Oxidative carboxylation of olefins with CO2: environmentally benign access to five-membered cyclic carbonates
The purpose of this focus review is to provide a comprehensive overview of the direct synthesis of five-membered cyclic carbonates via oxidative carboxylation of the corresponding olefins and carbon dioxide with particular attention on the mechanistic features of the reactions. The review is divided into two main sections. The first section is a discussion of the single-step reactions, while the second consists of an overview of one-pot, two-step sequential reactions.
1. IntroductionThe use of carbon dioxide (CO2) as a building block for the synthesis of high-value fine chemicals is receiving growing interest in the context of sustainable and green chemistry because it is a cheap, plentiful, non-toxic, non-flammable and renewable compound.1–10 However, due to its high thermodynamic stability and chemical inertness, the efficient fixation of this greenhouse gas into chemical products is a challenging issue as decarboxylation reactions can readily occur.11 A strategy to overcome the low reactivity of CO2 is based on the reactions with high free energy substrates.12 The cycloaddition of CO2 and energy-rich epoxides stands as one of the most attractive and efficient strategies for CO2 conversion and utilization.13 This reaction is exothermic and generates five-membered cyclic carbonates, which are extremely useful solvents in the chemical industries, electrolytes in Li-ion batteries, and intermediates in various organic and polymer synthesis.14 Interestingly, this synthetic strategy is one of the few industrially relevant reactions utilizing CO2.15 Nevertheless, most epoxides are toxic and mutagenic and there is evidence that they cause cancer.16 Recently, the direct synthesis of cyclic carbonates from alkenes and CO2, so-called one-pot “oxidative carboxylation”, have attracted considerable attention owing to the lower cost and greater availability of alkenes as compared to the corresponding epoxides.17 The strategies for this appealing synthetic approach can be typically classified into two categories: (i) one-pot, two-step sequential reaction consisting of oxidation followed by carboxylation; and (ii) direct oxidation and carboxylation in just a single-step.18 In 2011, Sun and colleagues highlighted these reactions in their interesting review paper entitled “Direct synthetic processes for cyclic carbonates from olefins and CO2”.19 In connection with our recent review articles on CO2 fixation reactions,20 herein, we will try to provide a comprehensive and updated overview of recent developments in the synthesis of five-membered cyclic carbonates through the oxidative carboxylation of olefins with CO2 (Fig. 1). The review is divided into two major sections. The first section focuses exclusively on single-step conversion of alkenes to cyclic carbonates, while the second covers stepwise processes. The sections were classified based on the type of catalysts (e.g., metal-catalyzed reactions, metal-free reactions, electrocatalytic reactions) which may help a good understanding of the reaction mechanisms.
2. Single-step approaches
2.1. Metal-catalyzed reactionsSynthesis of cyclic carbonates through the metal-catalyzed oxidative carboxylation of olefins has experienced considerable growth in recent years as evidenced by the number of published articles. One of the earliest reports describes the usefulness of metal catalysts for this conversion was published by Aresta and colleagues in 2000,21 who showed that the reaction of styrene 1 with CO2 (45 atm) in the presence of molecular oxygen as an oxidant and a catalytic amount of Nb2O5 in DMF at 120 °C afforded the corresponding styrene carbonate 2 in only 5% along with styrene oxide, benzaldehyde, and benzoic acid side products (Scheme 1). Although only one poor yield example was disclosed, this paper represents the first example of a rare-earth metal-catalyzed synthesis of carbonates from olefins and CO2. Noteworthy, other metal oxides such as ZnO, Fe2O3, MoO3, Ta2O5, La2O5, and V2O5 as well as silica, alumina and molecular sieves were also found to promote this CO2 fixation reaction; albeit, in lower yields. It should also be mentioned that previously the same authors demonstrated the usefulness of rhodium catalysts for this reaction.22 The authors suggested that this reaction may proceeds through the formation of styrene oxide intermediate via oxidation of CC bond with O2 followed by cycloaddition with CO2. Subsequently, they found that the reaction temperature had a significant impact on the success of this transformation.23 Slightly increasing the temperature considerably increased the reaction rate. For example, performing the coupling of CO2 with styrene oxide at 135 °C, in the presence of above-mentioned catalytic system, gave the styrene carbonate in up to 80% yield, while for reaction at 110 °C, the desired product was obtained in only 11.2% yield. A similar example of the oxidative carboxylation of olefins by a metal catalyst was disclosed by Srivastava, Srinivas, and Ratnasamy in 2003, when they reported about the use of titanosilicate molecular sieve (TS-1; Si/Ti = 36) and DMAP combination to promote the reaction of olefins (i.e., allyl chloride, styrene) with CO2 (6.9 atm) in the presence of H2O2 as an epoxidizing agent in acetone.24 Although the reactions were carried out under a relatively mild condition, the product yields were still poor. In 2005, Arai’s research team improved the efficiency of this reaction in the terms of yield (42%), temperature (80 °C) and time (4 h) by utilizing Au/SiO2–ZnBr2–TBAB (tetra-n-butylammonium bromide) combination as a catalytic system in the absence of any organic solvent.25 They suggested that Au/SiO2 was active for the epoxidation of styrene, and ZnBr2 and TBAB cooperatively catalyzed the subsequent cycloaddition of CO2 to the in situ generated epoxide.
2.2. Metal-free reactionsThe direct metal-free oxidative carboxylation of olefins and CO2 provides a very valuable approach to a wide range of five-membered cyclic carbonates. This process generally rely on the use of halogen-containing catalysts/promoters to stabilize the ring-opening intermediates. One of the earliest mention of the synthesis of carbonates from olefins and CO2 under metal-free conditions can be found in a 2004 paper by Arai et al.37,38 although only one low-yielding example was described. Considering the catalyst, oxidant, temperature and CO2 pressure, the optimized conditions of this CO2 fixation reaction involved using TBAB as a catalyst and aq TBHP as an oxidant in supercritical CO2 at 80 °C. Under the optimized conditions, styrene carbonate was produced in 33% yield from the corresponding olefin within 6 h. A possible process for the formation of compound 2 should involve the formation of hypobromite OBr−via the oxidation of Br− with TBHP, followed by bromination of the styrene 1 in the presence of H2O to give the bromohydrin intermediate A, which after hydrobromination in the presence of in situ generated OH− converts to the epoxide B. Next, ring opening of the epoxide through nucleophilic attack by Br− affords an oxy anion species C. Finally, nucleophilic attack of this intermediate to the carbon atom in CO2 leads to the carbonate salt intermediate D that cyclize to the cyclic carbonate 2 (Scheme 14). In 2013, Balzhinimaev and co-workers reinvestigated this reaction by performing the process at room temperature under 8 atm pressure of CO2 in DCM.39 However, only trace amounts of the desired carbonate were obtained.
2.3. Electrocatalytic reactionsElectrocatalytic synthesis of cyclic carbonates directly from the corresponding olefins and CO2 has been rarely studied; indeed, only one example of such a reaction was reported in the literature by Yuan–Jiang and co-workers.45 By employing styrene as the model reactant, the reaction variables such as electrodes, electrolyte, and solvent were carefully studied. The results indicated that an undivided graphite/Ni-cell was more effective than other cells (e.g., graphite/Al, graphite/Zn, graphite/Cu, Al/Ni, Zn/Ni) and compared to other electrolytes NH4I was the best choice for the conversion. DMSO was also found to be the best solvent over DMF, THF, DCM, MeCN, dioxane, and acetone. Noteworthy, the presence of a small amount of H2O was crucial for the success of this reaction. Under the optimized conditions, a large variety of terminal and internal olefins 25 were tolerated well and provided the target carbonates 26 in moderate to quantitative yields (Scheme 19). The procedures could also be adapted to large scale synthesis of various important carbonates. On the basis of mechanism studies, the authors proposed that this oxidative carboxylation reaction starts with the generation of molecular iodine and ammonia at the anode (2I− → I2 + 2e−) and cathode (2NH4+ + 2e− → 2NH3 + H2), respectively. Next, I2 reacts with olefin 25 to form iodonium intermediate A, which after reaction with water gives iodohydrin intermediate B. Subsequently, this latter undergoes deprotonation with NH3 to yield the anionic intermediate C, followed by reaction with CO2 to generate intermediate D. Finally, the intramolecular cyclization of D affords the expected carbonate 26 and regenerates I− (Scheme 20).
3. One-pot, two-step approaches
3.1. Metal-catalyzed reactionsIn 2007, Qiao and Yokoyama along with their colleagues developed a convenient one-pot two step methodology for the synthesis of styrene carbonate 2 form styrene 1 and CO2 employing methyltrioxorhenium(MTO)/urea hydrogen peroxide (UHP)/Br4[1-ethyl-3-methylimidazolium EMIm]Br4/Zn[EMIm]2Br4 combination as a catalytic system.46 Initially styrene, MTO, UHP, and [EMIm]Br4 were added to the reactor and after the epoxidation reaction (2 h), Zn[EMIm]2Br4 and CO2 (30 atm) were added to the same reactor to afford the expected carbonate in 83% within 2 h. Four years later, the group of Li–Hu synthesized a range of cyclic carbonates 28 in good to high yields from the corresponding olefins 27 through a similar one-pot two-step process in which after the epoxidation of the olefins by TBHP in the presence of MoO2 catalyst (100 °C, 1–2 h), CO2 and TBAB were added into the same reactor and heated to 140 °C for 1–6 h (Scheme 21a).47 Based on the literature, the authors proposed a mechanistic pathway to this reaction as shown in Scheme 21b. Later, Siewniak and co-workers revisited this reaction by performing the cycloaddition step in the presence of ZnBr2/PS-TBMAC (PS = polystyrene, TBMAC = tributylmethylammonium chloride) combination as a catalytic system.48
3.2. Metal-free reactionsIn 2016, L.-N. He and colleagues developed an efficient protocol for the metal-free synthesis of five-membered cyclic carbonates from the corresponding olefins via hydroxybromination–carboxylation reaction sequences.53 Thus, in the presence of 1 equiv. of K2CO3 as an inexpensive base in PEG1000, bromohydrins A, which were prepared through the hydroxybromination of olefins 35 by the composition of K2S2O8–NaBr, were treated with CO2 (30 atm) to give cyclic carbonates 36 in almost fair to good yields (Scheme 25). The results demonstrated that aromatic olefins provided better yields than aliphatic ones and electron-rich aromatic olefins gave relatively higher yields compared to the electron-deficient ones. To the best of our awareness, this is the first and only example of the metal-free synthesis of cyclic carbonates from olefins and CO2 through a one-pot two-step reaction.
4. ConclusionSince the industrial revolution, which began in 1850, human sources of carbon dioxide emissions into the atmosphere have been growing dramatically (almost 3% each year). This has caused serious environmental issues such as global warming, climate change, and ocean acidification. Therefore, the capture, utilization and storage (CCS) of this greenhouse gas have been attracting extensive attention worldwide. Chemical fixation of CO2 is considered as a promising route for the utilization of CO2 because it can be used as a renewable and environmentally benign C1 feedstock for the synthesis of many value-added chemicals. However, high thermal and kinetic stability of CO2 limit its application and a large energy input is usually required for its activation. A strategy to overcome the low reactivity of CO2 is based on the reactions with high free energy substrates. The cycloaddition of CO2 and energy-rich epoxides stands as one of the most attractive and efficient strategies for CO2 conversion and utilization. However, most epoxides are toxic and mutagenic. Moreover, their preparation requires an additional step form the corresponding olefins. An alternative or complementary route involves the direct oxidative carboxylation of olefins. As illustrated, both terminal and internal olefins were compatible with this page of cyclic carbonates synthesis. In addition, aliphatic, aromatic, as well as heteroaromatic olefins were applicable to this reaction and at least three different kinds of catalysts (metal-, organo-, and electro-catalysts) have already been found as active catalysts for this CO2-fixation reaction. However, most of the reactions covered in this review have been performed under harsh conditions (high reaction temperature and/or high CO2 pressure). Therefore, it is still necessary to develop novel and highly efficient catalytic systems that can allow this conversion under milder conditions.
Conflicts of interestThere are no conflicts to declare.
AcknowledgementsThis work was carried out with funding from the National Key R&D Program of China (Grant No. 2018YFC0808103) and Key Laboratory of Western Mine Exploitation and Hazard Prevention, Ministry of Education (Grant No. SKLCRKF1916) and National Natural Science Foundation of China (Grant No. 51874232).
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