1 2 3 4 "This is the peer reviewed version of the following article: Top. Curr. Chem. 2017, 375 (15), 1-28, which has been published in final form at DOI: 10.1007/s41061-016-0101-8. This article may be used for non-commercial purposes in accordance with the Terms and Conditions for Self-Archiving published by Springer at https://link.springer.com/." 1 Synthesis of Carbonates from Alcohols and CO2 1 2 Nicole Kindermann,[a] Tharun Jose,[a] and Arjan W. Kleij[a][b]* 3 4 5 6 7 8 9 [a] Institute of Chemical Research of Catalonia (ICIQ), the Barcelona Institute of Science and Technology, Av. Països Catalans 16, 43007 – Tarragona (Spain). [b] Catalan Institute of Research and Advanced Studies (ICREA), Pg. Lluis Companys 23, 08010 – Barcelona (Spain) Email: akleij@iciq.es 10 11 12 13 14 15 Abstract Alcohols are ubiquitous compounds in nature that offer modular building blocks for synthetic chemistry. Here we discuss the most recent development of different classes of alcohols and their coupling chemistry with carbon dioxide as to afford linear and cyclic carbonates, the challenges associated with their formation and the potential of this chemistry to revive a waste carbon feed stock. 16 17 18 19 Keywords Carbon Dioxide  Carboxylative Cyclization  Cyclic Carbonates  Diols  Heterogeneous Catalysis  Homoallylic Alcohols  Homogeneous Catalysis  Linear Carbonates  Propargylic Alcohols 20 2 1 Contents 2 3 1. Synthesis of Acyclic Organic Carbonates 4 1.1 The Importance of the Formation of Acyclic Carbonates from Alcohols 5 1.2 Organic Promoters 6 1.3 Metal-Based Homogeneous Catalysis 7 1.4 Metal-based Heterogeneous Catalysis 8 1.5 Prospects of Acyclic Carbonate Formation 9 2. Cyclic Organic Carbonates from Saturated Alcohols 10 2.1 Synthesis of Five-membered Cyclic Carbonates 11 2.1.1 Metal based Catalysts 12 2.1.2 Organocatalysts 13 3. Formation of Six-membered Cyclic Carbonates 14 3.1 Metal based catalysts 15 3.2 Organocatalysts 16 4. Cyclic Carbonates derived from Unsaturated Alcohols 17 4.1 Metal based catalysts 18 4.2 Organocatalysts 19 5. Cyclic Carbonates from Halo-Alcohols 20 6. Conclusions and Outlook 21 7. References 22 3 1 1. Synthesis of Acyclic Organic Carbonates 2 1.1 The Importance of the Formation of Acyclic Carbonates from Alcohols 3 Among various organic molecules which can in principle be derived from CO2 and alcohols, 4 acyclic carbonates such as dimethyl carbonate (DMC), diethyl carbonate (DEC) and diphenyl 5 carbonate (DPC) have attracted considerable attention over the past decades [1, 2]. Especially 6 DMC has been a focus of active recent research in the field, since it represent a multifunctional 7 molecule that can be applied as an apolar solvent, a fuel additive, or as an organic reagent in 8 the production of higher carbonates including polycarbonates, polyurethanes and isocyanates 9 [3–5]. 10 Conventionally, DMC has been produced either from oxidative carbonylation of methanol 11 or through a reaction between methanol and phosgene (Scheme 1) [4]. Both processes cannot 12 be regarded neither sustainable nor environmentally benign, since they use highly toxic and 13 corrosive reagents, and require expensive catalysts in the case of the oxidative carbonylation or 14 disposal of hydrogen chloride when using phosgene. A much more attractive way to produce 15 DMC (and related acyclic carbonates) would be the direct reaction between CO2 and methanol, 16 as shown in Scheme 1. The sole byproduct of this process is water, and its atom economy is 17 comparable to that of the oxidative carbonylation of methanol. 18 19 Scheme 1 Different routes for the synthesis of dimethyl carbonate (DMC) 20 21 One major drawback of this reaction, however, is its equilibrium limitation 22 (thermodynamics) providing only (very) low yields in DMC. High pressures of CO2 might help 23 to partially overcome these limitations [6] but at the cost of a high energy demand 24 accompanying this pressurization. Removal of the water formed in the DMC synthesis is 4 1 another way to shift the equilibrium towards the desired product DMC. Accordingly, the 2 development of efficient catalysts and in combination with a dehydrating agent has become one 3 major focus in the synthesis of acyclic carbonates [7]. Besides DMC, also DEC and DPC have 4 relevance in industrial processes [8, 9]. DPC has already found wide application in 5 polycarbonate synthesis being efficiently used for transesterification of Bisphenol A (BPA). 6 The derived polycarbonate is a thermoplastic polymer used on a large scale as a material for 7 numerous applications (including electrical insulation) with a production of more than a billion 8 tons per year; apart from the DPC route, the major process towards BPA based polycarbonates 9 still relies on the direct reaction of BPA with phosgene. DPC synthesis from phosgene or 10 through oxidative carbonylation, comparable to DMC synthesis, has technical and logistic 11 disadvantages. Transesterification of DMC with phenol is an attractive alternative to the 12 conventional synthesis methods, however, in order to make the overall process “greener”, DMC 13 needs to be produced in a sustainable way. 14 For the direct synthesis of DMC from methanol and CO2, efficient removal of water – beside 15 the adequate choice of a catalyst – seems to be crucial in order to achieve an improvement of 16 this promising methodology and to increase its relevance in industrial synthesis. Thus, the 17 following sections will concentrate on organic and inorganic promoters for DMC formation, 18 and the importance of dehydrating agents. 19 1.2 Organic Promoters 20 One approach to address the equilibrium limitation is based on organic molecules acting as 21 promoters of DMC formation. Activation of CO2 or water capture might both be performed by 22 the same organic molecule [10–12]. Especially in the latter scenario, a stoichiometric use of the 23 promoter is required, since water is usually bound irreversibly. For instance, Aresta and 24 coworkers established the use of dicyclohexylcarbodiimide (DCC; Scheme 2) for the direct 25 synthesis of DMC from CO2 and methanol under mild conditions (e.g. 80 ºC and 5.0 MPa of 26 CO2). Based on the use of DCC, they reported yields of up to 62% for DMC after only 6 h, but 27 the protocol could also be successfully employed in the conversion of ethanol or allyl alcohol 28 substrates thereby giving access to other dialkyl carbonates. Mechanistic and computational 29 studies led to a putative mechanism, with an isourea/hemi-carbonate adduct as the proposed 30 key intermediate (Scheme 2). 31 5 1 2 3 Scheme 2 Organic promotors for the direct formation of DMC from alcohols and CO2 4 5 A different synthetic route towards DMC synthesis, which gives access to symmetric and 6 asymmetric acyclic carbonates, is based on Mitsunobu’s reagent. This methodology, developed 7 by Chaturvedi et al. [11] can furthermore be successfully employed to convert primary, 8 secondary and even tertiary alcohols in a one-pot reaction with good to high yields (7098%) 9 in all reported cases. Recently, the successful application of DBU for the synthesis of acyclic 10 carbonates was demonstrated by Jang and coworkers [12]. The scope comprised the synthesis 11 of various cyclic and acyclic organic carbonates, including DMC, which could be obtained in 12 48% yield under comparatively mild reaction conditions (70°C, 10 bar CO2). 13 1.3 Metal-Based Homogeneous Catalysis 14 Among the homogeneous metal catalysts considered, metal alkoxides have been intensively 15 studied in DMC synthesis since they have been shown to absorb CO2 to form organic carbonates 16 [13, 14]. Besides titanium, zirconium and niobium compounds [14–18], tin(IV) complexes have 17 been investigated in detail with respect to DMC formation from methanol and carbon dioxide. 18 Tetraalkoxy [Sn(OR)4] as well as dialkoxydialkyl [R12Sn(OR2)2] tin compounds are classes of 19 organometallics both active in DMC formation, even though the efficiency of the reported 20 systems remains rather low [18–20]. Significant improvements of the methodology were made 21 by Sakakura et al. by investigating the effect of different drying agents on the DMC formation 22 catalyzed by organometallic tin compounds. Initially starting with orthoesters, DMC yields of 23 48% (based on the orthoester reagent) and a selectivity of 85% (DMC) were found under high 24 pressures of CO2 (300 atm) and reaction temperatures of 180°C, using [Bu2Sn(OMe)2] as 25 catalyst [21]. Notably, a substantial enhancement of the catalyst performance by the addition 26 of onium salts was observed. One drawback of this approach, however, was the required 6 1 stoichiometric use of orthoesters. As shown in Scheme 3, they can capture water under the 2 release of an alcohol and an ester, but recycling of the desiccant is not feasible. 3 4 5 Scheme 3 Organic desiccants that have been employed as water-capturing agents 6 7 By contrast, drying agents such as acetals promised to be more sustainable, since they feature 8 recovery potential from the formed ketone that upon reaction with alcohols can regenerate the 9 acetal [22]. In comparison to the orthoester system, a combination of tin compound and acetal 10 performed slightly better with a DMC yield of 58% based on the acetal. Even though in this 11 case onium salts do not lead to improved catalyst performance, more recently it has been shown 12 that acidic co-catalysts have a pronounced influence on the efficiency in DMC formation, and 13 for instance the presence of small amounts of co-catalytic Ph2NH2OTf accelerated the reaction 14 substantially [15]. 15 Besides organic desiccants, also inorganic versions such as molecular sieves have been 16 successfully employed in the dehydration process [6]. Even though zeolites are not considered 17 to be very efficient under high reaction temperature conditions, yields up to 45% based on 18 MeOH were reported [6]. Mechanistic proposals are based on early structural findings [23], as 19 depicted in Scheme 4. After CO2 insertion into the metal methoxide moiety, the bridging 20 alkoxide reacts with the hemi-carbonate anion with subsequent DMC release. The active 21 species can be reestablished from the corresponding oxide or hydroxide by reacting with 22 methanol under the release of water [24]. Recent reinvestigations and density functional theory 23 (DFT) calculation, though, suggest that the actual active intermediates might be stannoxane 24 dimers (Scheme 4) [25, 26]. 7 1 2 3 Scheme 4 Mechanistic proposal based on structural findings (left); reactive intermediates suggested by computational and experimental studies 4 5 1.4 Metal-based Heterogeneous Catalysis 6 The use of heterogeneous catalysts in the synthesis of chemical compounds has several key 7 benefits if compared with homogenous catalysis. Separation of the catalyst from the products 8 is usually straightforward, e.g. by a simple filtration. At the same time, the ease of separation 9 is advantageous when it comes to recyclability of the catalyst. This makes heterogeneous 10 catalysts an interesting choice for industry, especially when similar selectivities and activities 11 as in the case of homogenous catalysts can be achieved. For DMC synthesis, the use of metal 12 oxides had a considerable impact on the field. Besides main group metal oxides such as MgAl 13 hydrotalcites [27, 28], mainly transition metal oxides have been employed. Among these are 14 vanadium oxides, doped with Brønsted acids [29] or copper/nickel [30], but the most widely 15 studied systems consist of zirconium and cerium oxides. 16 Early work by Tomishige and Fujimoto revealed the great potential of the amphoteric 17 materials ZrO2 and CeO2 (or solid solution mixtures) [31–35], being mainly attributed to 18 synergistic effects between their acidic and basic sites [31]. However, for the simple metal 19 oxides the equilibrium restriction did not allow for yields exceeding 2% [34] even under high 20 CO2 pressures of 6 MPa and temperatures around 127 ºC. Doping of the metal oxides with 8 1 Brønsted acidic sites using H3PO4 or H3W12O40 led to slightly improved yields or shorter 2 reaction times under comparable conditions [36–38]. 3 Major breakthroughs were only achieved, though, if dehydrating agents were added. In 4 contrast to orthoesters or acetals, which were formerly used by Sakakura, Tomishige and 5 coworker suggested the use of nitriles in 2009 [39, 40]. Water capture with nitriles leads to 6 amides, which can later be converted back to the corresponding nitrile and enable the 7 regeneration of the dehydrating species. The elegance of this synthetic route is based on the 8 simultaneous conversion of CO2 and methanol to DMC and the nitrile hydration to the 9 corresponding amide by CeO2. If using acetonitrile as desiccant at 0.5 MPa CO2 pressure and 10 150 ºC, the yield of DMC after 48 h reached about 9% but with only a mediocre selectivity for 11 DMC of 65%. Also the selectivity of amide formation upon water capture remained an issue, 12 especially in the light of recyclability of the dehydrating agent. Benzonitrile proved to be a 13 much better choice not only with respect to selective amide formation, but its use also increased 14 the DMC yield to a remarkable 47% (1 MPa, 150 ºC, 86 h), with a significantly improved 15 chemo-selectivity of 75% [41]. 16 As suggested by the authors, the reason for this improved reactivity/selectivity behavior 17 might be suppression of competitive alcoholysis of the formed amide, if compared to acetamide 18 that is in situ produced from acetonitile. A systematic screening for suitable nitrile-based 19 dehydrating agents [42, 43], which are efficiently hydrated by CeO2, finally led to the use of 2- 20 cyanopyridine as the preferred nitrile in combination with a cerium oxide catalyst. With this 21 system (5 MPa, 120 ºC, 12 h), yield of and selectivity for DMC were extraordinarily high, 22 reaching levels of 94% and 96%, respectively. The recycling of the formed amide was also 23 addressed, and the dehydration of 2-picolinamide by Na2O/SiO2 was shown to be feasible even 24 though the overall efficiency should be improved. The scope is not only limited to the formation 25 of DMC, but also ethanol or branched alcohols could be converted in a similar way albeit with 26 a drop in yield of the corresponding carbonate product. 27 In order to provide a lead for further improvement of the catalytic system, mechanistic 28 insights are inevitable. Tomishige et al. proposed a reaction cycle based on kinetic, 29 spectroscopic and computational studies, leading to an overall mechanism as depicted in 30 Scheme 5 [44]. It resembles the mechanism suggested by Bell et al. for the zirconium oxide 31 catalyzed formation of DMC from methanol and CO2 [45]. A molecule of CO2 inserts into the 32 CeOCH3 bond of surface bound methanol to yield a Cemethyl carbonate species. For the 33 next step the authors suggested a nucleophilic attack of another surface bound methoxy group 9 1 to give DMC. The formed hydroxide species on the cerium oxide surface can subsequently 2 react with 2-cyanopyridine and result in the formation of 2-picolinamide [46, 47]. Other 3 authors, however, claim the attack of gas-phase methanol [48], or the formation of a 4 carbomethoxide intermediate on the cerium oxide surface [49]. Therefore, the exact mechanism 5 concerning the cerium oxide mediated formation of DMC remains subject of ongoing debate in 6 the literature. 7 8 9 Scheme 5 Mechanism of cerium oxide catalyzed simultaneous formation of DMC and 2picolinamide formation from MeOH/CO2 and 2-cyanopyridine, respectively 10 11 Its superb performance regarding yield and selectivity towards DMC formation makes the 12 combination of cerium(IV) oxide and 2-cyanopyridine a promising candidate for further 13 commercial applications. This is reflected in the development of the first continuous flow 14 process using fixed bed reactors [50], as well as intensified research concerning the reusability 15 of the cerium oxide catalyst, which eventually suffers from deactivation by adsorption of the 16 formed amide [44, 51]. Interestingly, the scope of this system is not limited to the formation of 17 simple acyclic carbonates, but can also be employed to cyclic carbonates (see section 2 and 3 10 1 of this review) [52], as well as cyclic and acyclic carbamates and urea derivatives [5355]. Very 2 recently, cerium oxide and 2-cyanopyridine was furthermore used to synthesize polymeric 3 materials from CO2 and diols, as depicted in Scheme 6 [56]. Despite the low molecular weight 4 of the produced materials, this represents the first exciting example of a direct copolymerisation 5 between CO2 and diols. 6 7 Scheme 6 Direct polycarbonate formation from diols and carbon dioxide 8 9 1.5 Prospects of Acyclic Carbonate Formation 10 Over the last two decades, huge progress has been made regarding the yield and chemo- 11 selectivity in the production of DMC from methanol and carbon dioxide. Homogeneous and 12 heterogeneous catalyst systems have been improved continuously, going hand in hand with the 13 development of efficient dehydrating systems to shift the reaction equilibrium towards the 14 formation of DMC. Various studies were dedicated to homogeneous metal alkoxide 15 compounds, with tin(IV) complexes leading the way. In combination with different water- 16 removing agents, good yields and selectivities for the formation of acyclic carbonates were 17 obtained. Even better performances were achieved using heterogeneous, amphoteric metal 18 oxides such as zirconium oxide and cerium oxide, with the latter giving excellent yields and 19 selectivities towards DMC when combined with 2-cyanopyridine as desiccant. Although 20 product separation and recyclability of catalyst and dehydrating agent have been clearly 21 demonstrated, recovery of the nitrile from 2-picolinamide remains problematic for further 22 applications and is at the same time crucial to optimize the cost-benefit ratio as well as to design 23 a truly sustainable, atom-economical route for the preparation of acyclic organic carbonates. 24 New reactor technologies such as the use of flow chemistry with a clever use of recycle streams 25 may at some point in time allow to efficiently run DMC and related synthesis in continuous 26 mode at low conversion of the alcohol without the need for a dehydrating agent. 27 11 1 2. Cyclic Organic Carbonates from Saturated Alcohols 2 2.1. Synthesis of Five-membered Cyclic Carbonates 3 2.1.1 Metal based catalysts 4 From an industrial point of view, the transformation of ethylene glycol (EG) and propylene 5 glycol (PG) is of high interest. Both EG and PG are byproducts obtained in the 6 transesterification of ethylene carbonate (EC) and propylene carbonate (PC), respectively, 7 using MeOH as a reagent and with the aim to produce DMC. Thus, recycling of the EG or PG 8 by reaction with CO2 has the potential to lift the overall sustainability of DMC synthesis from 9 EC or PC (Scheme 7) [57]. 10 11 Scheme 7 Synthesis of cyclic carbonates from CO2 and the glycols EG and PG 12 13 Tomishige reported the first attempt for this transformation using a calcined CeO2ZrO2 14 catalyst system in the presence of acetonitrile [58, 59]. Conversions of 0.7 and 1.6% were 15 obtained under optimal reaction conditions using 0.36 mmol (total amount of Ce and Zr) of 16 catalyst weight, 200 mmol of CO2, 120 mmol acetonitrile, a reaction temperature of 150 °C for 17 2 h: these conditions led to the synthesis of EC and PC from EG and PG, respectively. 18 Interestingly, the EC and PC amount increased when the calcination temperature of the catalyst 19 system was increased, which resulted in lowering the amount of acid/base sites and the surface 20 area. As in the formation of linear carbonates, the reaction mediated by the CeO2ZrO2 catalyst 21 system is thermodynamically limited with a maximum conversion of 2%; removal of H2O 22 would be crucial to maximize the conversion/yield of the carbonate product. 23 Subsequently, He et al. reported two different catalyst systems such as dibutyltin 24 oxide/dibutyltin dimethoxide and low toxic magnesium and its oxide for the synthesis of PC 25 from PG and CO2 under supercritical conditions using N,N,-dimethylformamide (DMF) as 26 solvent [60, 61]. In both cases, the maximum conversion obtained for PG was <4% with 100% 27 chemo-selectivity. Also, alkali carbonates were used to catalyze the PC synthesis from PG and 12 1 CO2 with a relative high yield of 10.5% under supercritical conditions in the presence of 2 ammonium carbonate and acetonitrile [62]. Acetonitrile did not only act as a solvent here but 3 also as the dehydrating agent to eliminate the water produced during the reaction thereby 4 shifting the equilibrium towards the formation of PC. However, the hydrolysis of acetonitrile 5 may generate acetamide and can subsequently react with water to form acetic acid and 6 ammonia. Acetic acid can react with PG to produce propylene glycol-2-acetate, lowering the 7 overall selectivity towards PC. Therefore, introduction of ammonium carbonate into the 8 reaction system inhibited the hydrolysis of acetamide and improved the chemo-selectivity 9 toward PC. 10 11 Scheme 8 Synthesis of GC from Gly and CO2 12 13 The combination of its bio-based origin and wide reactivity has made glycerol carbonate 14 (GC) a versatile and renewable building block for organic chemistry. The direct carboxylation 15 of glycerol (Gly) and CO2 is a very interesting though challenging route that would convert two 16 waste materials from the chemical industry into a valuable product (Scheme 8). Mouloungui et 17 al. attempted to prepare GC under supercritical conditions but the reaction did not occur [63]. 18 Later on, Dibenedetto employed tin-based catalyst systems [n-Bu2Sn(OMe)2] and [n-Bu2SnO] 19 under solvent-free conditions using 6 mol% of catalyst [n-Bu2Sn(OMe)2], 5 MPa of CO2 20 pressure, a reaction temperature of 180 °C for 15 h to obtain a maximum conversion of 6.7% 21 [64]. Molecular sieves were introduced into the reactor to reduce the water content in the 22 homogeneous phase to favor the equilibrium towards the formation of GC. 23 Hereafter, Dibenedetto applied mixed oxide catalysts (CeO2/Al2O3 and CeO2/Nb2O5) under 24 the same reaction condition mentioned above in a biphasic system using tetra(ethylene glycol) 25 dimethyl ether (TEGDME) as solvent to obtain a maximum Gly conversion of 2.5%. The 26 catalyst was recyclable at least 3 times without any observable loss of activity [65]. Munshi et 27 al. showed that addition of methanol to the Dibendetto´s n-Bu2SnO-based catalyst system 28 enhanced the catalytic activity and as a result 30% yield for GC in 4 h at 80 °C and 3.5 MPa 29 CO2 pressure using 1 mol% of catalyst could be attained [66]. The reaction with diglyme, an 13 1 inert aprotic solvent, instead of methanol did not improve the yield of GC and also the use of 2 other alcohols such as ethanol, propanol and butanol slowed down the reaction rate. These 3 combined results indicated that the role of methanol is not just acting as a solvent but it is likely 4 also chemically involved. The proposed mechanism presumes the activation of n-Bu2SnO by 5 methanol to give n-Bu2Sn(OMe)2 which in turn reacts with Gly forming n-Bu2Sn(glycerol-2H) 6 and undergoes CO2 insertion, leading eventually to GC via a Sn(glycerolcarbonate) complex. 7 The catalytic cycle is completed by ligand exchange in the presence of methanol followed by 8 ring-closing, release of the GC product and the reformation of the Sn(OMe)2 complex species 9 as shown in Scheme 9. During the process, the monomeric species (n-Bu2Sn(glycerol-2H)) can 10 either incorporate CO2 or oligomerize causing catalyst deactivation. The addition of methanol 11 prevents the formation of an oligomeric species as it is actively involved in the formation of 12 GC whereas the water formed during reaction is removed continuously from the system. 13 14 Scheme 9 Proposed reaction path towards GC formation [66] 15 16 Similar to Tomishige´s work [58] using acetonitrile as a medium that helps to overcome the 17 thermodynamic limitation of the process, Sun and coworkers used acetonitrile as a sacrificial 18 coupling in the presence of a La2O2CO3ZnO catalyst system for the transformation of CO2 19 and Gly into GC [67]. The highest Gly conversion (30.3%) with a GC yield of 14.3% was 20 reported at 4 MPa of CO2 pressure, a reaction temperature of 170 °C after 12 h when the catalyst 21 system was calcined at 500 °C. The envisioned mechanism involves the activation of Gly by 22 Lewis acidic sites (Zn2+) forming a glyceroxide anion, and subsequently the oxygen atom of 23 the adjacent hydroxyl group attacks the zinc cation resulting in zinc-glycerolate species along 14 1 with the formation of a molecule of water which is converted to an amide reacting with 2 acetonitrile. 3 The uncalcined catalyst produces only a low yield of GC due to the higher content of crystal 4 lattice water, which favors the hydrolysis of acetonitrile (cf., amide formation) and results in 5 more byproducts. With the introduction of La2O2CO3, the amount of lattice oxygens (LaO 6 pairs) increases on the surface and leads to an increase of moderately basic sites, which in turn 7 enhance the activation of glycerol and CO2 and thereby exhibit higher catalytic activity than 8 pure ZnO. Likewise, various other catalysts systems such as Cu/La2O3 and Cu-supported 9 catalysts, Zn/Al/La and Zn/Al/La/M (M = Li, Mg and Zr) hydrotalcites, and Zn/Al/La/X (X = 10 F, Cl, Br) catalysts were also successfully employed for the direct carbonylation of Gly to 11 obtain maximum conversions of the substrate of <36% under typically harsh, supercritical 12 reaction conditions in the presence of acetonitrile [68–70]. 13 Very recently, He and co-workers obtained GC from the carbonylation of Gly and CO2 over 14 CeO2 catalysts with the hydrolysis of 2-cyanopyridine as the sacrificial dehydrating agent [71]. 15 Calcined CeO2 with three different morphologies pertinent to nanoparticles, nano-rods and 16 sponge-like nanomaterials were prepared corresponding to three different types of methods 17 being precipitation, hydrothermal treatment and sol-gel methodology, respectively. All three 18 samples showed excellent catalytic performance obtaining GC yields between 2034 % under 19 relatively mild reaction conditions (150 °C, 4 MPa, 5 h) with an activity order of nanorod 20 catalyst > catalytic nanoparticles > sponge-like catalyst. The CeO2 nano-rod type catalyst with 21 the most abundant basic sites and oxygen vacancies gave the highest yield of GC, and sponge- 22 like CeO2 with medium amount of basic sites and the least defects gave the lowest, indicating 23 that the oxygen vacancies play an important role in the catalytic system. Among various 24 dehydrating agents used, 2-cyanopyridine showed the best performance ascribed to the 25 relatively strong alkalinity and the formation of intramolecular hydrogen bonding in the 26 produced amide when the nitrile reacts with water. By optimizing the reaction conditions, the 27 GC yield could be increased to as high as 78.9% and the used catalyst could be easily 28 regenerated through the calcination process at 400 °C for 5 h, and was recycled five times 29 successfully. 30 15 1 2.1.2 Organocatalysts 2 Jang and co-workers introduced a metal-free carbonylation reaction in which alcohols are 3 converted into corresponding cyclic carbonates in the presence of an organic base and 4 dibromomethane 5 diazabicyclo[5.4.0]undec-7-ene (DBU), is assumed to deprotonate the OH unit of ethylene 6 glycol (EG), which would render it more nucleophilic. DBU is also known to form an adduct 7 with CO2, and can in this way increase the nucleophilic character of CO2 favoring reaction with 8 EG. In the presence of 2 equivalents of DBU at 0.5 M of CH2Br2 and at 0.5 MPa/70 ºC, EG 9 (0.5 mmol) was converted into EC (24% after 18 h). In the presence of an ionic liquid (IL: 10 bmimPF6), the EC-yield was enhanced to 54% and under higher CO2 pressure (1 MPa) it was 11 further improved to 74%. (CH2Br2) as the solvent [11]. The organic base, 1,8- 12 ILs are well-known to increase the solubility of CO2 in the reaction media. Under the 13 optimized reaction conditions, various other alcohols were also tested as substrates and it was 14 found that Gly underwent good conversion obtaining a GC yield of 86%. Similarly, other 15 methyl- and phenyl-substituted ethylene diols underwent smooth conversion to their cyclic 16 carbonates to afford yields of 6779%. A cyclic diol was also tested and displayed an excellent 17 yield of 73%. A set of additional experiments was performed to understand the operating 18 mechanism in these diol/CO2 coupling reactions. First, an 19 conducted with mono-18O-labeled styrene glycol and a 70% yield of the styrene carbonate was 20 achieved, where the 21 carbonate had the 18O-labeled atom incorporated. Second, the use of optically active (S)-styrene 22 glycol was examined, and the resultant styrene carbonate was analyzed by HPLC showing full 23 retention of the initial configuration. 18 O label did not exchanged with 16 18 O-labeling experiment was O during the reaction, i.e. the cyclic 24 The mechanism of the aforementioned reaction is shown in Scheme 10 in which the primary 25 alcohol first attacks the DBUCO2 adduct to form the carbonate intermediate I, which then 26 reacts with CH2Br2 to form reactive carbonate II. Then the intermolecular attack of the (pre- 27 activated) secondary alcohol unit onto the carbonate followed by the elimination of HOCH2Br 28 affords the styrene carbonate product. The intramolecular addition of the carbonate nucleophile 29 in intermediate III likely is not competitive under these conditions as supported by the 30 experimental result obtained using (S)-styrene glycol. 31 16 1 2 Scheme 10 Plausible mechanism for cyclic carbonate formation from styrene glycol and CO2 3 4 3. Formation of Six-membered Cyclic Carbonates 5 6 3.1 Metal based catalysts 7 The pioneering group of Tomishige has successfully performed various carboxylation reactions 8 of diols and CO2 to afford their cyclic carbonates. Recently, they employed the privileged CeO2 9 catalyst in combination with 2-cyanopyridine as dehydrating agent to yield various five- and 10 six-membered cyclic carbonates from CO2 and diols [52]. From a series of different metal 11 oxides combined with 2-cyanopyridine for the synthesis of PC from PG and CO2, CeO2 was 12 shown to be more active by 2 orders of magnitude compared with other metal oxides. 2- 13 Cyanopyridine was preselected as nitrile for its exceptional reactivity towards hydration 14 forming 2-picolinamide. Without the addition of 2-cyanopyridine, the PC yield was as low as 15 0.3% due to the unfavorable thermodynamics. Addition of 100 mmol of 2-cyanopyridine 16 provided an excellent PC yield of >99% (chemo-selectivity >99%) in just 1 h using 20 mol% 17 catalyst (CeO2), at 130 °C/5 MPa CO2 and this is the highest yield of PC from CO2 and PG to 18 date. 19 To extend the synthetic potential of this catalyst system, synthesis of six-membered ring 20 carbonates was carried out by employing various 1,3-diols with monoalkyl-, dialkyl-, and 21 phenyl-substitutions and generally the corresponding cyclic carbonates were obtained in high 22 yields of 6297% with good to excellent chemo-selectivity (7799%). The ester that is formed 23 by reaction of the diol starting material with 2-picolinamide in situ produced was spotted as the 24 major byproduct. Syntheses of six-membered ring carbonates, especially those having multiple 17 1 substituents, are difficult to realize using any methodology, despite the fact that they represent 2 useful chemicals and intermediates for, inter alia, biodegradable polymers for drug delivery 3 systems. The results obtained for this Ce-based catalyst system mediating the synthesis of 4 various six-membered carbonates are highly attractive compared to other methodologies 5 reported to date, except for the non-substituted trimethylene carbonate derived from oxetane 6 and CO2, for which Kleij et al. [72] reported a very high yield (95%) using a simple though 7 highly reactive Al-catalyst based on amino-triphenolate ligands. 8 After the reaction, the CeO2 catalyst was removed from the reaction mixture and the filtrate 9 was analyzed by coupled plasma atomic emission spectroscopy (ICPAES), which indicated 10 that no Ce species had leached into the filtrate (<0.1 ppm). The catalyst was therefore 11 successfully reused for three times without any loss of its high selectivity and yield; the BET 12 surface area and X-ray diffraction pattern of the CeO2 material before and after the reaction 13 remained virtually unchanged and thus indicated that the CeO2 catalyst is highly stable under 14 the experimental conditions. The mechanistic details are similar to the ones already discussed 15 for the formation of acyclic carbonates in section 1. Overall, this catalyst system shows the best 16 yields for cyclic carbonate synthesis (five- and six-membered ones) from diols and CO2 17 reported to date. 18 19 3.2 Organocatalysts 20 Buchard and co-workers [73] performed the synthesis of six-membered cyclic carbonates 21 directly from various 1,3-diols and CO2 at room temperature, 0.1 MPa of CO2 using standard 22 reagents. First, the selective mono-insertion of CO2 into one of the OH bonds of 1,3-butanediol 23 in various solvents was examined in the presence of DBU as catalyst. After the selective 24 formation of the mono-carbonate intermediate at low concentration, 1 equivalent of tosyl 25 chloride/triethylamine was added to the reaction mixture and stirred at room temperature. The 26 pure targeted product was isolated in an appreciable yield of 44%, which was increased to 68% 27 when a higher concentration of diol (going from 0.1 to 1.7 M) was applied. Investigation into 28 the scope of diol substrates revealed that various 1,3-diols were good reaction partners in this 29 organocatalytic approach, and the corresponding six-membered cyclic carbonates were isolated 30 in low to good yields (1170%). After initial insertion of CO2, into one of the OH bonds, 31 tosylation of the carbonate species or the remaining alcohol function can be envisaged. 18 1 Hereafter, the cyclization proceeds via either an addition/elimination sequence or an SN2 2 pathway, leading to retention or inversion of stereochemistry, respectively. 3 However, the exclusive formation and isolation of the (R,R)-configured cyclic carbonate 4 from (R,R)-2-4-pentanediol (yield: 53%) as well as the optical activities of the cyclic carbonates 5 obtained from enantiopure (R)- and (S)-1,3-butanediol, clearly indicated a preference for the 6 addition/elimination pathway (Scheme 11), with no observable racemization or inversion of 7 stereochemistry, which was further supported by DFT calculations. 8 9 10 11 Scheme 11 Preferred pathway for the conversion of (R)-1,3-butanediol into its six-membered carbonate [73] 12 13 4. Cyclic Carbonates derived from Unsaturated Alcohols 14 4.1 Metal based catalysts 15 Reaction of CO2 with propargylic alcohols typically affords α-alkylidene cyclic carbonates 16 through a carboxylative cyclization process (Scheme 12), which is also a promising and green 17 route to convert CO2. Moreover, α-alkylidene cyclic carbonates possess a wide range of 18 applications in organic synthesis being for instance building blocks in the formation of α- 19 hydroxy ketones and 5-methylene-oxazolidin-2-one derivatives. 20 Inoue et al. [74] performed the Pd(0)-catalyzed [Pd(PPh3)4] carboxylative cyclization 21 reaction of CO2 (1 MPa) with sodium 2-methyl-3-butyn-2-olate (prepared from the 22 corresponding alcohol and a slight excess of NaH) and iodobenzene in THF as solvent at 100 23 °C and obtained the cyclic vinylidene carbonate in 68% yield. Alternatively, the use of copper 24 catalysis proved to be highly beneficial to further develop this type of reaction. For instance, a 25 cationic copper complex derived from 2,5,19,22-tetraaza[6,6](1,1')ferrocenophane-l,5-diene 26 was an effective catalyst operating under 3.8 MPa of CO2 and 100 °C affording good yields of 19 1 the cyclic carbonates (>90%) using various substituted propargylic alcohols under neat 2 conditions [75]. Similarly, CuCl in the presence of the IL [BMIm][PhSO3] yielded the α- 3 alkylidene cyclic carbonate in 97% yield under a milder CO2 pressure of 1 MPa at 120 °C [76]. 4 5 6 Scheme 12 Conversion of CO2 into an α-alkylidene cyclic carbonate with an exo-cyclic double bond using propargylic alcohols 7 8 Substantial improvement in activity was reported for metal based catalyst systems when 9 Mizuno et al. communicated a tungstate based complex (i.e., TBA2[WO4], TBA = 10 tetrabutylammonium) as an efficient homogeneous catalyst for conversion of CO2 with 11 propargylic alcohols to give the corresponding cyclic carbonates under mild reaction conditions 12 [77]. DFT calculations allowed to optimize the tungstate structure and the basicities of oxygen 13 atoms in various polyoxometalates (POMs) were compared with the natural bond orbital (NBO) 14 charges; the simple [WO4]2- tungstate was found to be the most basic among the series. Under 15 only 0.1 MPa of CO2 pressure and at a relatively low temperature of 60 °C, propargylic alcohol 16 (2-methylbut-3-yn-2-ol) was coupled with CO2 in acetonitrile to give the corresponding 17 carbonate structure in 76% yield. Upon lowering the catalyst loading while increasing the CO2 18 pressure (2 MPa) and reaction time, the total turnover number could be enhanced to a significant 19 473 with a yield of 95%. In a similar way, more lethargic substrates such as propargylic alcohols 20 with internal triple bonds (cf., 1-ethynylcyclohexan-1-ol) were also effectively transformed to 21 their cyclic carbonates in excellent yield of up to 95%. 22 Having witnessed the enhancement in activity brought about by the [WO4]2- anion in 23 carbonate formation under much milder conditions, Song and co-workers [78] employed a 24 Ag2WO4/Ph3P dual catalyst system for the conversion of CO2 and propargylic alcohols to 25 provide the α-alkylidene carbonates under solvent-free conditions. In this dual catalyst system, 26 the [WO4]2- anion was envisioned to activate both CO2 and the propargylic alcohol generating 27 a carboxylate intermediate which could then be intercepted by Ag-activated C≡C triple bond to 28 afford the product (Scheme 13). 20 1 2 Scheme 13 Chemical fixation of CO2 through the dual activation pathway [78] 3 4 Using this system at 1 mol% of catalyst loading, 25 °C and 0.1 MPa of CO2 pressure an 5 optimum yield of 96% was obtained. Subsequently, other substrates were examined with this 6 dual catalyst and propargylic alcohols with alkyl and aryl substituents at the propargylic 7 position were also effective substrates to give the corresponding cyclic carbonates in good to 8 excellent yields. The method was, however, not effective for 1-isopropyl and 1,1- 9 cyclopentylene substituted substrates which showed (very) low conversions due to a 10 combination of steric hindrance and ring strain. Secondary propargylic alcohols failed to form 11 the carbonate product, while the internal propargylic substrate 2-methyl-4-phenylbut-3-yn-2-ol 12 required both higher CO2 pressure (1 MPa) and temperature (80 °C) to afford the corresponding 13 cyclic carbonate in good yield (82%) using longer reaction times (36 h). This catalyst system 14 was easily separated by extraction with hexane and after drying the catalyst could be 15 successfully recycled four times without observable loss of activity. 16 At room temperature and CO2 pressures <2 MPa, AgOAc/DBU [79] and F-MOP-3-Ag/ 17 DBU catalyst systems (F-MOPs = fluorinated microporous organic polymers having Ag(I) sites 18 incorporated) [80] in toluene gave good results. Notably, both catalyst systems were successful 19 in converting various terminal and bulky internal propargylic alcohols to their corresponding 20 cyclic carbonates in good yields. Heterogeneous metal systems have also been reported as 21 effective catalysts for α-alkylidene carbonate formation. Important limitations for these 22 supported catalyst systems, though, were primarily the required supercritical conditions and 23 high catalyst loadings and, moreover, a limited substrate scope allowing only the conversion of 24 terminal propargylic alcohols [81, 82]. Significant improvement of activity and reusability 25 features of heterogeneous metal catalysts was reported by Liu and co-workers [83] who used 26 porous organic polymers (POPs) as a solid support. This material allows the introduction of 27 various CO2-philic functional species inside its structure to obtain a more active, functional and 28 reusable catalyst. Specifically, the authors prepared porous poly(triphenylphosphine) with azo 21 1 (R‒N=N‒R) functionalities (i.e., a poly(PPh3)-azo material) with the Ag sites being coordinated 2 by the phosphine ligands. This system was efficiently used for CO2 transformations taking 3 advantage of cooperative effects between the functional porous polymer and the metal species. 4 The poly(PPh3)-azo-Ag/DBU catalyst converted at room temperature and a CO2 pressure of 1 5 MPa the benchmark propargylic alcohol 2-methylbut-3-yn-2-ol to its corresponding cyclic 6 carbonate with a yield of 56% in 3 h; a higher yield was achieved by further increasing the 7 reaction time to 18 h (>99%) with a remarkable total TON of 1563. The presence of a high local 8 concentration of PPh3 ligands and azo functionalities in the polymer (PPh3/Ag = 200:1, azo/Ag 9 = 300:1) facilitated cooperative effects towards the formation of α-alkylidene compounds. 10 Moreover, the catalyst system was also shown to be recyclable at least five times without loss 11 of activity. After several uses, transmission electron microscopic (TEM) analysis of the 12 catalytic material indicated that the metallic Ag particles were still highly dispersed without 13 changes in particle size after recycling. In order to establish possible leaching, the catalyst was 14 separated through centrifugation after the reaction performed for 1 h, and the filtrate was 15 analyzed by ICP-OES which demonstrated that there was no observable leaching of Ag species 16 (<10 ppb). Then, various terminal propargylic alcohols with both alkyl and aryl substituents 17 were examined as substrates and these reacted efficiently with CO2, though substrates 18 comprising bulky isopropyl groups or small rings required longer reaction times to obtain good 19 yields presumably due to steric effects. 20 The proposed mechanism for the formation of α-alkylidene cyclic carbonates mediated by 21 the poly(PPh3)-azo-Ag catalyst system is shown in Scheme 14 [83]. First, the propargylic 22 alcohol, activated by DBU, reacts with pre-activated CO2 to generate a carbonate intermediate. 23 An intramolecular ring-closing step is then followed by proto-demetallation to afford the 24 corresponding cyclic carbonate with the regeneration of the active Ag species and DBU. 25 Similarly, He et al. [84] used [(PPh3)2Ag]2CO3 (in situ formed from Ag2CO3 and PPh3) as a 26 robust and highly efficient single-component bifunctional catalyst for the coupling of 27 propargylic alcohols and CO2 at room temperature and atmospheric pressure. After only 2 h, 2- 28 methylbut-3-yn-2-ol and CO2 were converted to the cyclic carbonate in 93% yield under 22 1 2 3 Scheme 14 Proposed mechanism for the coupling reaction of propargylic alcohols with CO2 catalyzed by poly(PPh3)-azo-Ag/DBU [83] 4 5 neat conditions using the in situ prepared catalyst. The recovered precipitate after the reaction 6 could be reused in a subsequent carboxylative cyclization cycle. 7 Ag nanoparticles (NPs) have been immobilized to other solid supports such as sulfonated 8 macro-reticular resins (SMRs) forming an active AgNPs/SMR catalyst [85]. Alternatively, Ag 9 halides were supported on a porous carbon material (AgX@C, X = Cl, Br and I) [86] and the 10 resultant catalyst was effective at room temperature and atmospheric pressure in the 11 carboxylative cyclization of the propargylic alcohol 2-methylbut-3-yn-2-ol and CO2 in the 12 presence of DBU as co-promoter. The use of the AgNPs/SMR catalyst gave a 91% yield of the 13 cyclic carbonate after 10 h in DMF, and the catalyst was shown to be recyclable at least five 14 times. The AgI@C catalyst furnished the product in 99% yield within 4 h in acetonitrile, and 15 was used in a total of 10 cycles without any significant loss in activity. 16 Interestingly, a zinc salt (ZnI2) in combination with triethylamine (NEt3) was recently 17 proposed as a catalyst system in this area and showed excellent synergistic effects to promote 18 the solvent-free reaction of CO2 and 2-methylbut-3-yn-2-ol under a CO2 pressure of 1 MPa and 19 at 30 °C to obtain 4,4-dimethyl-5-methylene-1,3-dioxolan-2-one in 95% yield after 10 h [87]. 20 These results are promising to devise new catalyst systems based on earth-abundant and cheap 21 metals such as Zn. 22 23 1 4.2 Organocatalysts 2 As CO2 prevalently behaves as an electrophile, strong Lewis bases based on nitrogen 3 heterocycles have the potential to activate CO2 affording zwitterionic adducts. Thus, such 4 organocatalytic promotors can be utilized as convenient and cheap CO2 transformers to 5 accomplish its conversion by increasing its nucleophilic character. N-Heterocyclic carbenes 6 (NHCs) incorporating electron-donating heteroatoms have a strong basic character and this 7 enables strong σ-donor ability of the NHC useful for CO2 activation. Imidazolium-2- 8 carboxylates (with the integrated CO2 molecule in an activated state) have been 9 spectroscopically and structurally identified as NHC‒CO2 adducts. Ikariya and co-workers [88] 10 prepared NHCs (1,3-dialkylidazol-2-ylidenes) and their corresponding CO2 adducts (1,3- 11 dialkylimidazolium-2-carboxylates), and employed them as efficient catalysts for cyclic 12 carbonate synthesis using propargylic alcohols and CO2. 13 The NHC‒CO2 adducts showed comparatively superior activity under milder conditions 14 than the NHCs themselves, which required supercritical conditions to obtain good results for 15 the conversion of 2-methyl-3propyn-2-ol in the carboxylative cyclization with CO2 to form the 16 corresponding cyclic carbonate. Under solvent-free conditions at 4.5 MPa CO2 and 60 °C, 17 various NHC‒CO2 adducts, prepared by variation of the N‒substituents (di-isopropyl, di-tert- 18 butyl, diaryl) were tested for their catalytic activity. The NHC‒CO2 adduct 1,3-di-tert- 19 butylimidazolium-2-carboxylate gave the best yield (99%) for the cyclic carbonate product 20 among the adducts tested. If the catalyst loading, CO2 pressure and/or temperature were 21 lowered, a significant amount of an acyclic product (1,1-dimethyl-2-oxo-propyl-1´,1´- 22 dimethyl-2´-propynyl carbonate) was obtained along with the desired cyclic carbonate. The 23 carboxylative cyclization affording the cyclic carbonate and the subsequent addition of another 24 propargylic alcohol to the product is thought to lead to this 2:1 coupling product of both 25 substrates. 26 Various five-membered cyclic carbonates were prepared in good yields from different 27 propargylic substrates having disubstituted alkyne groups using the NHC‒CO2 catalyst. The 28 presence of electron-withdrawing groups conjugated to the triple bond in the substrate led to 29 the targeted products in faster rates and at lower reaction temperatures. The NHC‒based catalyst 30 also tolerates propargylic substrates equipped with heterocycles such as pyridine and thiophene, 31 whereas allylic compounds such as 2-methyl-3-buten-2-ol and 2-methyl-4-phenyl-3-buten-2-ol 32 did not give any cyclization product. In each product, the C=C double bond was found to have 33 a (Z)-configuration, indicating that the addition of the NHC‒carboxylate to the alkyne fragment 24 1 proceeded predominantly in an anti fashion. The postulated mechanism for the NHC‒CO2 2 mediated carboxylative cyclization of propargylic alcohols and CO2 involves the nucleophilic 3 addition of the imidazolium-2-carboxylate to the C≡C triple bond and subsequent 4 intramolecular cyclization of the alkoxide intermediate (Scheme 15). A significant positive 5 effect of electron-donating N‒alkyl substituents present in the NHC structure implies that the 6 intramolecular nucleophilic attack of the CO2 moiety, once bound to the NHC, onto the 7 substrates may be rate-limiting step in this catalytic cycle. 8 9 10 Scheme 15 Mechanism of the carboxylative cyclization catalyzed by a NHC‒CO2 adduct [88] 11 12 N-Heterocyclic olefins (NHOs) are compounds that are capable of further stabilizing the 13 positive charge that arises upon activation of CO2 due to aromatization of the heterocyclic ring 14 thereby making the terminal carbon atom of the initial olefin group more electronegative and 15 susceptible towards the activation of electrophilic reaction partners. Recently, Lu et al. [89] 16 prepared various NHO‒CO2 adducts and employed them as catalysts for coupling of 17 propargylic alcohols and CO2 to yield α-alkylidene carbonates. For comparative reasons, NHC‒ 18 CO2 adducts were also prepared and in situ decarboxylation experiments monitored by IR 19 spectroscopy in CH2Cl2 at 40 ºC revealed that decarboxylation of the NHO systems occurred 20 within 2 h, whereas only small amounts of NHC adducts decomposed under similar conditions. 21 This demonstrates that the decarboxylation of NHO‒CO2 adducts is relatively easy and 22 therefore of more practical use for cyclic carbonate synthesis. The relatively poor thermal 23 stability of NHO‒CO2 adducts therefore offers an opportunity to use these compounds as active 24 catalysts for CO2 transformations at low temperature. In the catalyst activity screening phase, 25 1 2-methyl-4-phenylbut-3-yn-2-ol was chosen as a model substrate and reacted with CO2 at 2 2 MPa pressure and 60 °C for 12 h. Among the various NHO-CO2 adducts, the isopropyl- 3 substituted NHO-CO2 adduct showed the best results leading to a 93% isolated yield of the 4 cyclic carbonate. The difference in catalytic activity observed for the NHO‒CO2 adduct in 5 comparison with its corresponding NHC‒CO2 was established for various propargylic 6 substrates, and typically the NHO adducts are about 10‒100 times more active than their NHC 7 analogues. Various terminal and internal propargylic alcohols smoothly underwent the 8 carboxylative cyclization reaction and were converted into their corresponding α-alkylidene 9 cyclic carbonates in moderate to excellent yields. Apparently only a slight structural difference 10 exists between the NHO and NHC adducts; the much higher reactivity for the NHO‒CO2 11 adducts was tentatively ascribed to the lower stability of the Ccarboxylate – CNHO bond. 12 13 14 Scheme 16 Proposed mechanism of the carboxylation reaction catalyzed by the NHO‒CO2 adduct [89] 15 16 The proposed mechanistic manifold begins with the zwitterionic compound NHO−CO2 that 17 adds to the triple bond of propargylic substrate through nucleophilic attack. Meanwhile, 18 hydrogen transfer of alcohol generates the new zwitterion Ia (Scheme 16, path A), and then the 19 alkoxide anion attacks the carboxylate carbon to release the desired product and regenerating 26 1 the NHO, which rapidly captures free CO2 to form the NHO−CO2 adduct to induce further 2 turnover. The higher thermal instability of the NHO-CO2 adducts favorably adds to the overall 3 kinetics of the reaction, thus creating higher turnover at lower temperatures as compared with 4 the reactivity of analogous NHC-CO2 adducts. The NHO with increased electronegativity at 5 the terminal carbon atom can also act as a Brønsted base able to abstract a proton from the 6 propargylic alcohol to form the intermediate IIa (path B) which subsequently reacts with CO2 7 to give intermediate IIb (Scheme 16, path B). Subsequently, the intermediate IIc is obtained 8 by intramolecular ring-closure within intermediate IIb, which abstracts a proton from the 2- 9 methyl imidazolium cation to release the desired product. The obvious difference between both 10 pathways A and B is that the hydrogen at the alkenyl position of cyclic carbonate originates 11 exclusively from the propargylic substrate (path A) or both substrate and catalyst (path B): this 12 aspect may be elucidated by a proper labeling of the NHO and/or propargylic substrate. 13 The same group also prepared various CO2 adducts of alkoxide-functionalized imidazolium 14 betaines (abbreviated as AFIBs) and explored the AFIB‒CO2 adducts as effective 15 organocatalysts within the context of carboxylative cyclization of propargylic alcohols with 16 CO2 [90]. The best result (97% yield) for the AFIB‒CO2 mediated formation of the cyclic 17 carbonate product was obtained under 2 MPa pressure at 60 °C using 2-methylbut-3-yn-2-ol as 18 substrate. The catalyst system proved to be more effective for the carboxylative cyclization of 19 terminal rather than internal propargylic substrates. 20 Minakata et al. [91] treated various allylic alcohols with stoichiometric tBuOI under 0.1 MPa 21 of CO2 pressure and low reaction temperature resulting in the synthesis of five-membered cyclic 22 carbonates containing a potentially useful alkyliodide group. The reagent, tBuOI, can be readily 23 prepared in situ from commercially available tert-butyl hypochlorite (tBuOCl) and sodium 24 iodide (NaI), and serves to iodinate an elusive and rather unstable alkyl carbonic acid that is 25 first generated from CO2 and an unsaturated alcohol. The introduction of the iodine atom 26 radically changes the position of the equilibrium of the initial CO2-trapping reaction (Scheme 27 17). The use of tetrahydrofuran (THF) as solvent and a reaction temperature of ‒20 °C resulted 28 in the conversion of prop-2-en-1-ol into the corresponding cyclic carbonate in 92% yield. To 29 further investigate the efficacy of tBuOI, other iodinating reagents such as bis(pyridine)iodine 30 tetrafluroborate (IPy2BF4), N-iodosuccinimide (NIS), I2 and a combination of I2 and 31 triethylamine were tested, but all these reagents failed to provide the desired product. 32 The main reason for tBuOI being the most appropriate iodinating reagent is related to the 33 liberation of a relatively weak acid (i.e., tBuOH) during the reaction of allyl-carbonic acid and 27 1 tBuOI. -Branched allylic alcohols also could be smoothly transformed into their corresponding 2 cyclic carbonates in good yields. Both (E)- and (Z)-allylic alcohols were transformed into their 3 corresponding cyclic carbonates. Allyl alcohols containing rigid cyclic olefins, hydroxyl, ester 4 or silyl groups, and homo-allylic alcohols allowing the formation of six-membered cyclic 5 carbonates were also compatible with this CO2 conversion reaction. Similarly, various internal 6 propargylic alcohols reacted with CO2 under similar conditions and permitted the synthesis of 7 the corresponding iodoalkyl derived carbonates in good yields. 8 9 10 11 Scheme 17 (a) Reaction of tert-butyl hypoiodite with weak acids. (b) Strategy for trapping carbonic acids with tert-butyl hypoiodite [91] 12 13 Recently, an efficient carboxylation/alkene functionalization reaction of homoallylic alcohols 14 was reported by Johnston et al. to produce chiral cyclic carbonates (Scheme 18) [92] using an 15 approach similar to the one reported by Minakata (vide supra). At low temperatures and 16 ambient pressure, a toluene solution of 3-phenylbut-3-en-1-ol was treated with CO2/N- 17 iodosuccinimide in the presence of various bases including NaH, DBU, TBD, DMAP as well 18 as hydrogen bond donors such as TFA and thiourea but these conditions failed to deliver the 19 desired carbonate product or gave only rise to low yields. A Brønsted acid/base combination 20 was then explored to promote the reaction and the use of a chiral pyrrolidine-substituted 21 bis(amidine) gave a promising 18% yield of the iodinated cyclic carbonate in 39% ee. An 22 analogous catalyst incorporating trans-stilbene diamine (StilbPBAM; Scheme 18) instead of 23 trans-cyclohexane diamine provided the product in 33% yield and 36% ee. Exploration of 24 strong Brønsted acid additives (HNTf2) (1 equiv) combined with the StilbPBAM organocatalyst 25 enhanced the activity to provide a 95% yield (91% ee) in the presence of molecular sieves (4 26 Å). Combined, the results suggest an important role for hydrogen-bonding in the key 27 selectivity-determining step, and a unique reactivity associated with the proper mutual 28 positioning of the Brønsted acid and base in the relevant transition state that controls the 28 1 asymmetric induction. Various other substituted styrene homoallylic alcohols were also tested: 2 -naphthyl substituted anisole derivatives (meta- and para-substituted), and halogen substituted 3 substrates were all converted into their six-membered carbonates with excellent 4 enantioselectivity and in good yields, whereas substitution near the alkene moiety was not 5 tolerated and no conversion was noted in these cases. 6 7 8 Scheme 18 An enantioselective method for the synthesis of cyclic carbonates from homoallylic alcohols and CO2 using a chiral StilbPBAM organocatalyst [92] 9 10 5. Cyclic Carbonates from Halo-Alcohols 11 Another approach for the preparation of cyclic carbonates from CO2 could be the use of 12 halohydrins (haloalcohols) as potential starting substrate in presence of a base. The obvious 13 drawback of this approach is the stoichiometric amount of halide waste that is produced 14 alongside, but in certain cases it may provide an alternative if other methods fail to deliver the 15 desired organic carbonate structure. 16 Using PEG-400 as an environmentally friendly solvent and K2CO3 as an easily accessible 17 base, various cyclic carbonates (yield 72-100%) were prepared by the group of Wu starting 18 from vicinal halohydrins such as chlorohydrin, bromohydrin, and phenyl and alkyl-substituted 19 halohydrins at low CO2 pressure (2 MPa) and temperature (50 °C) [93]. The choice of PEG- 20 400 as a reaction medium is beneficial in terms of solvation of the potassium cation to increase 21 the basicity of K2CO3, an increase in CO2 concentration in this specific medium accelerating 22 the reaction, and the ease of product separation. Similarly, Zhang and co-workers [94] reported 29 1 on the utilization of 1.1 equiv. of Cs2CO3 as base, obtaining both five- and six-membered cyclic 2 carbonates in good to excellent yield under relatively mild conditions (40 °C under 0.1 MPa 3 CO2, Scheme 19). By variation of different solvents and bases, the optimal conditions were 4 determined (Cs2CO3, DMF) and the haloalcohol substrate 3-chloro-1-propanol reacted 5 efficiently with CO2 giving 95% yield of trimethylene carbonate (TMC). One advantage of this 6 method, which seems a general approach if the haloalcohol is readily available, is the easy 7 formation of larger-ring size cyclic carbonates which remains a synthetic challenge in the area 8 of organic carbonates. The mechanism of this reaction is pretty straightforward with the Cs2CO3 9 first deprotonating the alcohol resulting in the formation of a cesium alkoxide. The latter in turn 10 reacts with CO2 to form a carbonate intermediate which in the final step undergoes an 11 intramolecular ring closing reaction affording CsCl as a byproduct and TMC. 12 13 14 15 Scheme 19 Reaction manifold for the formation of TMC using 3-chloro-1-propanol as the starting material and Cs2CO3 as base [94] 16 17 Even more recently, Buchard and coworkers developed a related protocol for the formation 18 of six-membered cyclic carbonates starting from 1,3-diols at low (0.1 MPa) CO2 pressure and 19 using DBU [95]. The procedure involves the activation of both the alcohol functions of the 20 substrate by tosyl chloride and DBU allowing the in situ formation of a pseudo haloalcohol. 21 This intermediate is then easily converted in the presence of NEt3 to the desired cyclic 22 carbonate. DFT analysis revealed that the mechanism most likely goes through an 23 addition/elimination sequence with intermediate formation of a tosylated carbonate species, and 24 subsequent attack of the other activated alcohol (by NEt3) onto this carbonate fragment 25 releasing the product with retention of configuration as was indeed experimentally observed for 26 various chiral substrates. As for the aforementioned methodologies, stoichiometric amounts of 30 1 DBU-HCl and TsOHNEt3 are produced, and some optimization regarding the atom-economy 2 will still be required. 3 4 6. Conclusions and Outlook 5 This overview of the latest developments in the area of cyclic carbonate synthesis from alcohol 6 substrates and CO2 shows several advances made over the last 510 years with a major focus 7 on the more recent achievements. Since the direct conversion of alcohols in the presence of CO2 8 is thermodynamically limited and only low equilibrium yields can be attained, several catalytic 9 processes have been developed to circumvent this issue by using dehydrating agents. This 10 approach has resulted in the high yield synthesis of both cyclic as well as acyclic carbonates in 11 good yields, though a crucial feature to optimize remains the regeneration of the dehydrating 12 species. If this agent can be efficiently recycled then such a process would be extremely useful 13 for larger scale preparation of carbonates, and commercial exploitation. Apart from bypassing 14 the thermodynamic limitations, other approaches that use more functional substrates such as 15 propargylic alcohols have also been proven to be effective, and the carboxylative cyclization 16 reaction is now a valuable tool in organic synthesis. Similar types of activation protocols where 17 the initial alcohol function can first react with CO2 to form a linear carbonate followed by 18 intramolecular attack onto a pre-activated alkyne fragment would be welcome in order to design 19 new conversions and amplify the role of mono- and polyalcohols as suitable platform molecules 20 in organic synthesis. Several useful protocols towards five- and six-membered carbonate 21 synthesis in the presence of stoichiometric amounts of often simple and cheap reagents have 22 already been developed. However, to answer to ever-growing need for more sustainable 23 manufacturing of bulk and fine chemicals, new catalysis protocols are warranted to address this 24 feature more effectively. 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