation of internal epoxides such as cyclohexene oxide (ChO).

As could be seen, the most active ILs were those that contained bulk lipophilic substituents (*N*-octyl-*N* -pentafluorophenyl-, *N*-butyl-*N* -pentafluorophenyl- or *N*-methyl-*N* octyl-imidazole). The observed catalytic activity correlates well with the measured sorption of CO2 in the studied ILs under the reaction conditions of this study. The substitution of the 2-position also significantly reduced the activity of the tested ILs [64] (Table 10).


**Table 10.** Synthesis of PC from CO2 and PO using IL catalysts [64].

The authors detected an interaction (hydrogen bond formation) between the acidic hydrogen of the used imidazole bromides (bound in position 2) and the oxygen atom of PO using FT-IR spectroscopy [64]. The mentioned catalyst was very effective even for the carbonation of internal epoxides such as ChO (Table 11).



Reaction conditions: catalyst: 1-(2,3,4,5,6-pentafluoro)benzyl-3-octylimidazolium bromide (10 mol%), 0.4 MPa, 70 ◦C, 22 h.

As was published by Yang et al., in the case of butylated DABCO (BuDABCO), the corresponding bromides, chlorides and hydroxides were recognized as the most effective cycloaddition catalysts [29] (Table 12, Entries 1–4). In contrast, non-nucleophilic anions such as NTf2 −, PF6 − or BF4 − caused the loss of the catalytical activity of the studied BuDABCO salts.

**Table 12.** PC synthesis catalyzed by DABCO-based Lewis basic ionic liquids [29].


Surprisingly, when ILs containing activated CO2 in their structures in *N*,*N* -di(alkyl)imidazolium-2-carboxylates were tested as CO2 cycloaddition catalysts by Kayaki et al. [65], the observed activity was quite low and a high CO2 pressure was required to obtain satisfactory conversion (Table 13). The hydrogen in position 2 on the imidazolium ring is, in all probability, important as an HBD for epoxide ring activation and substitution with -COO− causes a decrease in catalytic activity (Table 13).

**Table 13.** Cycloaddition of CO2 to epoxides catalyzed by 1,3-di-tert-butylimidazolium-2-carboxylate [65].


Interestingly, some of the attempts to boost the catalytic activity of onium salts constructing di- or tricationic ILs (Scheme 7) often fall flat (Figure 7) [66–68].

**Scheme 7.** Preparation of tested ILs (substituted BzMIMs) described by Yang et al. [60]. R = H = [BzMIM]Cl; R = 4-CH3 = [*p*-MBzMIM]Cl; R = 2-CH3 = [*o*-MeBzMIM]Cl; R = 4-NO2 = [*p*-NBzMIM]Cl; R = 2-Cl = [*o*-ClBzMIM]Cl; R = 4-Cl = [*p*-ClBzMIM]Cl; X = PF6 = *o*-MeBzMIMBF4 −; X = BF4 = *o*-MeBzMIMPF6.

**Figure 7.** Structures of studied task-specific dicationic ILs (amino-pyridinium-pyrrolidinium bromide [66], Quaternized nicotine based ammonium ILs [67] and CH2-bridged tertiary amines [68].

Isothiouronium salts (Scheme 8) were also chosen for the testing of catalytic activity for CO2 addition, providing encouraging results (over 90% yield with selectivity over 99%) at 2 MPa pressure of CO2 and 140 ◦C after 2 h of action using 1 molar % of catalyst. The corresponding thiourea was practically nonactive [55].

**Scheme 8.** Preparation of catalytically active S-alkylisothiouronium salt [55].

Apart from ammonium and sulfonium salts, the methyl-trioctylphosphonium-based ILs with organic anions were studied as cycloaddition catalysts. Their catalytic activity was remarkable even for cycloaddition reaction of less reactive styrene oxide (SO) with CO2 at ambient pressure [46].

Wilhelm et al. compared the action of different aromatic or heterocyclic alcoholates (phenolates or anions of hydroxypyridine regioisomers, Figure 8) used as anions in combination with tetrabutylphosphonium, tetrabutylammonium and *N*-ethyl-DBU-based cations [69] (Table 14). The authors discovered the cooperative effect of the alcoholate anion of 2-hydroxypyridine with the tetrabutylphosphonium cation in the case of a reaction of CO2 with epichlorohydrin. The catalytic activity, however, of other ILs containing Bu4N+ or Et-DBU+ cations was slender (Table 14).

4-OPY

**Figure 8.** Structures of hydroxypyridine anion OPYs [69].

**Table 14.** Cycloaddition of CO2 with EPIC catalyzed ILs [69].


The authors suggested the mechanism of this reaction based on activation of the epoxide ring with the tetrabutylphosphonium cation as the Lewis acid with the simultaneous activation of CO2 and phenolate [69]. The most active [Bu4P] 2-OPY was tested at ambient pressure for the carbonation of different terminal epoxides with satisfactory results, using 50 molar % quantity of [Bu4P] 2-OPY to an appropriate epoxide (Table 15).



Reaction conditions: <sup>a</sup> catalyst (10 mol%), 0.1 MPa, 30 ◦C, 20 h; <sup>b</sup> catalyst (50 mol%).

Wang et al. prepared ammonium salts in situ by alkylating tertiary amides (*N*,*N*dimethylformamide (DMF), *N*,*N*-dimethylacetamide (DMAc), *N*-formylmorpholine, *N*methylpyrrolidone, tetramethylurea and *N*-formylpiperidine) with benzyl halogenides. The prepared ammonium salts enabled the formation of cyclic carbonates even at an ambient pressure, especially those prepared from DMF using benzylbromide [70] (Table 16).

**Table 16.** The effect of organic bases on cycloaddition (compared with DBU in co-action with alkyl halides) [71].



**Table 16.** *Cont.*

Wang et al. attributed the high activity of the DMF + BnBr mixture in particular to the activation of the oxirane ring by benzyl cations and the contemporary nucleophilic activation of CO2 by DMF [70] (Scheme 9).

**Scheme 9.** Proposed reaction pathway for the formation of cyclic carbonates catalyzed by benzylbromide/DMF [71].

Similarly, the effectiveness tertiary amines described earlier as active catalysts (see Section 3.1) was satisfactorily proven for the reaction of epoxides with CO2 at an ambient pressure after in situ quaternization via benzylation [71] (Table 16).

For a comparison of the action with the ammonium salts formed using arylmethylbromide derivatives, Bu4NBr was employed as the bromide source using the same reaction conditions as the model reaction (Table 16, Entry 15).

As could be seen, the yield was lower than that using benzyl bromide as the bromide anion source, which was presumably due to the electrostatic interaction between the bromide anion and the ammonium center decreasing with the bulkiness of the cation. The authors stated, based on above-mentioned results, that the nucleophilicity of the bromide anion is weaker for Bu4NBr than for the salts (Bn-DBU+.Br<sup>−</sup>).

The successful utilization of tetrabutylammonium halides, especially bromide and iodide, in CO2 cycloaddition reactions was reported by Calo [72] (Table 17). The higher reactivity of Bu4NBr/Bu4NI in comparison with RMIMBr or RPYBr salts was explained by less coordination of halide with the bulkier Bu4N<sup>+</sup> cation [72]. In addition, the catalytic activity of cheap and commercially available Bu4NXs is high and quite comparable with much more expensive PPNXs salts (Figure 9).

**Table 17.** Cycloaddition of CO2 to glycidol producing hydroxymethyl ethylene carbonate (HMEC) [74].


<sup>a</sup> Methyl glycidyl ether as substrate; <sup>b</sup> PO as substrate; [PPN]Cl—bis(-triphenylphosphine)iminium chloride.

**Figure 9.** Structure of bis(triphenylphosphoranylidene)ammonium halide (PPNX) [73].

*3.5. Two Component Catalysts Containing HDBs and Onium Salts*

The cheap and easily available quaternary ammonium halides TBAB and TBAI are often combined with different HDBs with the aim of boosting catalytic activity for the insertion of CO2 in the oxirane ring.

It was observed that even the addition of glycidol to the Bu4NX significantly increased the yield of PO compared with Bu4NX used alone [74] (Table 18, Entries 4, 7–9).

Some mixtures of onium salts with HBDs produce low-melting eutectic solvents (DESs) that readily dissolve both CO2 and epoxide, enabling cycloaddition even at ambient pressure and low temperature [75]. DES is defined as a mixture of two or more compounds that are typically solid at room temperature, but when combined at a particular molar ratio, changes into liquid at room temperature [76].


**Table 18.** Comparison of the catalytic activity of various PILs and DESs for the carbonation of SO.

<sup>a</sup> 15% of CO2 and 85% N2. Abbreviations: TMGH—*N*,*N*,*N* ,*N* -tetramethylguanidinium; DEA—diethanolamine; Ch—choline.

They not only possessed comparable physicochemical properties to traditional ILs (designability, non-volatility and high thermal stability), but also had advantages such as low cost and a simple preparation process (mixing and melting) without the need for purification.

DESs prepared via the mixing of tertiary amines hydrogen halides (R3N.HX) and ethylene diamine or different aminoethanols were compared in the carbonation of SO at an ambient pressure, obtaining intriguing yields and selectivities of SC even in the case of DES prepared from hydrobromide of cheap triethyl amine and diethanolamine [75] (Table 18, Entries 10–13). DBU hydrobromide mixed with diethanolamine at a molar ratio of 2:1 was recognized as the most catalytically active. Testing this most effective DES, high yields of different carbonates were determined by GC-MS even at an ambient pressure and room temperature of CO2 after 48 h using 20 molar % of this DES. Testing the carbonation of internal ChO, the yield of CC was 43%. Applying a mixture of 15% CO2 with nitrogen (simulated flue gas) drops, however, the yield decreased from 92% (100% CO2 at an ambient pressure after 48 h at room temperature) to 10% (using 15%CO2 in nitrogen under the same reaction conditions, Table 18, Entries 14–21).

Comparing the catalytic activity of different DESs with various protic ILs, the DES (2 DBU.HBr + 1 DEA) is much more active than PIL DBU hydroiodide (DBU.HI) alone. The observed high catalytic activity was explained by the synergistic action of DEA (as HBD) and DBU.HBr as a source of highly nucleophilic naked bromide [75] (Table 18, Entries 16–21 and 24).

Similarly, high activity was observed using DES prepared from Bu4PBr with 2-aminophenol for the carbonating of terminal epoxides. The carbonation of internal epoxide CO to CC was, however, very slow [79] (Table 18, Entry 28).

Pentaerythritol as an aliphatic polyol-based HDB was recognized as effective for the carbonation of PO at elevated pressure [80]. Although completely inactive used alone or with KI, in a mixture with Bu4N<sup>+</sup> bromide or iodide, it is very active, obtaining a 97% yield of PC at 70 ◦C after 22 h of CO2 (0.4 MPa) action (Table 19).

**Table 19.** Comparison of catalytic activities of DESs based on pentaerythritol (PETT) and Bu4NXs for the preparation of PC [80].


Choline iodide together with *N*-hydroxysuccinimide forms DES, enabling the highyield carbonation of PO to SC at 30–80 ◦C and 1 MPa pressure of CO2. Instead of choline iodide, Bu4NX in a mixture with *N*-hydroxysuccinimide is applicable [81]. Using a 2 MPa pressure of CO2, a high yield of CC from internal ChO was obtained at 70 ◦C after 10 h of reaction (Table 18, Entry 29).

A broad set of aliphatic and aromatic alcohols in terms of their role as potential HDBs was studied by Alves et al. [82] in the co-action of Bu4NBr using pressurized CO2 (2 MPa) for PO carbonation. The authors observed that the most active HDBs were lowpolar polyfluorinated secondary alcohols such as tertiary alcohols HFTI or 1,3-bis-HFAB (Figure 10).

Aromatic polyols such as pyrocatechol, pyrogallol and gallic acid were less catalytically active. Aliphatic alcohols exhibited low cooperative activity in the case of the tested Bu4NBr, which was practically comparable with the catalytic effect of sole Bu4NBr. Interestingly, some of tested alcohols even exhibited inhibition effects.

**Figure 10.** Structures of the most effective HBDs in co-action with Bu4NBr for the carbonation of PO [81].

The high catalytical activity of RMIMs/phenols-based DESs was published by Liu et al. even at an ambient pressure of CO2 and at room temperature for SO [83]. Especially *N*-ethyl-*N* -methylimidazolium iodide (EMIMI) was recognized as a very suitable part of DES in co-action with phenols substituted with electron-donating groups such as –NH2, –C(CH3)3 and –Cl, –OH. The most effective DES contained EMIMI (2 mol) and resorcinol (1 mol). The authors explained its high catalytic activity as multifunctional HBD-based activation by acidic hydrogen bound in position 2 of EMIM salt together with hydrogen from the –OH group of resorcinol (Figure 11) and the subsequent action of iodide as a nucleophile. Interestingly, comparing the activity of (2 EMIMI + 1 resorcinol) DES with the much cheaper (2 Bu4NI + 1 resorcinol) binary system for SO carbonation, the obtained yields of PEC were very similar [83]. SO was the single epoxide studied, however, in this article. Another catalytically very effective DESs containing mixture of choline chloride and malic acid or choline iodide and glycerol published Vagnoni et al. [84].

**Figure 11.** Structures of resorcinol and gallic acid.

Additional very effective DESs were obtained as catalysts by mixing 2-hydroxymethylpyridine or 2,6-hydroxymethylpyridine with Bu4NI [40]. These DESs were able to catalyze the carbonation of EPIC to chloromethyl-ethylenecarbonate (CMEC) even at room temperature and ambient CO2 pressure. The carbonation of internal epoxide ChO was very slow, however, under ambient conditions even after 20 h using 8 molar % of catalyst [40].

Gallic acid (Figure 11), as a green, biobased and biodegradable HDB, was discovered by Sopena et al. as a more effective alternative of resorcinol in a binary Bu4NI + gallic acid catalytic system dissolved in 2-butanone [85]. Even internal epoxide was carbonated with a high yield at 80 ◦C and 1 MPa pressure of CO2 after 18 h [85].

Polycarboxylic acids such as citric acid were effectively applied as the HDB part of DES together with choline iodide [86]. The other tested carboxylic acids were less active HDBs compared with citric acid. Additionally, it was observed that the molar ratio of the used HBA and HDB is crucial. For DES obtained by the melting of choline iodide, citric acid at a molar ratio of 2:1 (excess of iodide source) is highly active. Changing the molar ratios significantly decreased the reaction yield (but not selectivity). ChO tested as an internal epoxide at 70 ◦C and 0.5 MPa of CO2 produced only 36% CC after6h[86].

The attempts to substitute ILs-based iodides or bromides as key parts of DESs were described by Wang et al. [87]. Applying boric and glutaric acids, together with BMIMCl, the authors described significant catalytic activity even without the presence of bromide or

iodide ions for the carbonation of terminal epoxides at 0.8 MPa of CO2 and 70 ◦C. [87]. The carbonation of internal ChO was below 40% after 7 h of CO2 action.

The most active HDB described until this time for the catalysis of epoxides' carbonation is ascorbic acid in co-action with Bu4NI [15] (Table 20). This mixture was effective even for the carbonation of internal epoxides, even at an elevated temperature (100 ◦C) and 2 MPa CO2 pressure [15] (Table 21).


**Table 20.** Comparison of various HBD/Bu4NI catalytic systems for the carbonation of EPIC.

<sup>a</sup> using SO as a substrate. <sup>b</sup> APAA—acetal protected ascorbic acid.


**Table 21.** Carbonation of methyl oleate using ascorbic acid (HBD) and different sources of nucleophile

<sup>a</sup> Using recovered catalysts.

Encouraged by the robustness of this catalytic system, Elia et al. tested the Bu4NI/ascorbic acid system for the cycloaddition of CO2 in epoxidized fatty acid esters [91]. Cyclic carbonates based on fatty acid esters seemed to be potential plasticizers for polyvinyl chloride instead of harmful phtalates, for example [92].

As can be seen in Table 21, the most effective catalytic mixture found contains Bu4NCl/ascorbic acid. Bu4NCl is superior because overly nucleophilic Bu4NI causes

18 L-ascorbic acid/[Bu4N]Cl 1.5/5 1 100 24 92 >99

undesirable Meinwald rearrangement producing ketones instead of cyclic carbonates, probably due to the sterical hindrance in the case of epoxidized oleic acid methyl ester [91]. In the case of epoxidized polyunsaturated fatty acid esters, allylic alcohols are produced as by-products using Bu4NI [91].
