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Catalytic strategies for the cycloaddition of pure, diluted and waste CO to epoxides under ambient conditions 2

Rafik Rajjak Shaikh, Suriyaporn Pornpraprom, and Valerio D'Elia ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b03580 • Publication Date (Web): 29 Nov 2017 Downloaded from http://pubs.acs.org on November 29, 2017

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Catalytic strategies for the cycloaddition of pure, diluted and waste CO2 to epoxides under ambient conditions Rafik Rajjak Shaikh, Suriyaporn Pornpraprom, and Valerio D’Elia* Department of Materials Science and Engineering, School of Molecular Science and Engineering, Vidyasirimedhi Institute of Science and Technology (VISTEC), 555 Moo 1, Payupnai, Wangchan, Rayong, 21210, Thailand.

ABSTRACT: Cyclic organic carbonates represent a relevant class of chemicals that can be prepared from CO2 by cycloaddition to epoxides. The application of efficient catalysts is crucial in allowing the cycloaddition reaction to proceed under very mild conditions of temperature, pressure and CO2 concentration thus resulting in a sustainable and carbon-balanced approach to CO2 conversion. This is particularly the case if impure waste CO2 could be employed as a feedstock. In this perspective, we have critically analyzed the burgeoning literature on the cycloaddition of CO2 to epoxides with the aim to provide state-of-the-art knowledge on the catalysts that can convert CO2 to carbonates under ambient conditions. These have been systematically organized in families of compounds and critically scrutinized in terms of catalytic activity, availability and mechanistic features. Finally, we provide an overview on the catalytic systems able to function using diluted and impure CO2 as a feedstock.

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KEYWORDS: carbon dioxide, CO2 fixation, cycloaddition reaction, ambient conditions, catalysis.

1. INTRODUCTION The development of chemical processes that can recycle CO2 as a chemical feedstock for the production of industrially attractive chemicals, plastics precursors and fuels is regarded as a viable method for contributing to the reduction of the net amount of anthropogenic carbon dioxide released in the atmosphere.1-9 Besides other limitations,10 one of the main hurdles to overcome for the implementation of this strategy is represented by the high indirect carbon costs necessary to power CO2 conversion to chemicals due to its inherent thermodynamic stability.11 These costs can offset the beneficial effects of CO2 transformation to chemicals by the indirect production of additional carbon dioxide.11-13 Remarkably, the synthesis of cyclic organic carbonates from CO2 and epoxides represents one of few14-16 processes that can be carried out under mild and even ambient conditions of temperature and pressure when highly active catalysts are employed. Indeed, this cycloaddition reaction is thermodynamically favored and its driving force is determined by the release of the ring-strain energy contained in the epoxide substrate.17 Cyclic carbonates are generally liquid and highly stable compounds that can guarantee a long-term sequestration of CO2 when compared to other commodity products such as urea that promptly releases CO2 upon utilization in agriculture.18 The synthesis of cyclic carbonates from CO2 is currently widely exploited in industry; in the Shell Omega process ethylene carbonate is formed from the cycloaddition of CO2 to ethylene oxide and is subsequently hydrolyzed to afford ethylene glycol.19 In the Asahi-Kasei process for the production of bisphenol-A polycarbonate, ethylene carbonate is cleaved with methanol to produce


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dimethyl carbonate (polycarbonate precursor) and ethylene glycol thus affording two commercial products in a single reaction step.20

Figure 1. Number of documents (darker green) involving propylene carbonate published for each five years’ time frame in the 1982-2016 period. The fraction of documents representing patents is shown in light green. The data were obtained from SciFinder® searches.

Furthermore, cyclic organic carbonates are increasingly in the focus of academic and industrial research (Figure 1, note the increasing fraction of patents) for their ability to serve as solvents in chemical processes21-28 and in batteries29-30 and as useful intermediates for the preparation of a large variety of products. The latter include acyclic carbonates,31 carbamates,32-33 polymers,34 methanol,35-36 cis-diols,37-38 spiro compounds,39 heterocyclic compounds,40-41 ionic liquids.42-43 In the light of these considerations, the synthesis of cyclic carbonates under ambient conditions could represent an environmentally friendly and more carbon-balanced approach to the preparation of useful chemicals from CO2 that minimizes the indirect carbon costs related to


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energy intensive processes such as power generation and CO2 gas compression. This is especially the case if waste and impure CO2 obtained from capture and separation plants or even directly from flue gas could be employed as a feedstock for this reaction.44 It is clear that this strategy requires the development of highly active and robust catalysts that are able to operate under challenging conditions (use of diluted CO2, reaction temperature close to ambient, presence of moisture and impurities).45-46 Overall, gaining a thorough overview on the catalysts able to carry out the cycloaddition of CO2 to epoxides under ambient conditions is crucial and propaedeutic to understand the challenges, opportunities and potential pitfalls of this approach. Therefore, whereas the cycloaddition of CO2 to epoxides has been carefully reviewed in other works,47-54 when dealing with the use of pure CO2, this manuscript will uniquely focus on providing a state-of-the-art knowledge on the homogeneous and heterogeneous catalysts operating at ambient conditions (atmospheric pressure, reaction temperature in the 20-30 °C range with the exception of a few catalytic systems operating at 0 °C when using propylene oxide as a substrate). Mechanistic details are discussed when deemed necessary for a better understanding of the origin of the observed catalytic activity under ambient conditions. Catalytic strategies that have shown the potential to employ waste CO2 as a feedstock for the title reaction are already emerging and will be discussed in detail in the final sections of this review.55-59 From a mechanistic standpoint, the cycloaddition of CO2 to epoxides can be carried out using nucleophilic catalysts, binary systems constituted by a nucleophilic and a Lewis acidic component or single component catalysts displaying both components in a multifunctional molecule. Nucleophilic compounds such as amidine base DBU (1,8-diazabicyclo[5.4.0]undec-7-ene),60-61 guanidine base TBD (1,5,7-triazabicyclo[4.4.0]dec-5-ene)62 and N-heterocyclic carbenes63


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(NHCs) are known to attack CO2 at its electrophilic carbon atom to produce the corresponding CO2-adducts. The formation of such adducts (A-1, Scheme 1, Cycle 1; the nucleophiles in this class are referred as Nu1) can be regarded as the initial step in the cycloaddition of CO2 to epoxides promoted by these nucleophiles.35, 64-69 In particular, A-1 can attack and ring-open a molecule of epoxide (or of a Lewis-acid (LA) activated epoxide A-2) leading to I-1. Cyclization of the latter intermediate leads to the target carbonate product and restores the nucleophile for a new catalytic cycle. Other nucleophiles such as P-ylide-CO2 adducts70 and the organic oxyanion betaine71 have also been proposed to act according to a similar mechanism. Through a different mechanistic route (Scheme 1, Cycle 2, the nucleophiles acting according to this mechanism are denoted as Nu2), quaternary ammonium and phosphonium salts and ionic liquids bearing nucleophilic counter anions can ring-open the epoxide substrate by a nucleophilic attack of the anion and catalyze the synthesis of cyclic carbonates after the steps of CO2 activation and cyclization.72-74 The presence of hydrogen bond donors or of Lewis acidic compounds such as metal-organic complexes and coordination compounds, that are able to activate the epoxide substrate, can facilitate the step of epoxide ring-opening often leading to a remarkable acceleration of the reaction rate. In this case, following the cooperative step of ring opening, CO2 inserts in the metal-alkoxide bond of I-2 to afford intermediate I-3 whose cyclization yields the cyclic carbonate product (Scheme 1, Cycle 2). As an effect, many catalysts for the cycloaddition of CO2 to epoxides are represented by binary systems with the Lewis acidic component being generally denoted as the “catalyst” and the nucleophilic component as the “co-catalyst”.75-78 However, this convention is potentially misleading (the Lewis acidic “catalysts” are generally not active in the absence of a nucleophilic “co-catalyst”, whereas, the latter class of compounds can often catalyze the cycloaddition reaction also in the absence of a Lewis acidic “catalyst”) and it is not used in this


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manuscript. Instead, we simply refer to the Lewis acidic and nucleophilic nature of the catalytic components. It is worth mentioning that variations from the main mechanistic pathways discussed above exist: Werner et al. have reported a convergent concept for the simultaneous activation of CO2 by a N-heterocyclic carbene (as in cycle 1 of Scheme 1) and of the epoxide by ring opening with a KI-crown ether complex; the potassium atom acting as a Lewis acidic center (as in cycle 2 of Scheme 1).79 In the azaphosphatrane-catalyzed reaction, the activation of CO2 proceeds by its cycloaddition to the P-N bond of the catalyst.80 Reaction mechanisms involving more than one Lewis acidic metal center have also been published.76, 81 Furthermore, for the case of internal epoxides, the formation of ketone by-products via Meinwald rearrangement may occur (Scheme 1).82


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Scheme 1. General mechanistic pathways for the cycloaddition of CO2 to epoxides. In the specific case of the cycloaddition of CO2 to epoxides under ambient conditions, most catalytic systems are binary (or single component catalysts displaying both Lewis acidic and nucleophilic moieties within the same molecule) and follow the mechanism shown in cycle 2 of Scheme 1. In this context, most of the literature reports describe the application or discovery of new Lewis acidic complexes and compounds that are applied in the presence of well-established sources of nucleophilic anions such as quaternary onium salts or ionic liquids. Therefore, this review is mainly organized according to the class of Lewis acid employed; these have been


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categorized according to their chemical structure and are critically compared and discussed within each class. As an effect of this analysis, we show that each class of compounds corresponds to a specific range of catalytic activity. The compounds within the same chemical class have been generally compared for catalytic activity in the cycloaddition reaction using the literature available or calculated turnover frequencies (TOFs) as reference values. In general, we refer to the use of propylene oxide (PO) as the substrate as it represents an industrially relevant epoxide and it is used in the vast majority of the studies discussed in this review. It is clear that, in the absence of benchmarking, the TOF values obtained using different concentrations, reaction times and ratios between the catalytic components is only indicative of the range of activity of each compound and family of compounds.83 Therefore, in order to minimize the potential bias due to the lack of benchmarking in the literature, we focused on those catalytic experiments for which the reaction time did not exceed 48 h, the loading of any catalytic component was not higher than 20 mol% and the reaction yield or conversion was close to or higher than 50 % (with the exception of a few cases for which this condition could not be applied). Only solventless reactions were considered. Furthermore, when available and significantly higher than the result obtained using PO, we highlight also the highest TON and TOF values reported using any other common terminal epoxide (See Chart 1 for a list of widely studied terminal epoxides and the relative abbreviation used in this manuscript) during the catalytic investigation. Besides catalytic activity, other factors such as availability, molecular weight, potential costs and sustainability have been taken into account in the discussion.


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Chart 1. Commonly employed terminal epoxides and relative abbreviations 2. HOMOGENEOUS CATALYSIS The application of homogeneous catalysts (metal-organic complexes, coordination compounds and organocatalysts) for the conversion of CO2 and epoxides to cyclic carbonates under ambient conditions is discussed in the following sections.

2.1. Monometallic Metal-organic Complexes 2.1.1. Salen Complexes Salen complexes and affine compounds (salphen, salalen etc.) are readily available, display a highly tunable structure and can be prepared in enantiomerically pure form thus allowing their application for asymmetric synthesis and kinetic resolution.84 Two independent reports by Jacobsen and Katsuki in 1990, where salen frameworks were used as ligands for manganese in asymmetric epoxidation reactions, contributed to the realization of the potential of this class of compounds in catalysis.85-86 Inspired by these findings, several authors have expanded the area of application of metallosalen complexes to several catalytic reactions.87-93 A prime attempt to apply salen complexes for CO2 fixation with epoxides was reported by Nguyen et al. in 2001 involving Cr(III) salen complexes used in combination with DMAP (N,N-dimethylaminopyridine) as a nucleophile.94 The authors applied this catalytic system at room temperature but under ca. 7 bar


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CO2 pressure observing low turnover frequencies. Following this seminal report, several authors have reported the application of salen complexes as Lewis acids for the cycloaddition of CO2 to epoxides95 with various catalytic systems able to carry out the title reaction under very mild (room temperature, PCO2: 2-7 bar)96-104 or ambient conditions (vide infra). Enantiomerically pure salen complexes have been efficiently used for the kinetic resolution of epoxides by cycloaddition of CO2.95, 98, 101, 105-107 Focusing exclusively on the catalytic systems active for the title cycloaddition reaction under ambient conditions, recent examples of Cr(III) salphen (salphen: salen complex with an aromatic bridge) complexes were reported by North and co-workers (Scheme 2).108 The authors initially demonstrated that complex 1, used in the presence of TBAB (tetrabutylammonium bromide), could catalyze the cycloaddition of CO2 to various terminal epoxides under ambient conditions. In the same work, the authors attempted the preparation of more active catalysts by finely tuning the pattern of substitution of the organic framework as previously observed in the literature. 95, 109 Replacing one of the t-butyl substituents at each phenyl ring with electron-donating methoxy groups (2) led to an increase of the catalytic activity compared to 1. The binary catalyst 2/TBAB afforded 100 % conversion of styrene oxide (SO) to styrene carbonate (SC) in 24 h and 45 % conversion in 6 h. The replacement of the t-butyl groups with electron-withdrawing nitro groups led, instead, to a strong decrease of the catalytic activity. Overall, the authors attributed this trend of activity to the destabilization of the M-O bond in the ring-opened intermediate (See I-2 in Scheme 1, Cycle 2) by electron-donating groups favoring the step of insertion of CO2. With complex 2 in hand, the authors further explored the effect of the counterion at the metal center. The catalytic activity was found to increase with the nucleophilicity of the counterion. Fully-


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optimized complex 3 bearing a bromide counterion was found as the generally most active catalyst for a broad selection of epoxides.

Scheme 2. Cr(III)-salphen complexes for the cycloaddition of CO2 to epoxides. North et al. studied the kinetic resolution of terminal epoxides using monometallic and bimetallic aluminum salen complexes in the presence of TBAB under ambient conditions (025 °C, 1 bar).110 The authors proposed that the use of aluminum salen complexes for this application could result advantageous because undesired side reactions such as the formation of polycarbonate and the hydrolysis of the epoxide by traces of moisture do not generally occur when using aluminum-based systems. Whereas the resolution of SO was not successful, the ability of the aluminum salen complexes to afford the kinetic resolution of functionalized epoxides such as glycidyl ether or N-(2,3-epoxypropyl)diphenylamine could be achieved. This result represents the first example of kinetic resolution of epoxides by cyclic carbonate synthesis by an aluminum-based complex. Among the complexes studied, 4 (Scheme 3, Left) resulted as the most enantioselective species affording the carbonate of N-(2,3-epoxypropyl)diphenylamine with an enantiomeric excess of 86 % at 0 °C for a conversion of 15 %. The performance of 4 for the resolution of the latter epoxide was superior to that of analogous Co and Cr-complexes based on the same ligand design and applied under identical reaction conditions.


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Scheme 3. (Left) aluminum salen complex used by North et al. for the kinetic resolution of epoxides by cyclic carbonate synthesis. (Right) Co(III), Al(III), Zn(II) salen complexes studied by Deng et al. In a theoretical and experimental study under ambient conditions, Deng et al. explored salen complexes 5-7 (Scheme 3, Right) based on cobalt, aluminum and zinc for the synthesis of propylene carbonate.111 All three complexes were catalytically active when used in the presence of a relatively high loading of TBAB (7.2 mol%) and provided comparable yields of the product with similar TOF values (about 3 h-1) in a long reaction time (48 h). Zhou et al. recently reported an innovative and elegant pathway for the activation of salen complexes by the addition of phosphoranes.112 The reaction between the latter compounds and salen complexes induces the formation of a cationic species and liberates the metal counterion to serve as a nucleophile (Scheme 4, Left). This method, that the authors originally developed for the cyanosilylation of ketones,113 was applied for the cycloaddition of CO2 to SO under ambient condition using salen complex 8 (Scheme 4, Right). As expected, 8 did not show any activity for the title reaction when used in the absence of a nucleophile. However, in the presence of phosphorane 9 a low yield of styrene carbonate (SC) was observed. Starting from this result, and taking into account that this method can be applied to any salen complex, the authors investigated the addition of 9 to various Al, Mn, Cr, Co salen complexes. The best catalytic activity was found for salen complexes bearing iodine as a nucleophile. Cobalt showed the highest catalytic efficiency among the metals screened (10, Scheme 4). Upon addition of an equivalent amount of


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phosphorane 9, Complex 10 (1 mol%), catalyzed the quantitative conversion of SO to SC under ambient conditions in 24 h and could efficiently convert a large variety of terminal epoxides to the corresponding carbonate. Furthermore, the authors demonstrated the proposed mechanism of activation by conductivity, mass spectroscopy and NMR studies and could apply this catalytic system also for the synthesis of N-aryl oxazolidinones from the parent aziridines and CO2 under ambient conditions.

Scheme 4. (Left) Proposed activation of salen complexes by phosphoranes via formation of a cationic complex. (Right) Salen complexes used in the presence of phosphorane 9 as a catalytic system for the cycloaddition of CO2 to epoxide. The catalytic activity of the salen complexes discussed in this section is displayed in Table 1. With respect to the large number of reports describing the application of this class of Lewis acids for the cycloaddition of CO2 to epoxides,95 relatively few monometallic salen complexes have been reported to function under ambient conditions. In general, moderate TOF values have been observed for the conversion of PO to PC (in the 0.95 to 4.1 h-1 range) with the highest value been reported for phosphorane-activated Co(III) salen complex 10 (Table 1, Entry 9).


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Table 1. Comparison of catalytic activity of monometallic salen complexes for the synthesis of carbonates under ambient conditions.

Entry

MSC/Nucleophile

Yield (%)

T

(Epoxide)

Loading (mol%)

TON/TOF(h-1)

Time

1 (PO)

1/TBAB

71

0 °C

2.5/2.5

28.4/1.2

24 h

1/TBAB

65

25 °C

2.5/5

25.8/4.3

6h

3/TBAB

57

0 °C

2.5/2.5

22.8/0.95

24 h

3/TBAB

71

25 °C

2.5/2.5

28.2/4.7

3h

4/TBAB

28

25 °C

2.5/2.5

11.2/3.7

3h

5/TBAB

75.8

25 °C

0.48/0.48

155/3.2

48 h

6/TBAB

73.2

25 °C

0.48/0.48

150/3.1

48 h

7/TBAB

72.1

25 °C

0.48/0.48

148/3.1

48 h

10/9

98

25 °C

1/1

98/4.1

24 h

2 (SO)

3 (PO)

4 (SO)

5 (SO)

6 (PO)

7 (PO)

8 (PO)

9 (PO)


Ref. 108

108

108

108

110

111

111

111

112

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Slightly higher TOF values (4.3 to 4.7 h-1) were observed by North et al. for the conversion of SO to SC with 1/TBAB and 3/TBAB (Table 1, Entries 2, 4). The latter binary catalyst provided slightly higher TOF values (5.5 to 5.9 h-1) when the reaction was carried out in 3 h, however, in this case, the epoxide conversion was lower than 50 %.

2.1.2 Non-salen Monometallic Complexes The porphyrin macrocyclic framework is highly versatile and allows elaborate functionalization as way of tuning the electronic properties of the metal center; it is easily metalated with numerous ions and displays a good thermal stability.114 Metalloporphyrins have found wide application in catalytic systems for the fixation of CO2 by cycloaddition to epoxides,115-117 currently representing one of the most active families of Lewis acids for such reaction. For instance, the work by Ema et al. has shown that single component Mg(II) and Zn(II) TPPs (TPP: tetraphenylporphyrin) can reach TON values in the order of hundred thousand at 120 °C and with a CO2 pressure of 1730 bar.118-120 With respect to the activity of metalloporphyrins under ambient conditions, Maeda et. al have investigated functionalized Zn(II) TPP 11 (Scheme 5), bearing eight quaternary ammonium moieties as pendants on the porphyrin scaffold.120 At 20 °C, they reported a yield 82 % for the cycloaddition of CO2 to HO with a TON of 1640 in 48 h. The calculated TOF (34.2 h-1) represents the highest value reported so far in homogeneous catalysis for the title cycloaddition reaction under ambient conditions. From a mechanistic standpoint, the authors could to justify the high catalytic activity observed for 11 by a DFT approach leading to the identification of a cooperative mechanism between the pendant tetraalkylammonium chains and the metal center in the rate determining step of epoxide ring opening. In this mechanism, the bromide anion is coordinated and prepared for the attack to the epoxide by the partially positively charged hydrogen


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atoms of the side chains (See Figure 2 for a model-study). The network of hydrogen bonds arising from the hydrogen atoms in the alkyl chains was suggested to stabilize various anionic species within the catalytic cycle. These findings match a previous study by the authors on analogously functionalized Mg(II) TPPs.119

Scheme 5. Single component Zn(II) TPP as a catalyst for the coupling of CO2 and epoxides.

Figure 2. Representative transition state for the ring opening of PO by a model Zn(II) TPP with a single pendant tetraalkylammonium chain. The stabilization of the bromide anion by a network of positively charged hydrogen atoms of the alkyl side chain is shown. Reproduced with permission from Ref 120 copyrights: Wiley-VCH Verlag GmbH & Co. KgaA, Weinheim.


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Inspired by their initial studies on organobismuth complexes able to form carbonate dimers upon reaction with CO2 (Scheme 6),121 Shimada and Yin synthesized two bismuth compounds, 12 and 13, bearing a sulfur-bridged bisphenolato ligand, and evaluated their catalytic activity for CO2 conversion into cyclic carbonates.122 When compounds 12 and 13 were screened in the absence of a nucleophile using PO as the substrate under ambient conditions, no conversion to PC was achieved. However, in the presence of a halide salt, both compounds afforded high to excellent conversion of PO to PC. Amongst the examined nucleophiles, LiI was found to form an efficient binary system with compound 13 as observed from the nearly quantitative conversion (98 % in 24 h using 0.12 mol% catalyst) of PO into PC. The reactions could be carried out under solventfree conditions.

Scheme 6. Bismuth carbonate dimer (Left) and bismuth complexes (Right) bearing a sulfurbridged bis(phenolato) ligand. Babu and Muralidharan synthesized four Zn(II), Cd(II) and Cu(II) complexes of 2,5-bis[N(2,6-diisopropylphenyl)iminomethyl]pyrrole.123 The complexes were obtained by using two pathways: from the homoleptic diamido precursors, Zn[N(SiMe3)2]2 for 14 or Cd[N(SiMe3)2]2 for 15 or from the base promoted reaction of metal halides (CdCl2 for 15·CH3OH and CuCl2 for 16) with 2,5-bis[N-(2,6-diisopropylphenyl)iminomethyl]pyrrole (scheme 7). Among the four synthesized complexes, 14 and 16 were found to convert a range of terminal epoxides and an


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internal epoxide to the corresponding cyclic carbonate at ambient temperature (10-25 °C) under atmospheric CO2 pressure in the presence of TBAB. When tested with PO as the substrate, 14/TBAB and 16/TBAB gave 84 % and 82 % isolated yield of PC respectively under ambient conditions in 20 h reaction. Complex 14 could be recovered and reused for 6 catalytic cycles before undergoing deactivation. However, given the homogeneous nature of the catalyst, its recovery required the distillation of propylene carbonate to afford a residue containing 14 and TBAB. Interestingly, the addition of a fresh batch of TBAB restored the catalytic activity of the catalyst.

Scheme 7. Zn(II), Cd(II) and Cu(II) complexes of diisopropylphenyl)iminomethyl]pyrrole applied for CO2 fixation to carbonates.

2,5-bis[N-(2,6-

Bharadwaj et al. reported the synthesis of a Co(II) complex by the reaction of a prolinederived cryptand (Figure 3, Left) and Co(II) perchlorate in the presence of KSCN.124 The resulting complex (Co-Cryptate, 17) was found to include three Co(II) centers in a distorted octahedral geometry and two cryptand molecules (Figure 3). Despite the trinuclear nature of the complex, each Co(II) ion appears as an isolated unit coordinated by two proline ligands and two nitrogen atoms from the thiocyanate anions. Therefore, from a catalysis standpoint, 17 is considered within this section.


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ACS Catalysis

Figure 3. Structure of the proline-derived cryptand (Left) and of the Co-Cryptate complex (right) resulting by the reaction of the ligand with Co(II) perchlorate in the presence of KSCN. Adapted with permission from reference 124, copyrights: American Chemical Society.

Complex 17 was tested as a catalyst for the enantioselective aldol reaction in water and for the cycloaddition of CO2 to epoxides in the presence of TBAB. Concerning the latter reaction, terminal epoxides were converted to the corresponding cyclic carbonates in low to moderate yields (3249 %) in 12-24 h at 20 °C and atmospheric pressure. The authors employed a low catalytic loading of 17 (0.05 mol%; 0.15 mol% Co) and a relatively high TBAB loading (5 mol%). Catalytic activity data for the monometallic metal-organic complexes discussed in this section are presented in Table 2. The single component Zn(II) TPP (11) developed by Maeda et al. shows the highest TOF value obtained for the cycloaddition of CO2 to HO (Table 2, Entry 1; the authors did not report the conversion of other substrates under ambient conditions by 11). Interestingly, the binary catalytic systems employing 12 and 13 in the presence of LiI show a similarly high TOF value as 11 but for the conversion of PO (Table 2, Entries 2, 3).


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Table 2. Comparison of catalytic activity of monometallic (non-salen) complexes for the synthesis of carbonates

Entry

Monom. Complex/Nucleophile

Yield (%),

T

(Epoxide)

Loading (mol%)

TON/ TOF (h-1)

Time

1 (HO)

11

82

20 °C

0.05

1640/34.2

48 h

12/LiI

92

25 °C

0.12/0.5

755/31.5

24 h

13/LiI

98

25 °C

0.12/0.5

805/33.5

24 h

14/TBAB

67

10 °C

2.5/5

26.8/2.7

10 h

14/TBAB

86

25 °C

2.5/5

34.4/3.4

10 h

16/TBAB

64

10 °C

2.5/5

25.6/2.6

10 h

17/TBAB

43

0 °C

0.15/5

287/23.9

12 h

2 (PO)

3 (PO)

4 (PO)

5 (DMO)

6 (PO)

7 (PO)

Ref. 120

122

122

123

123

123

124

2.2. Bimetallic Metal-organic Complexes 2.2.1. Bimetallic Salen Complexes The application of bimetallic complexes where both metal centers are found in a suitable geometric arrangement to give rise to a cooperative catalytic effect might result advantageous in


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terms of reaction rates and/or product selectivity.125 In principle, the cooperation between the metal centers might take place by the simultaneous coordination and activation of one or more reaction substrates or intermediates by the cooperating metals.125-126 The North group has devoted a significant amount of investigation to bimetallic salen complexes for the cycloaddition of CO2 to epoxides under ambient conditions. They had previously employed bimetallic salen complexes for the asymmetric cyanohydrin synthesis. 127 Expecting a cooperative effect also for the cycloaddition of CO2 to epoxides, they explored the activity of several aluminum based bimetallic salen complexes with different patterns of substitutions at the salen scaffold (Scheme 8).128 Complex 18 could catalyze the cycloaddition of CO2 to various terminal epoxides under ambient conditions in the presence of TBAB as a nucleophile. Using 2.5 mol% of each catalytic component, PO could be converted to PC in 77 % yield in just 3 h of reaction at 0 °C under atmospheric pressure of CO2.

Scheme 8. General structure of the bimetallic aluminum salen complexes studied by North et al. for the cycloaddition of CO2 to epoxides under ambient conditions. The substitution pattern for complex 18 is indicated.


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Encouraged by the catalytic efficiency of the 18/TBAB system, the authors carried out systematic kinetic and mechanistic investigation in order to gain deeper mechanistic understanding of the catalytic activity of the bimetallic aluminum complexes under ambient conditions.

76, 129

It

was observed that various monometallic analogues of 18 could not catalyze the cycloaddition reaction under ambient conditions, thus highlighting the importance of the bimetallic nature of the complex. Importantly, the cycloaddition reaction was found to be of second order in the TBAB concentration and the in situ formation of tributylamine during the catalytic cycle was suggested. The authors proposed a mechanism in which the in situ generated tributylamine and CO2 form a carbamate that is coordinated to one of the aluminum centers (Scheme 9). The epoxide is ringopened on the neighboring aluminum center by the cooperative effect of the Lewis acidity of the metal and of the nucleophilic attack of a bromide anion from TBAB. The two reaction intermediates, supported on the same side of the salen complex, can then readily combine to form the desired carbonate product. The authors attributed the catalytic efficiency of 18/TBAB to this cooperative effect.

Scheme 9. Dual-site mechanism for the synthesis of cyclic carbonates by two adjacent aluminum centers of a bis-aluminum salen complex in the presence of TBAB. Nevertheless, it should be noted that the authors did not present a theoretical study on this subject. Furthermore, the formation of the tributylamine carbamate on one of the aluminum center under the experimental reaction conditions was not proven. In a separate study, mainly concerning monometallic salen complexes, Deng’s group explored the feasibility of the bimetallic catalytic mechanism as proposed by North et al. by DFT calculations.111 Based on the proposed mechanism


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(Scheme 9), the molecular fragments leading to the carbonate product should be formed on the same side of the catalyst. However, Deng et al. could not find any stable transition state corresponding to the mechanism in Scheme 9 because of strong steric effects. Their results support a single-site catalytic mechanism for 18/TBAB. More recently, North et al. showed that complex 18 can catalyze the cycloaddition of CO2 to epoxides even in the absence of a nucleophile, albeit at elevated temperature and pressure, and could be recycled at the end of the process.130 Mechanistic investigation led the authors to the discovery of the formation of an unprecedented carbonato-bridged bimetallic Al(III) salen complex in which CO2 is activated by insertion between the two aluminum centers.131 Whereas complex 18/TBAB exhibited high catalytic activity under ambient conditions, the undesirable need for two separate catalytic entities prompted North and co-workers to develop single component catalysts (Scheme 10).132 Several bimetallic salen complexes were synthesized using tertiary amino-functionalized salicylaldehydes as synthons. The amino groups were subsequently quaternarized by reaction with benzyl bromide. The catalytic results obtained using these single component catalysts under ambient conditions were comparable with those obtained with the binary catalytic system 18/TBAB. Complex 19, functionalized with four tetraalkylammonium bromide units, was found as the most effective catalyst for the conversion of terminal epoxides into cyclic carbonates.


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Scheme 10. Bimetallic single component aluminum salen complexes functionalized with tetraalkylammonium bromide moieties. Furthermore, a lower molecular weight analogue of 19 (20, Scheme 10) without tert-butyl substituents and cyclohexyl bridges, showed comparable, albeit slightly lower efficiency than 19 when screened using SO as the substrate. These catalysts represent a prime example of single component molecular catalysts active under ambient conditions. To further explore the potential of such single component catalyst systems, immobilized versions of the bimetallic aluminum salen complexes were prepared by anchoring them via an ammonium group to solid supports and will be discussed later in this review. In a related work, North et al. explored the possibility of using phosphonium salts instead of ammonium salts to prepare bimetallic single component catalysts analogous of 19 (Scheme 11).133 The presence of tert-butyl moieties on the salen scaffold was found to be necessary because of solubility issues, moreover, the catalysts with bromide as a counterion were more effective than those bearing a chloride. Complex 21 displayed a good catalytic performance for the cycloaddition of CO2 to terminal epoxides under ambient conditions. For instance, 74 % SO could be converted


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to SC in 6 h using a catalytic loading of 2.5 mol%. Higher molecular weight complex 22, bearing tert-butyl substituents was slightly more active than 21 as it led to a quantitative conversion of SO in 6 h. The further introduction of cyclohexyl bridges in the salen motif (23, Scheme 11) did not lead to a further improvement of the catalytic performance.

Scheme 11. Bimetallic aluminum complexes with quaternary phosphonium halide functionalities as single component catalysts. Wu and North developed a bimetallic aluminum salphen complex where the two salen scaffolds are connected by the phenyl bridge (24, Scheme 12).134 Preliminary studies with SO as a benchmark substrate showed that the combination of 24 (2.5 mol%) and TBAB (5 mol%) afforded 96 % conversion to SC at 25 °C and 1 bar CO2 under solvent-free condition. Increasing the temperature to 50 °C and the CO2 pressure to 10 bar allowed the authors to use a much lower loading of 24 (0.25 mol%) and TBAB (0.5 mol%) achieving a similarly high degree of conversion as under ambient conditions. When compared to its monometallic counterpart under identical reaction conditions for the carbonation of 1-decene oxide, the latter complex afforded lower


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carbonate yields than bimetallic complex 24. This observation, and the fact that modifying the 24/TBAB ratio from 2:1 to 1:1 did not led to a strong decrease in catalytic activity, was considered by the authors as a hint of a possible intramolecular cooperative effect between the adjacent salen units in complex 24 as observed for 18/TBAB.

Scheme 12. Bis-aluminum salphen complex developed by Wu and North.

A selection of the results obtained in various works by North et al. using bimetallic aluminum salen complexes under ambient conditions is displayed in Table 3. Based on these data, bimetallic aluminum complex 18 (Table 3, Entries 1, 2) appears considerably more active than any monometallic salen complex applied under ambient conditions (Table 1). Interestingly, catalyst 19 is comparably as active as 18/TBAB but with the advantage of being a single component system. The high molecular weight of these complexes represents a potential drawback. For instance, the molecular weight of single component complex 19 (Scheme 10) is about 1600 g/mol. Based on this value, one would need 4 g of 19 for the conversion of 5.8 g (100 mmol) of PO considering a typical loading of 2.5 mol%. To tackle this drawback, the authors have been working at strategies aimed at reducing the cost of preparation of complex 18 and they have prepared lower molecular weight bimetallic systems (vide infra).135 Nevertheless, the preparation of supported analogues of


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18 and 19 appears as the most promising strategy for cost reduction and it is discussed later in the section dedicated to the heterogeneous systems.

Table 3. Comparison of catalytic activity of bimetallic salen complexes for the synthesis of propylene carbonate.

Entry

BSC/Nucleophile

Yield(%),

T

(Epoxide)

Loading (mol%)

TON/TOF (h-1)

Time

1-(PO)

18/TBAB

77

0 °C

2.5/2.5

30.8/10.3

3h

18/TBAB

87

25 °C

2.5/2.5

34.8/11.6

3h

19

50

0 °C

2.5

20/6.7

3h

19

84

25 °C

2.5

33.6/11.2

3h

21

98

0 °C

2.5

39.2/1.6

24 h

21

99

25 °C

2.5

39.6/13.2

3h

24/TBAB

96

25 °C

2.5/5.0

38.4/1.6

24 h

2-(HO)

3-(PO)

4-(ECH)

5-(PO)

6-(BO)

7-(SO)


Ref. 128

128

132

132

133

133

134

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2.2.2. “Non-salen” Bimetallic Complexes Iron is an inexpensive, readily available and ecofriendly metal. The development of ironbased catalysts is of a high interest as an approach towards more sustainable chemical processes.75, 77-78, 136-138

In 2010, Williams et al. reported an air-stable bimetallic iron catalyst based on a

macrocyclic ligand (25, Scheme 13) for the alternate copolymerization of cyclohexene oxide (CHO) and CO2.139 When applied in the absence of a nucleophile at 80 °C under atmospheric pressure, 25 afforded 29 % CHO conversion producing PCHC ((poly)cyclohexene carbonate) and cyclohexene carbonate (CHC) in a 93:7 ratio. The selectivity for PCHC increased with pressure. Interestingly,

when

25

was

used

in

the

presence

of

PPNCl

(PPNCl:

bis(triphenylphosphoranylidene)ammonium chloride), 41 % CHO was converted into CHC with 100 % selectivity. The general applicability of 25/PPNCl was tested with terminal epoxides such as PO and SO under ambient conditions, however, only PO could be efficiently converted to PC at room temperature: using 25/PPNCl (0.5:1 mol%) 91 % PO conversion to PC was observed within 48 h.

Scheme 13. Bimetallic Fe(III) complex by Williams et al.


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North

et

al.

investigated

bimetallic

aluminum

acen

complexes

(acen:

N,N'-

ethylenebis(acetylacetoneiminate)) for the cycloaddition of CO2 to epoxides (Scheme 14).140 With respect to previous work with bimetallic salen complexes such as 18, the use of acen ligands would lead to less expensive complexes with a lower molecular weight by avoiding the use of salicylaldehyde. Complexes 26-28 showed activity for the conversion of epoxides to cyclic carbonates in the presence of TBAB under ambient conditions. The degree of catalytic efficiency among these catalysts followed the trend 28>26>27. Complex 28/TBAB (2.5 mol%) afforded the quantitative conversion of several terminal epoxide in 24 h. It is noteworthy that catalyst 26, prepared from less expensive reagents (pentan-2,4-dione and ethylenediamine), displayed a catalytic perfomance close to that of 28. The functionalization of the organic framework of 26 by the introduction of ester groups or the attempt to prepare a single component catalyst by the introduction of a quaternary ammonium moiety were not successful.

Scheme 14. Bimetallic Al(III) acen complexes. Metal complexes with heteroscorpionate ligands have been extensively studied for several catalytic applications.141 North and Otero evaluated the catalytic potential of aluminum heteroscorpionate complexes for the conversion of CO2 to cyclic carbonates. Thioamide-derived complexes 29 and 30, and imidate-derivatives 31a-31c (Scheme 15) were tested for the cycloaddition of CO2 to epoxides in the presence of TBAB.142 Under ambient conditions, the results obtained from the initial screening using SO as the substrate indicated


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bimetallic complex 29 (5 mol%) as the most active compound affording 69 % SO conversion to SC in 24 h. Thioamide-derived complexes 29 and 30 were comparatively more active than imidate complexes 31a-31c. Analysis of the reaction kinetics for 29/TBAB revealed a first order dependency of the reaction rate on the concentration of TBAB that is at variance with the results obtained for bimetallic aluminum complex 18/TBAB and that is indicative of a lack of cooperativity between the two aluminum centers in the complex. The authors verified the formation of tributylamine during the reaction as observed for 18/TBAB and attributed the absence of cooperativity to the geometric disposition of the aluminum centers on opposite faces of the complex, as observed from the crystal structure of 29. The promising results with 29/TBAB inspired further work on aluminum heteroscorpionates by the preparation of trinuclear aluminum complexes 32 and 33a-33d (Scheme 15).143 When employed under ambient conditions in the presence of TBAB (5 mol%), complexes 32, 33a-33c (5 mol%) afforded moderate to high conversion of SO (73, 77, 52, and 77 %, respectively) in 24 h. Interestingly, complex 33d provided quantitative conversion of SO to SC under the same reaction conditions with a TOF of 0.83 h-1. Using PO as the substrate under ambient pressure at 0 °C, complex 33d afforded PC in 85 % yield with a TOF of 0.71 h-1 which is higher than the TOF (0.5 h-1) obtained using bimetallic complex 29 under 10 bar CO2 pressure.


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Scheme 15. Heteroscorpionate aluminum complexes prepared in several studies by North and Otero for the synthesis of cyclic carbonates.


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As an extension of the work involving multimetallic heteroscorpionate complexes, μ-oxobridged aluminum complex 34 (Scheme 15) was prepared.144 34/TBAB (5 mol%) provided PC in 75 % yield with a TOF of 0.63 h-1 under ambient conditions that is in line with the values observed in previous studies using catalysts 29/TBAB or 33d/TBAB. Since this catalyst contains moisturesensitive aluminum-alkyl groups, the effect of the presence of water on the catalytic efficiency was investigated. Interestingly, it was found that low amounts of water had a beneficial effect on the reaction’s yield, however, when larger amounts of water were added, the catalytic activity dropped. Despite the presence of a μ-oxo aluminum bridge in 34 as in 18, the catalytic activity of 34/TBAB was by far lower than that of 18/TBAB. As for the case of complex 29, the authors ruled out the existence of a cooperative catalytic effect between the two aluminum centers. Further development saw the synthesis of complex 35 (Scheme 15), functionalized with a quaternary ammonium moiety, as a single component catalyst.145 35 was studied for the cycloaddition of CO2 to SO under ambient conditions, however, low conversion rates were observed (38 % SO conversion in 24 h using 5 mol% catalyst) that were attributed to the poor solubility of the catalyst in the reaction medium. When the reaction temperature was raised to 80 °C under atmospheric pressure, highly efficient conversion of internal and terminal epoxides was observed.


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Table 4: Comparison of catalytic activity of bimetallic (non-salen) complexes for the synthesis of cyclic organic carbonates under ambient conditions.

Entry

Bimet. Complex/Nucleophile

Yield (%),

T

(Epoxide)

Loading (mol%)

TON/ TOF (h-1)

Time

1-(PO)

25/PPNCl

91

25 °C

0.5/1

182/4

48 h

26/TBAB

42

0 °C

2.5/2.5

16.8/5.6

3h

26/TBAB

97

25 °C

2.5/2.5

38.8/12.9

3h

29/TBAB

69

25 °C

5/5

13.8/0.6

24 h

33d/TBAB

85

0 °C

5/5

17/0.71

24 h

33d/TBAB

100

25 °C

5/5

20/0.83

24 h

34/TBAB

75

25 °C

5/5

15.1/0.63

24 h

35

38

25 °C

5/5

7.6/0.32

24 h

2-(PO)

3-(HO)

4-(SO)

5-(PO)

6-(SO)

7-(SO)

8-(SO)

Ref. 139

140

140

142

143

143

144

145

The catalytic activity of the bimetallic complexes discussed in this section is displayed in Table 4. The bimetallic Al(III) complex 26 by North et al., used in the presence of TBAB (Table 4,


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Entries 2, 3), displays the highest TOF values under ambient conditions for this class of catalysts. Interestingly, this complex has a catalytic activity comparable to that of its bimetallic Al(III) salen analogues (Table 3) with the advantage of much lower molecular weight and synthesis costs. For the heteroscorpionate aluminum complexes only the most active species for each report are shown in Table 4. These complexes are generally much less active than the bimetallic aluminum complexes in Table 3 or of 26. This difference of activity has been generally attributed to the lack of cooperativity between the aluminum centers due to the geometry of the complexes.

2.3. Coordination Compounds Readily available coordination compounds of rare-earths and early transition metals such as Sc(OTf)3,146 NbCl5,147-148 YCl3,149 ZrCl4,150-151 have been employed as popular catalyst for various transformations due to their high Lewis acidity.152 In reactions that are strictly related to the cycloaddition of CO2 to epoxides, coordination compounds of yttrium and scandium have been reported as efficient catalysts for cycloadditions reactions of various dipolarophiles to epoxides.153154

Indeed, rare earth metal complexes are highly active Lewis acids for the cycloaddition of CO 2

to epoxides under mild conditions.155-157 Besides early transition metal derivatives, coordination compounds of zinc158-159 and of main group metals such as aluminum,160 indium,161 and tin,162 represent useful Lewis acid in catalysis. The clear advantage of this class of compounds, compared to metal-organic catalysts, is that they can be employed off-the-shelf and do not require the multi-step synthesis and purification of sophisticated organic ligands as well as complexation steps. Potential drawbacks related to the absence of an organic framework are the lack of appended functional groups that could be used to engineer the tethering of the complex to a solid support163 and/or for the preparation of single


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component catalysts.133 Unlike salen complexes, that can be used for the enantioselective cycloaddition of CO2 to epoxides in virtue of their chiral scaffold, coordination compounds do not allow for the chiral resolution of racemic epoxides.106 Moreover, coordination compounds such as metal halides are characterized by a high halogen to metal ratio that is undesirable from the environmental and corrosion-prevention standpoints.164 Despite these limitations, metal coordination compounds form inexpensive, highly efficient, low molecular weight catalysts for the conversion of CO2 to carbonates under extremely mild conditions when used in the presence of nucleophiles. Already in 1980 Kisch et al. reported the synthesis of cyclic carbonates from CO2 and epoxides under ambient conditions pioneering a concept that was to become highly popular in years to come “All the catalytic processes known so far for the synthesis of 4-methyl-1,3-dioxolan2-one (propylene carbonate) operate at elevated CO2 pressure (5-200 atm) and elevated temperature (100-200 °C). We now wish to report catalysts that are active at room temperature and normal pressure”.165 However, the most active catalysts identified in this early study (MoCl5/tributyl- or triphenylphosphine, PPh3) were able to provide moderate to good yields of PC only after 7 d of reaction at 20 °C and 1 atm CO2. Subsequently, the same author reported a study using zinc halides in the presence of quaternary ammonium iodide salts.166 The most active combination of salts (ZnCl2/TBAI, TBAI: tetrabutylammonium iodide) allowed the quantitative synthesis of PC at room temperature under 1-1.2 atm CO2 in 24 h (TOF: 20.4 h-1). In a more recent application of zinc salts for the cycloaddition of CO2 to epoxides under ambient conditions, Shi et al. employed zinc halides in the presence of K2CO3 and N,N-disubstituted imidazolium salts.167 The latter serve, at the same time, as pre-catalytic moieties for the formation of N-heterocylic carbenes (NHCs) upon deprotonation by the base and as a source of nucleophilic


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bromide anions for the ring opening of the Zn-coordinated epoxide (Scheme 16). NHCs are able to form a nucleophilic adduct with CO2.35, 67-69 The authors were able to show that the catalytic efficiency of the reaction was not strongly influenced by the kind of zinc halide or base added, however, the nucleophilicity of the counterion of the imidazolium salt played, obviously, a crucial role. Applying halides of other metals (Al, Fe, Mg) led to a much lower catalytic performance. Quantitative conversion of SO to SC was observed in 20 h under ambient conditions using 3 mol% ZnBr2/K2CO3 and 5 mol% of imidazolium bromide 36 (Scheme 16). The zinc loading could be reduced to just 0.5 mol% and the reaction time to 2 h but in the presence of a more-thanstoichiometric amount (200 mol%) of 36. Remarkably, in the presence of 2 equiv. 36, this ternary catalytic system showed the unique ability to quantitatively convert internal epoxides to the corresponding carbonates under ambient conditions.

Scheme 16. Plausible reaction mechanism for the cycloaddition of CO2 to epoxides catalyzed by the ternary system 36/K2CO3 (Base)/ZnBr2.

Shibata et al. reported the synthesis of cyclic carbonates under ambient conditions using a high loading of InBr3 (5 mol%) and PPh3 (10 mol%); a TOF of just 3.3 h-1 was obtained.168 Interestingly, by treating InBr3 and PPh3 with equimolar amounts of propylene oxide, the authors


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were able to isolate the phosphonium salt of the ring-opened epoxide thus supporting a cooperative reaction mechanism by InBr3 and PPh3 (Scheme 17); the epoxide is activated by the metal complex whereas the ring is opened by PPh3. A bromide anion must be displaced from the metal center upon ring opening.

Scheme 17. Phosphonium salt intermediate isolated by Shibata et al. and the plausible mechanism of formation. A μ-oxo-tetranuclear zinc cluster (37, Scheme 18) by Yang et al. was the first polyatomic metal cluster reported to catalyze the cycloaddition of CO2 to epoxides under ambient conditions.169 The authors initially tested trifluoroacetate salts (M(O(C=O)CF3)2) of various metals (Zn, Co, Fe, Mn) for the cycloaddition of CO2 to propylene oxide under ambient conditions (25 °C, 1 atm) in the presence of TBAI as a nucleophile. These highly Lewis acidic salts performed similarly well with the 1H NMR conversion of the epoxide in the 70-80 % range and with zinc being the most active metal. In virtue of its lower Lewis acidity Zn(OAc)2 performed less actively than its trifluoroacetate analogue under the same reaction conditions. Zinc and cobalt were selected for the preparation of tetranuclear μ-oxo trifluoroacetate clusters (M4(O(C=O)CF3)6O). The zincbased cluster (37) was slightly more active than Zn(O(C=O)CF3)2 (83 % conversion in 6 h reaction versus 79 %). The trend of activity of the trifluoroacetate versus the acetate ligand was confirmed as the acetate-based cluster (38) was much less active than 37. The cobalt based tetranuclear cluster (Co4(O(C=O)CF3)6O, 39) was less active than the corresponding mononuclear trifluoroacetate complex (50 % epoxide conversion for 39 versus 72 % for Co(O(C=O)CF3)2).


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The researchers investigated the effect of the presence of impurities in CO2 on the performance of 37 using a 95:5 mixture of CO2 and an additional gas (Ar, O2, Air, CO, NO2, SO2). Although they observed a reduction in the catalytic activity (from 83 % conversion in the presence of pure CO2 to 50-56 % when using gas mixtures), the catalyst performance was substantially the same when the added impurity was Ar or a potentially more reactive pollutant such as NO2 or SO2. This hinted at the absence of a specific effect on the catalyst by these common flue gas pollutants.

Scheme 18. μ-oxo-tetranuclear zinc and cobalt clusters prepared by Mashima et al.

Werner et al. explored the combination of 18-crown-6 ether (18C6) and calcium halides to afford highly active catalytic systems for the cycloaddition of CO2 to a broad range of epoxides.170 The use of calcium as a metal center appears attractive because it is highly abundant in nature and environmentally benign.171 An initial screening using tert-butylglycidyl ether as a benchmark substrate revealed that the combination of CaI2 and 18C6 led to the in situ formation of complex 40 (Scheme 19) as an unprecedented calcium-based catalyst for the coupling of CO2 to epoxides under ambient conditions without the addition of further co-catalytic species. Expectedly, the combination of 18C6 with calcium salts of non-nucleophilic counterions led to catalytically inactive complexes because of the lack of an efficient nucleophilic agent for the ring-opening step (Scheme 1). Complex 40 (5 mol%) could be efficiently applied for the cycloaddition of CO2 to


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several terminal epoxides derived from environmentally benign glycidol and terminal aryl and alkyl epoxides. In nearly all cases, quantitative yields of the desired carbonate products were obtained in 24 h. Disubstituted terminal epoxides, displaying a quaternary carbon at the threemembered ring, could be also efficiently transformed to the target carbonates under the same conditions. To further emphasize the high catalytic potential of 40, the authors could show that challenging internal epoxides could be converted to the corresponding carbonate under very mild conditions (45 °C, 10 bar CO2). Finally, based on the observed cis-diastereoselectivity for the conversion of internal epoxides, the authors deduced that the reaction mechanism would proceed through a SN2 pathway with retention of the cis configuration of the epoxide in the carbonate product. Only in the case of substrates bearing substituents capable of stabilizing cationic intermediates, the formation of amounts of trans-cyclic carbonates was observed, likely, through an SN1 mechanism.

Scheme 19. In situ prepared Calcium-18C6 complex for the cycloaddition of CO2 to epoxides.

D’ Elia et al. renewed the investigation on the catalytic activity of coordination compounds of early transition metals. The authors carried out a systematic screening of groups IV to VI metal halides and oxychloride compounds under mild conditions (50 °C, 5 bar) using DMAP as a nucleophile and PO as the substrate.172 NbCl5 emerged as the most active compound in this series and, when DMAP was replaced by TBAB, a TOF over 200 h-1 could be achieved under the same reaction conditions. Encouraged by these results, the authors tested the NbCl5/DMAP and


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NbCl5/TBAB binary systems under ambient conditions. When using industrially attractive ethylene oxide as the substrate, NbCl5/TBAB displayed a high TOF value (30.3 h-1). Despite being less active, NbCl5/DMAP allowed a study on the reaction mechanism by in situ IR spectroscopy by monitoring the variation of the reaction spectrum in the 1600-1700 cm-1 region.81 The latter region contains the signals relative to the asymmetric ring stretching of the pyridine ring (sensitive to the coordination environment of the aromatic nitrogen atom) and the C=O stretching of the hemicarbonate compound formed upon CO2 activation at 1685 cm-1 (Scheme 20).

Scheme 20. Representative IR stretching bands as identified for DMAP and various reaction intermediates in the cycloaddition of CO2 to PO catalyzed by NbCl5/DMAP. The individuation of the reaction intermediates by IR spectroscopy, along with judicious kinetic studies, allowed the authors to propose a reaction mechanism that involves the participation of two molecules of DMAP to the rate determining step of the cycloaddition reaction (ring closure): the nucleophile is involved in the formation of the crucial hemicarbonate intermediate and, in its free form, attacks the metal center of the hemicarbonate intermediate to facilitate cyclization. This hypothesis was supported by DFT calculations that highlighted an additional intriguing aspect in the mechanism of CO2 activation: it was found that the barrier for CO2 insertion in the Nb-O bond of the ring-opened alkoxide to form the crucial hemicarbonate intermediate


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could be reduced to just 15.9 kcal/mol from ca. 35 kcal/mol when considering a bimetallic insertion mechanism of CO2 activation. This observation could be exploited in a subsequent study to afford an unprecedented cooperative effect between surface immobilized complexes.173 Different kinds of supported niobium complexes (isolated monopodal, isolated bipodal, neighboring monopodal, Scheme 21) were prepared on the surface of dehydroxylated silica by the methodology of surface organometallic chemistry (SOMC).174-178 This was carried out by reacting stoichiometric amounts of the NbCl5·EtO2 precursor with silica dehydroxylated at various temperature and, therefore, displaying different surface densities of the reactive silanol moieties (≡SiOH).

Scheme 21. Main surface species present in the different catalysts reported in reference 173. The isolated monopodal complex was prepared on silica dehydroxylated at 700 °C; the isolated bipodal and neighboring monopodal complexes were prepared on silica dehydroxylated at 200 °C. The different surface complexes (Scheme 21) were employed for the cycloaddition of CO2 to propylene oxide under relatively mild conditions (60 °C and 10 bar). The material presenting isolated surface complexes was found to perform poorly. As expected, the best catalytic activity was found for the material presenting neighboring niobium complexes and was attributed to the ability of the closely associated complexes to cooperate in the step of CO2 insertion and in the subsequent step of cyclization. DFT calculations confirmed a reaction barrier of just 15.9 kcal/mol


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for the formation of the supported hemicarbonate intermediate for the bimetallic pathway. The corresponding barrier was as high as 31.5 kcal/mol for an isolated niobium complex (Scheme 22).

Scheme 22. Different barriers (kcal/mol) for the insertion of CO2 in the niobium alkoxide complex formed upon epoxide ring opening. Monometallic pathway on an isolated niobium complex (Left, CO2 insertion barrier 31.5 kcal/mol) and bimetallic pathway on neighboring monopodal complexes (Right, CO2 insertion barrier 15.9 kcal/mol). Prompted by the excellent activity displayed by niobium halide complexes under ambient conditions, additional studies were carried out using binary catalysts involving niobium coordination compounds. Wilhelm et al. employed a broad selection of imidazolium bromides (bearing different substitution patterns at the nitrogen atoms and at the C2-position) as nucleophiles in the place of quaternary ammonium salts.179 They were able to show that a more nucleophilic bromide anion could be obtained by introducing an alkyl chain at the C2-position of the imidazole scaffold. This substitution had the effect of weakening the interaction between the imidazolium cation and the bromide anion resulting in a more nucleophilic halogen anion.


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Therefore, an enhancement of the catalytic activity of the C2-substituted imidazolium bromides was observed compared to their C2-unfunctionalized analogues. In the search for more active Nb-based catalytic systems, Dutta et al. screened various niobium complexes for the cycloaddition of CO2 to PO under ambient conditions in the presence of various quaternary ammonium salts or N-nucleophiles.180 When TBAB was used as a nucleophile, the screening confirmed NbCl5 as the most active complex within the following order of activity: NbCl5> NbBr5> Nb(OEt)5>> NbOCl3> NbF5> Nb(NMe2)5 based on the yield of PC obtained within four hours of reaction. Nevertheless, Nb(OEt)5/TBAB displayed the highest initial reaction rate among all catalytic pairs being more than twice as fast as NbCl5/TBAB. The authors were able to explain this apparent discrepancy by in situ IR investigation by demonstrating the formation of a linear carbonate by-product in the Nb(OEt)5/TBAB catalyzed reaction according to the mechanism depicted in Scheme 23. Hemicarbonate intermediate I was found to evolve into two possible reaction products: the target propylene carbonate and an acyclic carbonate (IV), as demonstrated by the formation of a product with a C=O stretching at 1747 cm-1 in the IR spectrum of the reaction mixture (Scheme 23). The formation of the latter by-product was unavoidably accompanied by the decomposition of the catalyst thus justifying the observed drop in catalytic activity for Nb(OEt)5/TBAB after the initial reaction turnovers. Concerning the order of activity of the nucleophilic component, TBAB resulted more active than other quaternary ammonium salts such as tetrabutylammonium chloride and TBAI. Nitrogen nucleophiles were generally less active than the quaternary ammonium salts leading to poor yields of the desired carbonate. Interestingly, the use of Nb(OEt)5/DMAP led, exclusively, to the formation of the acyclic carbonate analogous of IV (Scheme 23) as an effect of a sterically hindered cyclization step.


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Scheme 23. Competitive reaction pathways arising from the hemicarbonate intermediate (I) in the Nb(OEt)5/TBAB catalyzed cycloaddition of CO2 to PO. Rare-earth metal halides such as YCl3 have been found to display a catalytic activity comparable or higher than that of NbCl5 when used in the presence of TBAB181 and have been employed for the conversion of CO2 from diluted flows including actual industrial flue gas. 57 This aspect is analyzed later in section 5. A comparison of catalytic activity for the cycloaddition of CO2 to epoxides for the coordination compounds reviewed in this section is presented in Table 5. The highest TOF values (about 20 h-1, Table 5, Entries 3, 9, 12) for the case of PO were reported using ZnCl2/TBAI, NbCl5/TBAB and YCl3/TBAB. In case of EO, D’ Elia et al. reported a TOF of about 30 h-1 (Table 5, Entry 10). Interestingly, the TOF values obtained using these readily available catalytic systems under ambient conditions are higher than for the metal-organic complexes in Tables 1-4 with the exception of some monometallic complexes in Table 2 (Entries 1-3).


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Table 5. Comparison of catalytic activity of metal coordination compounds in the cycloaddition of CO2 to epoxides under ambient conditions.

Entry

CC/Nucleophile

Yield (%),

T

(Epoxide)

Loading (mol%)

TON/ TOF (h-1)

Time

1-(PO)

MoCl5/PPh3,

78,

20 °C

0.7/3.5

76/0.45

168 h

ZnCl2/TBAI,

90,

25 °C

4/16

22.5/6.4

3.5 h

ZnCl2/TBAI

98,

25 °C

0.2/0.8

490/20

24 h

ZnBr2/36/K2CO3

83,

20 °C

3/20/3

27.7/13.8

2h

InBr3/PPh3,

82,

25 °C

5/10

16.4/3.3

5h

37/TBAI,

94,

25 °C

1/3

94/15.7

6h

CaI2/18C6

85

23 °C

5/5

17/0.71

24 h

CaI2/18C6

97

23 °C

5/5

17/0.81

24 h

NbCl5/TBAB,

74,

25 °C

1/2

74/18.5

4h

NbCl5/TBAB,

64,

23 °C

2-(PO)

3-(PO)

4-(SO)

5-(PO)

6-(PO)

7-(PO)

8-(HO)

9-(PO)

10-(EO)


Ref. 165

166

166

167

168

169

170

170

172

172

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11-(PO)

12-(PO)

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0.18/0.35

365/30.3

12 h

Nb(OEt)5/TBAB

43,

25 °C

1/2

43/10.7

4h

YCl3/TBAB

75

25 °C

1/2

75/18.8

4h

180

181

2.4. Organocatalysts Organocatalysts are widely regarded as potentially inexpensive, non-toxic and robust compounds. They are generally insensitive to the exposure to moisture and air. In many cases they can be obtained from readily available and renewable sources such as amino acids,182 peptides,183184

alkaloids,185 or sugars.186 Their application appears as an advantageous and safe strategy for the

metal-free preparation of commodity chemicals.187-189 These features have led to an increasing attention for the application of organocatalysts for the conversion of CO2 to value-added products, including the cycloaddition of CO2 to epoxides.48, 50 The organocatalysts that have been applied for this purpose include nitrogen nucleophiles,190-192 ionic liquids,193-194 ammonium and phosphonium salts,195 N-heterocyclic carbenes (NHCs)69, 196 and hydrogen bond donors (HBDs). The latter family of compounds displays the ability to activate the epoxide substrate for nucleophilic ring opening by establishing a network of hydrogen bonds (Scheme 1, Cycle 2). Accordingly, they generally require the presence of a nucleophile or of a “built-in” nucleophilic moiety for the opening of the epoxide ring; a typical example being represented by single and dual component catalytic systems based on phenols and ammonium or phosphonium salts.197-202 Other active HBDs include a large variety of hydroxy compounds203 and their derivatives such as alcohols functionalized with ammonium204-205 or phosphonium moieties,206 fluorinated alcohols,207-208 hydroxyl-functionalized N-heterocycles,209-211 silanediols212 and boronic acids.213


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Azaphosphatranes,80 carboxylic acids214 and ureas have also been employed as hydrogen bond donors.215 Another relevant class of organocatalysts is represented by strong nucleophiles able to activate CO2 by the formation of a zwitterionic adduct (Scheme 1, Cycle 1). The use of organocatalysts for the cycloaddition of CO2 to epoxides has been analysed in various reviews.48, 50, 53 In agreement with the scope of this review, only a restricted selection of organocatalytic systems that have been reported to show catalytic activity under ambient conditions will be discussed. Silanediols are known to provide electrophilic activation of substrates via hydrogen bonds216 and can be applied for studies on molecular recognition.217 Mattson et al. envisaged that epoxide activation via multiple hydrogen-bonding with silanediols could facilitate ring-opening by a nucleophile.212 As expected, when different silanediols (41-43, Scheme 24) were tested using TBAI as a nucleophile for the cycloaddition of CO2 to SO, catalyst 41 provided the highest yield of SC (93 % in 18 h) under ambient conditions using a high catalyst loading (10 mol% of both components). Under optimized reaction conditions, good to excellent carbonate yields were observed using several terminal epoxides as the substrates. The molecular recognition of the iodide anion and of the epoxide by the diol functionality of 41 could be demonstrated via 1H NMR by monitoring the upfield shift of the signal relative to the hydroxyl protons of 41 in the presence of increasing concentrations of TBAI or of epoxide.


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Scheme 24. Silanediols as hydrogen-bond donors for the metal-free cycloaddition of CO2 to epoxides.

A more recent example of the application of a multiple hydrogen bond donor for the cycloaddition of CO2 to epoxides under ambient conditions has been presented by D’ Elia et al. with a study on ascorbic acid.218 Besides catalytic activity, the authors sought to identify a HBD that could be non-toxic, readily available, inexpensive and renewably sourced. Therefore, they screened several commercially available non-toxic compounds bearing different hydrogen bonding moieties (hydroxyl, carboxyl, enediol) for the cycloaddition of CO2 to ECH under ambient conditions. The catalytic system ascorbic acid/TBAI (2:4 mol%) appeared as the most active pair providing the desired carbonate product with 70 % conversion after 23 h. This value increased to 94 % by raising the temperature to just 40 °C under atmospheric CO2 pressure. Interestingly, this study represented the first truly catalytic application of ascorbic acid instead of its common use as a stoichiometric reagent in catalysis.219 Several terminal epoxides could be converted to the corresponding carbonates using this catalytic system under atmospheric pressure albeit more sterically hindered substrates generally required a mild reaction temperature of 60 °C. Moreover, the catalytic system was highly resilient to the presence of water in the reaction environment


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showing only a slight drop in catalytic performance after the addition of 10 mol% water. The authors performed a mechanistic investigation on this catalytic system by a DFT approach. They could show that different hydroxyl moieties within the ascorbic acid scaffold take part to the activation of the substrate and to the stabilization of the transition states. In particular, a cooperative mechanism by the hydroxyl functionalities in the enediol and ethyldiol moieties was observed in lowering the energy barrier for the step of CO2 insertion thus highlighting ascorbic acid as a bifunctional HBD for the activation of CO2 (Figure 4). This observation was corroborated by a study on an acetal protected analogue of ascorbic acid (APAA): in the absence of free ethyldiol hydroxyls, lower catalytic activity and higher reaction barriers were respectively observed and calculated for the latter compound (See Figure 4 for the transition state of CO2 activation for ascorbic acid and APAA).

Figure 4. Calculated transition states for the step of CO2 activation (TS-CO2) in the cycloaddition of CO2 to PO catalyzed by ascorbic acid (Left) and APAA (Right). In the absence of the stabilizing effect by the ethyldiol moiety, the barrier for CO2 activation for APAA is 5.5 kcal/mol higher in energy. Selected distance are provided in Å. Reproduced with permission from reference 218: copyrights: American Chemical Society. Endo et al. reported the use of quaternary phosphonium salts in the presence of alcohols as catalysts for the cycloaddition of CO2 to epoxides under ambient conditions.220 The authors observed that when PPh3·HI (5 mol%) was used for the conversion of phenyl glycidyl ether under


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ambient conditions in a solvent-free reaction the carbonate product would be formed in just 43 % yield (TOF: 0.36 h-1). However, the authors discovered that the use of protic solvents such as alcohols would lead to a strong increase of the reaction yield to nearly quantitative levels. The role of the alcohol solvent (ca. 2 equiv. relative to the substrate) was clarified by 1H NMR experiments on diluted mixtures of alcohol and epoxide highlighting substrate activation by the shift of the signal relative to the hydroxyl proton. The authors carried out a systematic optimization of the catalysts that led to the identification of PPh3·MeI as a slightly more active system than PPh3·HI. Furthermore, the authors discovered that the activity of the catalytic system was affected by the structure of the alcohol solvent employed (with 1-methoxy-2-propanol resulting as the most active), an observation that they attributed to the different degree of solvation of the iodide anion. The optimized catalytic system could afford the cycloaddition of CO2 to several epoxides in the optimized alcohol solvent under ambient conditions in quantitative yields within 24-48 h. Since CO2 may behave as an electrophile, zwitterionic CO2 adducts can be obtained from strong nucleophiles (Scheme 1, A-1).69 This approach was adopted by Zhou et al. using preformed phosphorous ylides to generate adducts 44-49 (Scheme 25).70 These compounds were initially tested as metal-free catalysts for the conversion of CO2 to cyclic carbonates at 100 °C in dichloromethane under 20 bar CO2. The authors found out that the ylides with electron-donating alkyl groups (46-48) would be, in general, more efficient catalysts. Compounds 44 and 45 displayed no or very low catalytic activity under identical conditions. Although catalysts 46-48 displayed similar activity in the initial screening, 47 (5 mol%) was selected for activity studies under ambient conditions because of its higher solubility in PO: 90 % conversion of PO to PC was observed with this catalyst within 6 h without requiring any solvent or additional nucleophiles.


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When enantiomerically pure terminal epoxides were used as substrates, 47 afforded the corresponding optically active cyclic carbonates with retention of configuration.

Scheme 25. CO2 adducts 44-49 obtained from phosphorous ylides and CO2 and proposed mechanism of formation of cyclic carbonates using these compounds as catalysts. Kinetic analysis of the reaction mixture by in situ IR spectroscopy provided interesting mechanistic information: no carbonate product could be formed by the reaction of 47 with the epoxide in the absence of CO2 because of its high stability and, likely, also because of the steric hindrance at the phosphorous atom. However, carbonate formation started when free CO2 was added to the reaction. Furthermore, a reaction order of about two was found experimentally for 47, implying the participation of two molecules of this adduct to the rate determining step of the process. The authors proposed the formation of a new 47-CO2 adduct upon introduction of CO2 and a reaction mechanism involving two molecules of the latter intermediate for the activation of the epoxide (Scheme 25). Hydrohalogenide salts of DBU were used by Endo et al. as catalysts for the cycloaddition of CO2 to epoxides under ambient conditions.221 In particular, 5 mol% DBU·HI could catalyze the conversion of phenyl glycidyl ether to the corresponding carbonate in 24 h in 95 % yield. The initial screening employed 2-methyltetrahydrofuran as a solvent. The authors demonstrated the


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importance of the bicyclic structure of the amidine base by showing that monocyclic and acyclic amidines formed less active catalysts with HI than DBU or its DBN analogue (1,5Diazabicyclo(4.3.0)non-5-en). The activity of the hydrohalogenide salts declined in the order HI>HBr>HCl, an observation that the authors attributed to the better leaving group activity of the iodide anion within the cyclization step of the reaction mechanism (See Scheme 1). The absence of shift of the 1H NMR signals of the epoxide in the presence of the catalytic system led the authors to exclude its activation by the Brønsted acidic moiety; the amidinium salts acting merely as a source of nucleophilic iodide anions. Interestingly, the authors did not consider the mechanistic pathway where CO2 is activated by free DBU to form a carbamate (See the following example for additional discussion). Finally, it was shown that DBU·HI could catalyze the cycloaddition of CO2 to terminal epoxides under ambient conditions in solvent-free reactions: ECH was converted to the corresponding carbonate in 95 % yield under ambient conditions in 24 h using 5 mol% of amidinium salt. By taking into account that strong organic bases can react with electrophilic CO2 to form carbamates222 and with alkyl halides to form zwitterionic compounds, Hirose and co-workers have used a combination of DBU and benzyl bromide (BnBr) as a binary catalytic system for the cycloaddition of CO2 to epoxides under mild or ambient reaction conditions.192 When evaluated for the carbonation of ECH at 65 °C under atmospheric CO2 pressure, DBU/BnBr was found to form an efficient catalyst affording ECH carbonate in 95 % isolated yield. Other bases such as DABCO (1,4-Diazabicyclo[2.2.2]octane), pyridine, triethylamine, DMAP formed less efficient catalysts with BnBr under identical conditions. When the reaction temperature was lowered to 25 °C, the yield of ECH carbonate dropped to 54 %. The proposed mechanism involves the in situ formation of an amidinium salt, [Bn-DBU+Brˉ]. The bromide anion can act as a nucleophile for


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the ring-opening of the epoxide to provide an oxyanion species. The latter can eventually attack a carbamate formed by the reaction of CO2 and a free amidine base to deliver the final cyclic carbonate product (Scheme 26).

Scheme 26. Proposed reaction mechanism of epoxide and CO2 activation by the organocatalytic system DBU/BnBr. The same group reported 2-pyridinemethanol/TBAI as an organocatalytic system active under ambient conditions.210 This catalytic system was able to afford 92 % conversion of ECH and 86 % conversion of PO into the corresponding cyclic carbonates under ambient conditions using a relatively high loading of the binary system (8 mol%) in 20 h of reaction. The pendant hydroxyl group of 2-pyridinemethanol was proposed to activate the epoxide for the nucleophilic ring opening by TBAI by formation of a H-bond. This mechanistic aspect could be demonstrated by 1

H NMR studies by monitoring the shift of the signal of the hydroxyl proton in the presence of the

epoxide. The pyridine nitrogen was also proposed to take part to the reaction by providing activated CO2 via formation of a carbamate. However, the formation of this intermediate in solution under ambient conditions was not experimentally proven. The performance of the restricted selection of organocatalytic systems reported to be active under ambient conditions is displayed in Table 6.


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Table 6. Comparison of catalytic activity of different organocatalysts for the synthesis of propylene carbonate

Entry

Organocatalyst Loading

Yield (%),

T

(Epoxide)

(mol%)

TON/ TOF (h-1)

Time

1-(SO)

41/TBAI

93

23 °C

10/10

9.3/0.52

18 h

Ascorbic acid/TBAI

64

25 °C

4/8

16/0.7

23 h

Ascorbic acid/TBAI

70

25 °C

2/4

35/1.5

23 h

47

90

25 °C

5

18/3

6h

DBU/HI

95

25 °C

5/5

19/0.79

6h

DBU/BnBr

54

25 °C

5/5

10.8/0.45

24 h

2-pyridinemethanol/TBAI

86

25 °C

8/8

10.7/0.53

20 h

2-pyridinemethanol /TBAI

97

25 °C

8/8

12.1/0.6

20 h

2-(PO)

3-(ECH)

4-(PO)

5-(ECH)

6-(ECH)

7-(PO)

8-(ECH)

Ref. 212

218

218

70

221

192

210

210

Analysis of the TOF values suggests that the degree of activity achieved, thus far, using dual or single component organocatalysts under ambient conditions is still far from that obtained by the most active metal-based systems (See Tables 2, 5). With the exception of the best TOF value of


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3 h-1 obtained by Zhou et al. using phosphorous ylide-CO2 adduct 47 (Table 6, Entry 4),70 and of ascorbic acid (Table 6, Entry 3), the catalytic systems displayed afforded less than a turnover per hour. This observation is, in part, a consequence of the lower intrinsic catalytic activity of organocatalysts when compared to metal based system.223 Furthermore, it should be considered that the vast majority of the studies dedicated to the application of organocatalysts for the cycloaddition of CO2 to epoxides is relatively recent48 when compared to metal-based systems and there might still be plenty of space for further improvement. Indeed, this view is supported by the recent development of highly active organocatalytic systems for the cycloaddition of CO2 to epoxides under mild conditions.224 Given the potential benefits in terms of costs, sustainability and availability of organocatalytic upon metal-based systems, the development of highly active organocatalysts for the title cycloaddition reaction under ambient conditions remains highly desirable.50 3. HETEROGENEOUS CATALYSTS Heterogeneous catalysis plays a key role in transformations of industrial interest,225-228 including processes that use CO2 as a feedstock.229-232 The application of heterogeneous catalysts is advantageous because of potential recyclability and facile separation and recovery from the reaction products. Most heterogeneous catalytic systems for the cycloaddition of CO2 to epoxides reported in the past have been applied under harsh conditions of pressure and temperature due to their intrinsically lower catalytic activity when compared to their homogeneous counterparts.233 Moreover, the majority of the literature reported heterogeneous systems is represented by heterogeneous Lewis acids used in the presence of large amounts homogeneous nucleophiles such as quaternary ammonium salts. This drawback is due to the difficulty to achieve an efficient coimmobilization of both catalytic components in a way that preserves their ability to act in a


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cooperative fashion. In recent years, some efficient heterogeneous or partially heterogeneous systems for the cycloaddition of CO2 to epoxides under ambient conditions have emerged and are discussed in the following sections.

3.1. Immobilized Bimetallic Al(III) salen complexes In the homogeneous phase, the combination of bimetallic aluminum complexes and TBAB and their single component analogues have been shown to form active catalytic systems for the cycloaddition of CO2 to epoxides under ambient conditions (See Section 2.2.1 and Table 3).128 The development of heterogeneous versions of these catalysts, that could be readily recovered from the reaction mixture and reused, appears as a necessary strategy given their high molecular weight and the relatively high synthetic costs.135 North et al. prepared immobilized versions (50, 51, Scheme 27) of single component bimetallic Al-salen complex 19 (Scheme 10) 132 by anchoring the salen scaffold to the Merrifield resin via a pendant ammonium moiety.


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Scheme 27. Single component bimetallic aluminum salen complexes immobilized on different supports for the heterogeneous catalytic CO2 fixation into cyclic carbonates under ambient conditions. Complex 50 (2.5 mol%), displaying a single quaternary ammonium bromide moiety with respect to homogenous catalyst 19, afforded 100 % conversion of SO to SC under ambient


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conditions in 24 h (TOF: 1.67 h-1) with fairly successful recyclability affording 94 % and 70 % SO conversion in two successive runs. Single component catalyst 50 represented the first heterogeneous catalyst to show catalytic activity under ambient conditions. Catalyst 51, with four quaternary ammonium moieties, afforded satisfactory conversion of SO to SC with yields of 79, 71, 67 and 64 % in four consecutive runs, however, no improvement in performance with respect to 50, arising from the presence of the additional quaternary ammonium moieties on the salen scaffold, was observed. These heterogeneous catalysts required the addition of propylene carbonate as a solvent for swelling the resin beads prior to use. The promising results obtained using Merrifield resin-supported complexes prompted North’s group to develop several single component immobilized bimetallic catalysts on amorphous silica, MCM-41, aluminium pillared clay and sol-gel supports (52-56, Scheme 27).55 When evaluated under ambient conditions using propylene carbonate as a solvent, silica-supported complex 55 provided the highest conversion (78 %) of SO to SC in 24 h whereas 52 afforded 69 % conversion of SO to SC. The SC yield afforded by 52 increased to 86 % (TOF: 1.43 h-1) in a solventless reaction. The reusability of complex 52 was investigated using PO as model substrate under atmospheric CO2 pressure. The authors observed a progressive loss of catalytic activity after eleven consecutive reactions using 52. This loss of activity was explained by the dequaternization of the ammonium moieties via a reverse Menschutkin reaction. The authors were able to show that 52 could be fully reactivated by restoring the quaternary ammonium group by the addition of benzyl bromide. Upon reactivation, catalyst 52 could be used for 32 consecutive reactions thus increasing the overall catalyst lifetime and making the whole process more attractive from an industrial standpoint. Furthermore, 52 was found to be stable up to 170 °C. The robustness, reusability and single component nature of this class of catalysts inspired their application for the


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conversion of CO2 to cyclic carbonates in flow reactors using simulated flue gas as an impure source of CO2. These aspects are discussed later in this review (Section 4). 3.2. Porous Organic Polymers Porous organic polymers (POPs) of various morphologies have emerged in recent years as valuable materials displaying unique properties in gas separation,234 photovoltaic applications235 and catalysis in virtue of their tunable, highly stable structure.236 These compounds have been praised for high wettability and active site accessibility by the reagents. 237 Recently, they have emerged as a class of excellent heterogeneous catalysts for the coupling of epoxides with CO 2 by showing the capability to provide high TOF values under mild238 and ambient conditions. In 2013, Deng et al. reported partially heterogeneous catalytic systems based on aluminumand cobalt-salen conjugated microporous polymers for CO2 capture and conversion to carbonates at ambient temperature and pressure (Scheme 28).239 These compounds were prepared via Sonogashira coupling by the combination of a pre-formed salen complex and a polytopic ligand able to react with three different salen units in order to generate a tridimensional structure. Both polymers, Co-CMP (57) and Al-CMP (58) showed good and reversible CO2 adsorption properties despite comparably low surface areas. This observation (although the experimental conditions for gas adsorption and CO2/epoxide cycloaddition are very dissimilar) was considered by the authors as a crucial factor for achieving high catalytic activity in the cycloaddition reaction of CO 2 to PO under ambient conditions. In combination with TBAB as a homogeneous nucleophile, Co-CMP and Al-CMP afforded PC in isolated yields of 81.5 % and 78.2 %. Co-CMP could be reused at least 11 times under ambient conditions until a slight loss of activity became evident that was attributed to Co leaching.


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Scheme 28. Al(III) and Co(III)-functionalized conjugated microporous polymers for CO2 capture and conversion under ambient conditions. With the goal to explore the catalytic potential of this class of porous organic polymers in detail, Eddaoudi et al. prepared Co-, and Cr-based porous organic polymers 59 and 60 (Scheme 29) by a similar approach as used by Deng et al. for the preparation of 57 and 58, but without tbutyl groups on the aromatic rings of the salen scaffold.109 59 and 60 had a lower surface area (370 m2 g-1 and 732 m2 g-1 respectively) than 57 (965 m2 g-1). Both 59 and 60 were tested as heterogeneous Lewis acids for the cycloaddition reaction of CO2 to PO in the presence of TBAB. These porous polymers showed excellent yields of PC and optimal recyclability at relatively high (75-100 °C) temperature and CO2 pressure (20 bar). However, under ambient conditions, the conversion was found to be just around 20 % for both compounds in 48 h. When compared with porous polymers 61-63 by Chun et al. (Scheme 29) with a lower surface area,240 both 59 and 60 displayed a slightly lower catalytic activity. Overall, these results indicate that both the substitution pattern at the salen scaffold and the polymer surface area may have a strong influence on catalytic activity by respectively influencing the Lewis acidity of the metal center and its accessibility to the reaction substrate.


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Scheme 29. Co(III), Cr(III) and Al(III) salen-based porous organic polymers. As discussed in section 2.1.2, metalloporphyrins show excellent catalytic activity for cyclic carbonate synthesis in the homogeneous phase (See also Table 2). Dai et al. synthesized TPPbased porous organic polymers (POP-TPPs), Co/POP-TPP, Zn/POP-TPP and Mg/POP-TPP (6466, Scheme 30) by radical polymerization of vinyl-functionalized TPP units followed by metalation and evaluated their heterogeneous catalytic activity for the cycloaddition of CO2 to ECH under ambient conditions.241 The three polymeric compounds behaved comparably well and afforded high yields of ECH carbonate in the presence of TBAB as a homogeneous nucleophile. Co/POP-TPP (64) could be reused 18 times without any significant loss of catalytic activity. At 29 °C, under atmospheric pressure of CO2, various substrates were converted to the corresponding cyclic carbonates with excellent selectivity. Under these conditions, 95.8 % PO conversion was reported using a very low metal loading (1.6 mg of Co per 12.5 mol of PO). Due to the proposed contribution of porosity in concentrating CO2 in proximity of the catalytically active center,


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Co/POP-TPP generally provided higher conversion and product selectivity than its homogeneous counterpart Co-TPP.

Scheme 30. Metalated TPP-based porous organic polymers (POP-TPPs): Co(II)/POP-TPP, Zn(II)/POP-TPP, and Mg(II)/POP-TPP as heterogeneous Lewis acidic components for the conversion of CO2 to cyclic carbonates. Wang et al. disclosed a further example of TPP-based microporous organic polymers for CO2 capture and conversion under ambient conditions (HUST-1-Co, 67, Scheme 31).242 67, prepared by Friedel-Crafts alkylation of the TPP scaffold with dichloromethane, displayed abundant ultra-micropores (with a size of about 0.7 nm), a high surface area (1360 m2 g-1) and a CO2 uptake capacity of 21.39 wt% at 0 °C and 13.17 wt% at 25 °C. Initial screening, using the corresponding homogeneous Co-porphyrin as a benchmark, revealed 67/TBAB as the most active catalytic system when using PO as the substrate (TOF: 103.4 h-1). As for the case of 64, the authors attributed this observation to the capability of the porous matrix to concentrate CO2 in proximity of the catalytically active centers. When employed for the carbonation of various epoxides, 67/TBAB afforded nearly quantitative yields of the corresponding cyclic carbonates under ambient conditions in a relatively long reaction time (30-48 h). The catalyst showed excellent recyclability:


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under ambient conditions: HUST-1-Co could be used for 15 cycles and no loss of activity was noted.

Scheme 31. Synthesis of metalloporphyrins-based microporous organic polymer HUST-1-Co for CO2 capture and conversion. Valuable additions to the study of TPP-based porous organic polymers for the cycloaddition of CO2 to epoxides was reported by Ji, Luo and co-workers. They initially developed TPP-based hypercrosslinked polymers (M-HCPs) containing Al, Co, Fe, and Mn centers (Scheme 32).243 As a result of a preliminary catalytic screening, Al-HCP (68) was found to form a better catalytic system with TBAB than Co-HCP, Fe-HCP and Mn-HCP. 68/TBAB could catalyze the cycloaddition of CO2 to PO under ambient conditions with an isolated PC yield of 99 % in 5 h using 0.25 mol% 68 (relative to the aluminum loading) in the presence of 2 mol% TBAB.


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Scheme 32. Synthesis of aluminum-based hypercrosslinked polymer (68) and of ionic microporous organic polymers (69, 70). 68 could be recovered by filtration or centrifugation and almost no decrease in catalytic activity was observed through 10 catalytic cycles using 68/TBAB. Other substrates such as SO, ECH etc. were successfully converted to the corresponding cyclic carbonates by using the 68/TBAB catalytic system at 40 °C and 10 bar CO2. The following contribution by the same group involved single component aluminum based ionic microporous organic polymers prepared by the nickel-catalyzed Yamamoto-Ullmann coupling of bromoaryl compounds (Al-iPOP 69, 70, Scheme 32).244 These catalysts were found to exhibit a catalytic activity comparable to that of 68/TBAB despite a much lower surface area (52 m2 g-1 for 69 and 86 m2 g-1 for 70) and CO2 uptake capability (5.8 wt% for 69 and 6.6 wt% for 70) that were attributed to the presence of the polar


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ionic liquid moieties within the pores of the organic framework. More importantly, the polymers were found to swell in the reaction substrate and in the carbonate product to form an organogel. The authors emphasized that this behavior could play an important role in creating a suitable microenvironment for CO2 conversion. Al-iPOP-2 (70) was a slightly better catalyst than its AliPOP-1 (69) homologue (the authors attributed this observation to the conjugated structure of the former) and afforded PC in 99 % isolated yield under ambient conditions with a TOF of 124 h-1. The TOF value was as high as 350 h-1 for conversion of PO below 40 %. In a comparable approach, Wang et al. prepared an additional example of TPP-based porous organic polymers as single component catalysts for the cycloaddition of CO2 to epoxides (71). The porous polymer was formed via solvothermal synthesis at 200 °C initiated by AIBN by combining a vinyl functionalized triphenylphosphonium bromide-based ionic liquid with a magnesium porphyrin scaffold functionalized with four vinyl moieties.245 This method had been previously adopted by the same group for the preparation of active catalysts for the cycloaddition of CO 2 to epoxides.246 When 71 was applied in the cycloaddition of CO2 to PO at 140 °C and 30 bar CO2 pressure the authors observed a TOF of 15600 h-1. This value represents the state-of-the art TOF obtained for a heterogeneous catalyst in the title reaction. The catalyst could be reused for five consecutive experiments. The authors studied the catalytic performance of 71 also for the cycloaddition of CO2 to PO under ambient conditions reporting a TOF of 50 h-1, however, the yield of PC under these conditions was just 12 % in 48 h of reaction. Wang, Zhou et al. reported the synthesis of hydroxyl-functionalized mesoporous poly-ionic liquids (72) by free radical polymerization of divinylbenzene (DVB) and of an epoxy-containing imidazolium ionic liquid (Glycidyl-3-vinylimidazolium bromide, [GVIM]Br) followed by epoxide ring-opening in hot water (Scheme 33).247 The vicinal diol moiety was designed to act as an


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organocatalytic hydrogen bond donor as discussed in Section 2.4, whereas, the bromide anion of the ionic liquid, should act as the nucleophile for the ring-opening of the substrate. When exploring the potential of 72 for the cycloaddition of CO2 to SO under mild conditions (70 °C, 4 bar), the authors were able to demonstrate the importance of the diol moiety and of the porous nature of the polymer through several control experiments. Prompted by the positive results obtained under mild conditions, the authors investigated the activity of 72 under ambient conditions using several internal epoxides. Interestingly, despite the presence of bromide anions in the structure of the polymer, the reaction under ambient conditions required the addition of a large amount (8 mol%) of TBAI. Under these conditions, ECH resulted as the most reactive substrate requiring 18 h for nearly quantitative conversion. Other terminal epoxides required long reaction times (48-120 h). A dual role of the vicinal hydroxyls was demonstrated by DFT calculations; besides acting as hydrogen bond donors, their presence led to a decrease of the positive charge of the imidazolium ring thus delivering a more nucleophilic bromide anion.

Scheme 33. Synthesis of hydroxyl-functionalized porous organic polymer 72 The catalytic performance of the porous organic polymers reviewed in this section for the cycloaddition of CO2 to epoxides under ambient conditions is displayed in Table 7. To notice, heterogeneous TPP-based POPs afford higher TOF values than any compound discussed in this review (Table 7, Entries 3-5). A TOF value as high as nearly 200 h-1 has been reported for 68/TBAB (Table 7, Entry 4). The excellent performance of these catalysts is likely to attribute to


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the contribution of the high intrinsic catalytic activity of metalated TPPs120 and to the judicious design of microporous structures able to capture CO2 and to concentrate it in proximity of the catalytically active centers. Furthermore, TPP-POPs can be designed as single component catalysts such as 70 (Table 7, Entry 5) representing a highly active and truly heterogeneous system (the addition of soluble nucleophiles is not required) for the transformation of CO2 under ambient conditions. Table 7. Catalytic activity of porous organic polymers for the synthesis of cyclic carbonates under ambient conditions

Entry

POP/Nucleophile,

Yield (%),

T

(Epoxide)

Loading (mol%)

TON/ TOF (h-1)

Time

1-(PO)

57/TBAB

81.5

25 °C

0.488/7.2

167/3.47

48 h

64/TBAB

95.8

29 °C

0.22/7.2

435.5/18.1

24 h

67/TBAB

94.6

25 °C

0.12/7.2

3101/103.4

30 h

68/TBAB

99

25 °C

0.25/2

992/198.4

5h

70

99

25 °C

0.1

992/124

8h

72/TBAI

91

25 °C

1.9/8

47.9/2.7

18 h

2-(PO)

3-(PO)

4-(PO)

5-(PO)

6-(ECH)


Ref. 239

241

242

243

244

247

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3.3. Metal-Organic Frameworks Metal-organic frameworks (MOFs), being the combination of metal ions or clusters and interconnecting organic ligands,248 are regarded as promising porous materials for application in catalysis249-250 including CO2 conversion.251-252 Their unique features include tunable structures that can be easily functionalized, large surface area and well-ordered porosity that play a crucial role in granting high CO2 uptake and its efficient conversion to useful chemicals.253 In this section, the MOF-based catalytic systems used for the cycloaddition of CO2 to epoxides under ambient conditions are discussed with a focus on their catalytic performance but not on their preparative techniques and morphological aspects. Given the relatively limited number of reports on MOFbased systems for the synthesis of cyclic carbonates strictly under ambient conditions, some examples will be discussed for which the cycloaddition reaction is carried out at atmospheric pressure and at moderate temperatures (T ≤70 °C) or at ambient temperature and CO2 pressure below 10 bar. In 2009, Song et al. reported that MOF-5, prepared from Zn4O and BDC (BDC = benzene1,4-dicarboxylate) could provide SC in 92 % yield when used in combination with TBAB as a nucleophile at 50 °C under ambient CO2 pressure.254 Balzhinimaev and co-workers prepared CrMIL-101 and evaluated its catalytic performance at ambient temperature and 8 bar CO2 in the presence of TBAB using PO as substrate.255 In comparative experiments including several MOFs, Cr-MIL-101 and Fe-MIL-101 provided PC in 82 % and 95 % yield respectively, whereas, MOF5 and HKUST-1 afforded moderate to very poor yields (67 % and 3 % respectively) of PC under identical reaction conditions. Recycling studies of Cr-MIL-101 revealed that the catalyst’s surface area decreased dramatically from 3270 m2 g-1 to 860 m2 g-1 after the second catalytic cycle and, consequently, the carbonate yield decreased from 95 % to 63 %.


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Scheme 34. Chemical diagrams of MOFs MMPF-9 and MMCF-2 as active Lewis acids for the cycloaddition of CO2 to epoxides under ambient conditions. Ma et al. reported two copper based MOFs, MMPF-9256 and MMCF-2257 (Scheme 34) for the catalytic fixation of CO2 with epoxides. Both systems worked as efficient catalysts for the cycloaddition of CO2 to PO under ambient conditions in the presence of a high loading of TBAB (7.2 mol%) and were found more active than other MOFs such as MOF-505 and HKUST-1. MMPF-9 exhibited a larger surface area (850 m2 g-1) than MMCF-2 (450 m2 g-1), nevertheless, MMCF-2 was slightly more active than MMPF-9. Indeed, under identical conditions, PC was obtained in 95.4 % yield using MMCF-2/TBAB as a catalytic system whereas, for MMPF9/TBAB, a PC yield of 87.4 % was observed. MMPF-9 could be successfully recycled.


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Lanthanide-based MOFs have attracted increasing the attention from the research community due to their unique properties for CO2 capture and as catalysts.258 Zhao and co-workers reported

an

europium-based

MOF,

([Eu(BTB)(phen)]·4.5DMF·2H2O;

BTB:

1,3,5-

benzenetribenzoate; phen: 1,10-phenanthroline).259 In combination with TBAB, this Eu-MOF could efficiently carry out the fixation of CO2 via cycloaddition to epoxides under atmospheric pressure. PO and SO could be efficiently converted to their carbonates in nearly quantitative yields at 70 °C. Based on the dissociation enthalpies of the Hf-O and Zr-O bonds, Beyzavi et al. predicted a higher oxophilicity for Hf and envisaged that Hf-based MOFs could act as stronger Lewis acids for the activation of epoxides. Therefore, they prepared Hf-NU-1000 as the Hf-analogue of ZrNU-1000 and evaluated its catalytic activity for the cycloaddition of CO2 to SO in the presence of TBAB.260 As expected, Hf-NU-1000 showed higher catalytic activity than Zr-NU-1000 under ambient conditions affording SC in quantitative yield whereas Zr-NU-1000 provided just 46 % conversion under identical conditions. The heterogeneous character of the reaction was confirmed by removing the catalyst and allowing the reaction to proceed in its absence and by ICP analysis of the product confirming the lack of Hf leaching from the MOF. Hf-NU-1000 could be reused in five consecutive reactions without any significant loss of activity. Bhaumik et al. prepared a strongly paramagnetic cerium-based MOF (Ce2NDC3) using amino tetrazoles and naphthalene dicarboxylic acid as organic ligands.261 Despite the relatively low surface area observed (112 m2/g), Ce2NDC3 displayed high catalytic activity for the conversion of terminal epoxides such as SO and ECH to the corresponding carbonates under ambient conditions in the presence of TBAB (1.8 mol%). ECH carbonate could be prepared in 92 % yield using


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0.24 mol% cerium in 8 h with a TON of 372 and a TOF of 46.5 h-1. To note, these results were obtained using a large excess of dichloromethane as a solvent.

Table 8. Comparison of catalytic activity of selected MOFs for the synthesis of cyclic carbonates under mild and ambient conditions.

Entry

MOF/Nucleophile,

Yield (%),

T

(Epoxide)

Loading(mol%), time (h)

(TON/ TOF (h-1))

P(CO2)

1-(PO)

Cr-MIL-101/TBAB

82

25 °C

1.1/3.3; 48

73.8/1.5

8 bar

Hf-NU-1000/TBAB

100

r.t.

4/10; 26

25/0.96

1 bar

MMCF-2/TBAB

95

r.t.

0.125/7.2; 48

763/16

1 bar

MMPF-9/TBAB

87.4

r.t.

0.125/7.2; 48

700/15

1 bar

Eu(BTB)(phen)/TBAB

97.8

70 °C

3.5/5; 12

27.9/2.3

1 bar

2-(PO)

3-(PO)

4-(PO)

5-(PO)

Ref. 255

260

257

256

259

The catalytic activity results obtained using MOFs in combination with TBAB are presented in Table 8. In general, PO is the most reactive substrate when using this kind of catalysts and their activity is often found to decline for more sterically demanding substrates, likely, as an effect of hindered substrate diffusion in the pores of the materials. A few MOFs can efficiently promote the cycloaddition reaction under ambient conditions with low to moderate TOF values (Table 8,


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Entries 2-4). The highest TOF values under ambient conditions were obtained using a low catalyst loading of MMCF-2 and MMPF-9 for a prolonged reaction time of 48 h (Table 8, Entries 3, 4) albeit in the presence of high loadings of TBAB as a homogeneous catalytic component. The TOF values reported using these MOFs as heterogeneous Lewis acids are comparable, or higher than those obtained using most of the homogeneous catalysts in Tables 1-6 but they are still far from what observed for the most active porous organic polymers in Table 7. Currently, there are no reports of single component MOFs catalyzing the title reaction under ambient conditions.

3.4. Metal Nanoparticles Besides the direct application of MOFs as heterogeneous Lewis acids for the cycloaddition of CO2 to epoxides, they were further explored as templates to generate metal oxide nanoparticles distributed in a porous carbon matrix with a relatively high surface area. Jiang and co-workers exploited ZIF-8 as a precursor to access ZnO nanoparticles through pyrolysis at different temperatures followed by oxidation with hypochlorite. 262 This process afforded ZnO nanoparticles incorporated into N-doped porous carbon (ZnO@NPC-Ox-T, T=pyrolysis temperature, 700, 800, 900 or 1000 °C). The surface area of the newly developed composites was found in the 77-150 m2 g-1 range whereas the CO2 uptake capacity was between 20 mg g-1 and 30 mg g-1 at 0 °C. When evaluated for catalytic activity in combination with TBAB as a nucleophile, ZnO@NPC-Ox-700 (Zn loading, 11.1 wt%) afforded 98 % SO conversion in 48 h under 1 bar CO2 at 60 °C with a TOF of 0.23 h-1. At 25 °C, 85 % PO was converted to PC with a TOF of 0.135 h-1. Ghosh et al. reported the preparation of microporous iron-phosphonate nanoparticles (HFPF-1) and their application for the cycloaddition of CO2 to epoxides under ambient conditions.263 The HFPF-1 nanoparticles were prepared by the reaction of a multidentate phosphonate precursor


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(hexamethylenediamine-N,N,N′,N′-tetrakis-(methylphosphonic

acid))

and

FeCl3

under

hydrotermal conditions. The resulting nanoparticles presented a diameter in the 45-60 nm range and a micropore size of 1.5 nm. The nanoparticles (50 mg) were employed in the presence of 5 mol% TBAB as a catalytic system for the cycloaddition of CO2 to SO. A high SC yield was obtained in 12 h under ambient conditions. Similarly, several terminal epoxides could be converted to the corresponding cyclic carbonate under identical conditions. In the same study, the HFPF-1 nanoparticles behaved also as versatile catalysts for the esterification of levulinic acid at 60 °C.


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4. OVERVIEW OF CATALYTIC ACTIVITY USING PURE CO2

For an overview, Figure 5 was prepared by taking into account a selection of the most active Lewis acids, organocatalysts and single component catalysts for each of the main classes of compounds discussed in this review (the TOF values generally refer to the reactions using PO as a substrate). It is interesting to notice that each class of compounds seems to correspond to a defined range of maximum catalytic activity. Similar results are obtained if the highest TOF values obtained for any epoxide are considered (See Figure S1). Based on these data, TPP-based porous organic polymers appear as the most performing Lewis acids under ambient conditions with the further advantage of being heterogeneous, recyclable compounds that can be developed into single component catalytic system (70, Table 7, Entry 5). In the homogeneous phase, a single component TPP complex (11, Table 2, Entry 1) resulted as the species with the highest reported TOF, albeit this value is very close to those observed for the binary sulfur-bridged bis(phenolato) bismuth complex/LiI system (13, Table 2, entry 3). Interestingly, simple coordination compounds of niobium, yttrium and zinc display a range of catalytic activity superior to that of various families of more sophisticated metal-based Lewis acids such as MOFs, mono- and bimetallic salen complexes.


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Figure 5. Graphical comparison of catalytic activity for the most active systems for each class of compounds reviewed in this work. The striped columns refer to heterogeneous catalytic systems, red numbers refer to single component catalysts.


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5. CATALYSTS FOR THE CONVERSION OF DILUTED, IMPURE AND FLUE GAS CO2 In the perspective of a future utilization of waste CO2 for the cycloaddition to epoxides, we analyze in this chapter the application of some of the catalytic systems highlighted in the previous sections using diluted CO2, simulated flue gas (pure CO2 doped with impurities) and actual flue gas. In this context, we focused our attention on a series of parameters that should be considered of relevance for evaluating the potential of a catalytic system for application with waste CO2: a) Nature of the catalyst (i.e. homogeneous versus heterogeneous and the number of catalytic components): it is clear that the application of heterogeneous, reusable catalysts is convenient in terms of costs, recyclability and for the sake of ease in product purification. Furthermore, the nature of the catalyst and the number of components are crucial to the reaction design; single component heterogeneous catalysts can be used in plug-flow reactors55 whereas, homogeneous or partially heterogeneous catalysts (i.e. constituted by a heterogeneous Lewis acid and a homogeneous nucleophile), need to be employed either under batch conditions or for the reactive capture and conversion of CO2 flows passed through the reaction mixture.57, 264 b) CO2 source: flue gas CO2 is emitted at atmospheric pressure and in a diluted form. The concentration of CO2 can be as low as 5-15 % for emissions from power generation plants56 up to nearly pure CO2 if oxy-fuel combustion technologies are implemented.265 Besides being diluted, CO2 in flue gas contains various impurities such as SOx, NOx,


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CO and moisture. The latter species might lead to detrimental effects on catalytic activity by poisoning the metal center and/or by leading to the formation of acidic impurities266 able to affect the support and its bond to the catalyst.55-56 Therefore, experimental studies that use samples of actual flue gas or diluted CO2 doped with pollutants provide more reliable information on the suitability of the investigated system for application in a scenario where impure CO2 is used as a feedstock. c) Reaction conditions: the conditions applied might have a strong impact on the sustainability of the process,11 therefore, catalytic systems able to use impure CO2 without requiring energy-intensive reaction setups are preferable. The reaction conditions applied in the selected studies are provided along with details on the reactions’ outcome. We initially present an overview of catalysts used with diluted CO2. These studies are presented in Table 9 and discussed in the following paragraphs.


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Table 9. Overview of catalytic systems used for the conversion of diluted CO2 to cyclic carbonates. Catalyst Type of catalyst (Components)

CO2 source and [CO2]

Conditions

Yield (%)/ TON / TOF (h-1) /

Ref.

Substrate 1) NbCl5/TBAB Homogeneous (2) PO

2) 36/K2CO3/ZnBr2, Homogeneous (3) SO 3) 64/TBAB, Heterogeneous (2) ECH 4) 68/TBAB, Heterogeneous (2) PO 5) 70, Heterogeneous (1) PO a

Diluted CO2 (0.15 bar) in Ar, [CO2]=12 %

25 °C, 1.15 bar in a batch reactor refilled with CO2 upon drop of pressure (semibatch)

94/ 94 /5.9

172

Diluted CO2 (0.5 bar) in N2; [CO2]=50 %

r.t., 1 atm, 2 h (Batch reaction)

89/ 29.7/ 14.8

167

Diluted CO2 (0.15 bar) in N2, [CO2]=15 %

29 °C, 1 atm, 12 h (Batch reaction)

(30)a/ 136 / 5.7

241

Diluted CO2 in N2, [CO2]=15 % (PCO2 = 4.5 bar)

40 °C, 30 bar, 5 h (batch)

81 / 324 / 64.8

243

Diluted CO2 in N2, [CO2]=15 % (PCO2 = 4.5 bar)

40 °C, 30 bar, 7 h (batch)

98 / 980 / 140

244

Value extrapolated from Figure 6 of reference 241.

Metal halides have been succesfully investigated for application with diluted CO2. NbCl5/TBAB could carry out the formation of PC (leading to 94 % PO conversion in 16 h) at room temperature using CO2/Ar mixtures with a CO2 concentration as low as 12 % (v/v), therefore resulting as a promising candidate for application with flue gas (Table 9, Entry 1).172 In an in situ IR study using NbCl5/DMAP as a model catalyst, the formation of the hemicarbonate complex (I-3, Scheme 1, Cycle 2) arising from the insertion of CO2 in the Nb-alkoxyde bond (I-2, Scheme 1, Cycle 2) was observed under a CO2 pressure of 0.5 bar CO2 (33 % CO2) over the reaction mixture.81


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In the course of their study on the catalytic system 36/K2CO3/ZnBr2,167 Shi et al. tested the ternary catalyst for the conversion of CO2 from CO2/N2 mixtures under ambient conditions. Using SO as the substrate in the presence of 3 mol% K2CO3 and ZnBr2 and of a non-catalytic amount of 36 (2 equiv.) the authors observed the formation of styrene carbonate in 89 % yield when 50 % CO2 in N2 was used (Table 9, Entry 2). The carbonate yield decreased to just 31 % when 33 % CO2 in N2 was used as a feedstock. Although the catalytic system could be recycled, the requirement of more than stoichiometric amount of 36 can potentially affect the sustainability of this process. TPP-based porous organic polymers have emerged as powerful heterogeneous Lewis acids and catalysts for the cycloaddition of CO2 to epoxides under ambient conditions )See

Table 7(. Thus far, they have not been applied for the conversion of CO2 from

actual or simulated flue gas. Cobalt-containing polymer 64 has been applied, in the presence of TBAB, for the cycloaddition of CO2 to ECH at 29 °C under atmospheric pressure using 15 % CO2 in N2 (Table 9, Entry 3).241 Under these conditions, about 30 % substrate conversion (with a selectivity of about 90 %) was observed in 24 h down from 96 % when using pure CO2. Interestingly, the authors reported that lower epoxide conversion was obtained when the corresponding homogeneous Co(III) TPP was used as a Lewis acidic under identical conditions. This observation was attributed to the ability of the porous matrix of 64 to capture and concentrate CO2 in the proximity of the metal center. The binary system 68/TBAB,243 (Table 9, entry 4) and single component catalyst 70,244 (Table 9, Entry 5) were applied using diluted CO2 (15 %) in N2 showing high to quantitative yields of PO. To notice, these reactions were carried


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out in batch reactors at 30 bar total pressure. Albeit these experiments demonstrate the suitability of these catalysts for use with diluted CO2 under the applied conditions, the partial pressure of CO2 (4.5 bar) was significantly higher than for actual flue gas as emitted. The application of highly active single component catalyst 70 for the conversion of flue gas under dynamic conditions would represent an important test bed for this promising catalyst. The catalytic systems that have been applied for the title reaction using impure CO2 (simulated or actual flue gas) as a feedstock are listed in Table 10.


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Table 10. Overview of catalytic systems used for the conversion of impure CO2 to cyclic carbonates. Catalyst Type of catalyst (Components)

CO2 source [CO2]

Conditions

ECH 2) ZrCl4/SiO2/TBAB Heterogeneous (2) PO 3) 18/TBAB, Homogeneous (2) Glycidol 4) 52 Heterogeneous (1) EO

a

Ref.

Main outcome

Substrate 1) YCl3/TBAB Homogeneous (2)

Yield (%)/ TON /TOF (h-1)/ CO2 conversion (%)

Actual flue gas (Cement factory),a [CO2]=10.2 %

r.t., 1 bar, 5 h, Flue gas was bubbled in the reaction mixture (semi-batch)

8/ 8 /1.6/ 92; Impurities in flue gas do not affect the catalytic activity.

57

Actual flue gas (Cement factory),a [CO2]=15 %, PCO2: 1.0 bar

60 °C, 7 bar, 18 h, Using a 10-fold excess of PO (batch)

9.9/ 13 /0.7 / 99 (first catalytic run) CO2 from compressed flue gas is converted.

267

Gas flow from oxycombustion [CO2]=5 % in He

26 °C, 1 bar, 80 h (semi-batch)

100/ 8.6 / 0.1 / 0.5 About 0.5 % of the CO2 in the stream is absorbed for the initial 2000 min of reaction

268

Simulated flue gas [SO2] (1700 ppm), or, [NO] (661 ppm) and [NO2] (36 ppm), [CO2]=16 % in N2

100 °C, 1 bar, 12 d, Plug-flow reactor, 18 % ethylene oxide (dynamic)

15-20 % CO2 conversion; No effect of impurities on catalytic activity in up to 12 d on-stream.

55

Cooling flue gas below its dew point during sampling operations led to condensation of large part of its

moisture content that was, therefore, excluded from the reaction environment.

The homogeneous catalyst YCl3/TBAB has been applied for the synthesis of cyclic carbonates using a flow of actual industrial flue gas as a feedstock.57 In preliminary experiments, Barthel et al. explored the ability of metal halides and TBAB to dynamically convert diluted CO2 ([CO2]: 50 % in Ar (v/v)) to cyclic carbonates by bubbling a flow of gas through a solution of binary catalyst in ECH under ambient conditions.57 The authors observed the nearly quantitative conversion of the amount of CO2 passed through the solution when group III metals (YCl3 and ScCl3, entries 6, 7 of


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Figure 6) were used as the Lewis acidic components of the catalytic pairs. ZrCl4 afforded, as well, high CO2 conversion rates slightly below those of the rare-earth compounds (Figure 6, entry 5).

Figure 6. Behavior of various early transition metal and rare-earth halides in the dynamic conversion of CO2 from a diluted flow (50 % CO2) bubbled through a solution of ECH, metal halide and TBAB. Adapted with permission from Ref. 57, copyrights: Royal Society of Chemistry. A strong drop in catalytic activity under identical conditions was observed when moving to group V and VI coordination complexes (TaCl5, NbCl5; MoCl5 Figure 6, entries 1, 3, 4); despite the good performance displayed in batch reactions, NbCl5/TBAB was not a good catalyst for application with flows of diluted CO2. Encouraged by these results, the authors applied YCl3/TBAB for the conversion of CO2 from increasingly diluted flows of CO2 (in O2) and from samples of flue gas ([CO2]=10.2 %). Flue gas was sampled directly from the fumestack of a cement company and was found to contain various impurities (air, NOx, SOx, CO, moisture). Most of the moisture present in the flue gas was excluded during the sampling operations as an effect of cooling the gas flow below its due point thus confirming the


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difficulty of sampling this component as discussed by North et al.56 From an experimental standpoint, the authors bubbled increasingly diluted flows of CO2 in O2 through a solution of ECH and YCl3/TBAB under ambient conditions. The authors plotted the dependency of the degree of CO2 conversion and ECH conversion on the concentration of CO2 in the gas flow (Figure 7).

Figure 7. Dependency of the degree of CO2 and ECH conversion in 5 h on the concentration of CO2 in the gas flow. The circle in the horizontal axis and the vertical dashed line denote the concentration of CO2 (10.2 % v/v) in the sample of flue gas employed. Adapted with permission from Ref. 57. Copyright: Royal Society of Chemistry. It was shown that the epoxide conversion achieved in five hours would decrease linearly with the dilution of CO2 in the gas flow as an effect of the lower number of equivalents of CO2 added to the reaction mixture. Nevertheless, the ratio between CO2 added and converted increased linearly with the dilution of CO2 up to 86 % of the CO2 passed through the reaction for [CO2] = 12.5 %. Remarkably, when a flow of actual flue gas was employed, the degree of epoxide and CO2 conversion matched the trend defined by the experiments with the pure gas mixtures. The authors highlighted this observation as proof of the resilience of the catalytic system towards the pollutants


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present in flue gas and of the capability of the binary catalyst to convert highly impure and diluted CO2. From a sustainability-friendly standpoint, the reaction could be carried out under ambient conditions and without the need of compressing the flue gas. Under the applied conditions, the epoxide represented the limiting reagent and this method can be regarded as a reactive equivalent of CO2 scrubbing from flue gas but with the advantage of transforming CO2 into a value-added product. As a possible drawback, the binary YCl3/TBAB catalyst is homogeneous and the catalytic components would need to be separated from the product at the end of the reaction. Moreover, halogen-based Lewis acid could decompose to HCl and have potentially corrosive effects if flue gas moisture is not carefully excluded from the reaction. Recently, the same group developed several zirconium-based catalysts supported on silica by a SOMC approach targeting the formation of surface complexes differing in terms of podality (number of bonds with the silica surface) and surface coverage (Scheme 35).267 Systematic spectroscopic investigation allowed the authors to demonstrate that the reaction of the ZrCl4(OEt2)2 precursor with Aerosil silica dehydroxylated at different temperature (200 °C (SiO2-200); 700 °C, (SiO2-700)) led to the formation of isolated zirconium complexes (for SiO2-700) and of a mixture of monopodal and bipodal zirconium complexes (for SiO2-200). When these complexes were tested for the cycloaddition of CO2 to PO (60 °C, 10 bar pure CO2) and ECH (60 °C, 1 bar pure CO2) in the presence of TBAB, quantitative yields of the corresponding carbonate products were obtained in 6-24 h with a TOF up to 40 h-1 (atmospheric pressure) and 46.5 h-1 (10 bar CO2) for ECH. At variance with comparable


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work on supported niobium complexes (See Scheme 22),173 the authors did not find any specific influence of the density of the zirconium complexes on the support surface on catalytic activity.

Scheme 35. Different surface complexes generated on two kinds of dehydroxylated silica (SiO2-200, SiO2-700) by SOMC grafting of ZrCl4(OEt2)2. The supported zirconium complexes in Scheme 35 were employed for the cycloaddition of CO2 to PO using flue gas sampled from the fumestack of a cement company as an impure source of CO2. The reaction was carried out under batch conditions at 60 °C and 7 bar total pressure (corresponding to a partial CO2 pressure of 1 bar) using PO in a large excess (about 10 times) compared to the amount of CO2 in the reaction vessel. Under these conditions, the authors observed the quantitative conversion of the amount of CO2 present in the flue gas based on the quantity of cyclic carbonate isolated at the end of the reaction. The amount of CO2 converted was found to decline slightly when the recycled catalyst was reused under identical conditions as


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the pristine catalyst. This study represents, therefore, the first example of a heterogeneous, reusable catalyst applied for the direct conversion of actual flue gas CO2 to cyclic carbonates. As possible drawbacks, it should be noted that a) this catalytic system requires the presence of a homogeneous component (TBAB) thus hindering its application in a flow reactor; b) In recycling studies, the observed catalytic activity was found to decline for all zirconium catalysts to ca. 70-80 % of the initial performance after five reuses. This was attributed to a combination of effects such as the loss of some weakly bound zirconium species after the first catalytic run and the progressive dehalogenation of the zirconium complexes to afford slightly less active surface complexes. In an early example of diluted CO2 conversion to cyclic carbonates, North et al. applied the 18/TBAB system for the cycloaddition of CO2 to glycidol using a flow of gas generated in an oxycombustion reactor by the complete oxidation of diluted methane (Table 10, Entry 3).268 The resulting flow, containing exclusively CO2 (5 %) in helium, was continuously added to a solution of glycidol, 18 and TBAB at 26 °C in a semibatch setup. The substrate was completely consumed within about 80 h thus providing a proof of feasibility for this kind of approach. In a different study, the authors explored also the application of 18/TBAB for the synthesis of cyclic carbonates under various conditions including the use of compressed air as a diluted source of CO2.38 Interestingly, 61 % SO conversion was noted at 50 °C and 10 bar pressure in 24 h using 2.5 mol% catalyst loading whereas, at 20 °C and 25 bar pressure, the conversion


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dropped to 25 %. This study demonstrated the capability of bimetallic Al-salen complexes to operate under extremely low partial pressures of CO2 (300-400 ppm). North et al.55 investigated the application of single component catalyst 52 as a heterogeneous system for the conversion of diluted and/or impure CO2 doped with common industrial pollutants from fossil fuel combustion. The single component nature of the catalyst allowed its application under dynamic conditions in a plug-flow reactor, a solution that appears highly suitable for industrial application. The activity of 52 under dynamic conditions was initially studied in the presence of a diluted flow of CO2 and epoxide containing 21 % carbon dioxide, 25 % ethylene oxide (EO) and 54 % nitrogen (total flow rate 4.7 mL/min) passed through a 15 cm long reactor filled with the silicasupported catalysts. The conversion rate of CO2 was found to increase with the temperature; when the temperature was increased from 20 °C to 60 °C, the conversion of CO2 increased from 6 % to almost 100 %. Although the MCM-41-supported analogous of 52 (53, Scheme 27) displayed four times higher activity than 52 under the same conditions, in view of the high cost of MCM-41, complex 52 was selected as the most suitable choice for possible industrial application. Concerning its stability under flow conditions, complex 52 was irreversibly deactivated in 18 h at 150 °C. Nevertheless, at 100 °C, the catalyst could be used continuously for an extended time while retaining 50 % of its initial activity after one week. Furthermore, it could be reactivated by treatment with benzyl bromide to regain its initial catalytic activity. Subsequently, the authors investigated the effect of flue gas contaminants such as SOx and NOx on the catalytic activity and lifetime of catalyst 52. 55 A flow-reactor was


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designed where the supply of CO2 and N2 was arranged from a cylinder containing either SO2 (1700 ppm), NO (661 ppm) or NO2 (36 ppm, Table 10, entry 4). The concentrations of these pollutants are significantly higher than usually found in flue gas. The concentrations of epoxide and CO2 in the gas mixture fed to the catalyst were set to 18 % and 16 % respectively. The latter value is comparable to what typically found (17 %) in actual flue gas from a coal-burning power station. The experiments using PO and EO as substrates revealed nearly no adverse effect of any of these impurities on the catalytic activity of 52 as well as no effect on catalyst reactivation (Figure 8).

Figure 8. Graphic representation of the catalytic activity of 52 in the cycloaddition of CO2 from a flow of simulated flue gas (containing one among possible impurities such as NOx or SO2) to PO (solid lines) and EO (dashed lines) at 100 °C. The catalytic performance observed using the gases doped with impurities is similar as when using the CO2/N2 mixture before and after catalyst reactivation. Adapted with permission from Ref. 55, Copyrights: Royal Society of Chemistry. Targeting the application of catalysts 52 with waste CO2, North et al. set to study the effect of actual flue gas on its catalytic performance. Due to various limitations in the


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collection of flue gas directly from the source point, the authors opted for exposing a batch of fresh catalyst to flue gas from a combustion test facility.56 Interestingly, all the catalyst samples that were exposed to flue gas were found to have aluminum concentrations similar to the pristine control sample as determined by ICP-MS analysis, indicating that no catalyst leaching had occurred as a consequence of the exposure to flue gas. The catalytic activity of 52 was investigated in the laboratory for the synthesis of SC in batch reactions after exposure to flue gas from the combustion of various fuels. Concerning the batch of catalyst exposed to flue gas from natural gas combustion, a lower catalytic activity than for the unexposed catalyst was observed. This effect was attributed to an exchange of the bromide anion of the quaternary salt by some other less nucleophilic anion (possibly arising from NOx or SOx) produced by the contact with flue gas. The reactivation of the catalyst with benzyl bromide led to a full recovery of its catalytic activity up to the levels of the pristine catalyst. The batches of catalyst exposed to flue gas from coal combustion displayed a lower catalytic activity than the control catalyst sample even after the attempts of reactivation. As all catalysts had shown to not leach aluminum, the authors tentatively attributed the lower catalytic efficiency to the deterioration of part of the silica support due to the action of the acidic components in flue gas. Based on these results, catalyst 52 appears as an efficient and robust system for the cycloaddition of impure CO2 to epoxides. As a single component catalyst, it is suitable for application under dynamic flows with diluted or impure CO2 in plug-flow reactors and it is the only heterogeneous complex so far reported to work under such conditions.


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As a potential drawback, this catalyst displays a large organic framework leading to a high molecular weight, high synthetic costs and carbon footprint. Therefore, particular attention has been paid to reducing the preparation cost of 52.135 To the best of our knowledge, this catalyst has yet to find application in industry. Hong et al. reported the use of CO2 from combustion processes as a feedstock for various reactions such as alkyne carboxylation, the Grignard reaction and the synthesis of cyclic carbonates from epoxides.35 CO2 was captured using an aqueous ethanolamine solution and subsequently released in the reaction environment upon heating. In particular, the synthesis of some terminal cyclic carbonates was accomplished in high yields using a ternary catalyst (ZnBr2 (2 mol%), K2CO3 (2 mol%), 1,3-bis-(2,6-diisopropylphenyl)imidazolinium chloride (2 mol%)) at 80 °C and at atmospheric pressure in DMSO. The authors did not specify the purity of CO2 after liberation from the amine solution. This experiment demonstrates that the processes of CO2 sequestration by amines and the synthesis of cyclic carbonates by cycloaddition to epoxides can be coupled, however, such post-combustion capture processes are known to involve a significant energy penalty.269 Based on the literature discussed above, it appears that the catalytic systems applied for the cycloaddition of CO2 from impure sources to epoxides are generally insensitive to the impurities commonly present in flue gas. It is worth noting that in the related reaction of CO2/epoxide copolymerization, Williams et al. found that Zn and Mg dinuclear catalysts could be used for the production of poly-(cyclohexene carbonate) polyols using impure CO2 captured from a demonstrator plant as a feedstock.59 This


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advance confirms that the chemistry of CO2 and epoxides is, in general, particularly suitable for the utilization of waste CO2. Interestingly, no literature report has yet surfaced on the application of organocatalysts for the conversion of diluted or impure CO2. It is clear that if the synthesis of cyclic carbonates from waste CO2 is to be carried out in a large scale, the application of inexpensive and renewable organic molecules would be desirable. Despite the generally lower activity of these systems under ambient conditions with respect to metal based catalysts, the recent discovery of highly performing organocatalysts,201, 224 constitutes a promising ground for further development.

6. CONCLUSION

Judiciously designed catalytic pathways for CO2 fixation are highly sought after in order to bolster the appeal of CO2-to-chemicals strategies for industry.2 Replacing high energy intensive processes with greener and milder ones is key to escape the recurring pitfalls of indirect carbon emissions.11 In this context, the catalytic cycloaddition of CO2 to epoxides is an atom-economic and industrially relevant process that can be carried out under ambient conditions. Herein, we have reviewed the entire literature on this burgeoning topic to outline the state-of-the-art on the catalytic systems able to promote the title reaction under ambient conditions and using impure/diluted CO2. Several categories of homogeneous and heterogeneous compounds have been identified for the cycloaddition of pure CO2 to epoxides under ambient conditions and


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the active systems have been compared in terms of catalytic performance (TON/TOF) within each class (Tables 1-8). Furthermore, an overview of the range of catalytic activity of the different families of compounds is displayed in Figure 5. We would like to emphasize that the exclusive use of the literature TOF values to validate the families of catalysts in this study would be reductive; other factors such as structural features, costs, availability, molecular weight, stability towards moisture and pollutants and toxicity should be taken into account. For instance, thanks to their tunable chiral structure, salen complexes represent the most convenient choice for the kinetic resolution of epoxides under mild conditions (See Section 2.1.1). Despite generally affording lower reaction rates, organocatalysts with the capability to convert CO2 to cyclic carbonates at ambient conditions have recently surfaced (Table 6). The development of increasingly active and readily available organocatalysts remains a viable strategy to access more economic, sustainable and easy-to-handle catalysts.48, 50, 53

. The promising results obtained under ambient conditions using pure CO2 have

inspired some authors to use diluted, impure or flue gas CO2 as a feedstock for the cycloaddition reaction to epoxides as described in Section 4. These advances have demonstrated the potential of this reaction to act as one of the few transformations that can be applied for the direct conversion of CO2 from flue gas;270 the other processes being the non-catalytic mineralization of CO2 to solid inorganic carbonates44 and CO2 tri-reforming.271 Moreover, at variance with the above-mentioned applications, the title reaction could be carried out, at least in some examples,57, 268 from actual flue gas under


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ambient conditions. With the current industrial production of cyclic organic carbonates being just 4 Mt/year,18 it is evident that this approach is far from representing a single strategy to the solution of the global CO2. However, it could offer a unique opportunity for the reactive capture of waste CO2 through a process that circumvents the energy demanding steps of power generation, gas separation, purification, compression and transportation. Thus, via the synthesis of cyclic carbonates, waste CO2 could be reinvested for the production of plastics, solvents and chemical precursors in place of its geological dumping. Replacing oil-based epoxides with substrates derived from renewable sources would lead to a further improvement of the sustainability and carbon neutrality of the process.45, 272-273 It is desirable that the next years might lead to the development of more active, inexpensive and robust heterogeneous catalysts for the cycloaddition of diluted CO2 to epoxides and to the incipit of their industrial application using waste CO2 as a feedstock.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. SUPPORTING INFORMATION Supporting tables for the catalytic data in Tables 1-8; supporting data for catalytic performance comparison.


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ACKNOWLEDGMENT V.D.E. thanks the Thailand Research Fund (Grant No. RSA6080059) for funding. R.R.S. acknowledges financial support from the Vidyasirimedhi Institute of Science and Technology.

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For Table of Contents only

The synthesis of cyclic carbonates from CO2 and epoxides is reviewed with a specific focus on the catalytic systems that are active at ambient conditions.


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Figure 1. Number of documents (darker green) involving propylene carbonate published for each five years’ time frame in the 1982-2016 period. The fraction of documents representing patents is shown in light green. The data were obtained from SciFinder® searches. 338x190mm (96 x 96 DPI)


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Figure 2. Representative transition state for the ring opening of PO by a model Zn(II) TPP with a single pendant tetraalkylammonium chain. The stabilization of the bromide anion by a network of positively charged hydrogen atoms of the alkyl side chain is shown. Reproduced with permission from Ref 120 copyrights: John Wiley & Sons. 338x190mm (96 x 96 DPI)


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Figure 3. Structure of the proline-derived cryptand (Left) and of the Co-Cryptate complex (right) resulting by the reaction of the ligand with Co(II) perchlorate in the presence of KSCN. Adapted with permission from reference 124, copyrights: American Chemical Society. 338x190mm (96 x 96 DPI)


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Figure 4. Calculated transition states for the step of CO2 activation (TS-CO2) in the cycloaddition of CO2 to PO catalyzed by ascorbic acid (Left) and APAA (Right). In the absence of the stabilizing effect by the ethyldiol moiety, the barrier for CO2 activation for APAA is 5.5 kcal/mol higher in energy. Selected distance are provided in Å. Reproduced with permission from reference 218: copyrights: American Chemical Society. 338x190mm (96 x 96 DPI)


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Figure 5. Graphical comparison of catalytic activity for the most active systems for each class of compounds reviewed in this work. The striped columns refer to heterogeneous catalytic systems, red numbers refer to single component catalysts. 270x152mm (120 x 120 DPI)


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Figure 6. Behavior of various early transition metal and rare-earth halides in the dynamic conversion of CO2 from a diluted flow (50 % CO2) bubbled through a solution of ECH, metal halide and TBAB. Adapted with permission from Ref. 57, copyrights: Royal Society of Chemistry. 338x190mm (96 x 96 DPI)


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Figure 7. Dependency of the degree of CO2 and ECH conversion in 5 h on the concentration of CO2 in the gas flow. The circle in the horizontal axis and the vertical dashed line denote the concentration of CO2 (10.2 % v/v) in the sample of flue gas employed. Adapted with permission from Ref. 57. Copyright: Royal Society of Chemistry. 338x190mm (96 x 96 DPI)


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Figure 8. Graphic representation of the catalytic activity of 52 in the cycloaddition of CO2 from a flow of simulated flue gas (containing one among possible impurities such as NOx or SO2) to PO (solid lines) and EO (dashed lines) at 100 °C. The catalytic performance observed using the gases doped with impurities is similar as when using the CO2/N2 mixture before and after catalyst reactivation. Adapted with permission from Ref. 55, Copyrights: Royal Society of Chemistry. 338x190mm (96 x 96 DPI)


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Graphical Abstract (Full size, the 3.11x1.75 version is found at the end of the manuscript) 338x190mm (96 x 96 DPI)


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