Catalytic transformation of CO2 into the value-added organic compounds is a very important and hot research topic in organic synthetic chemistry and green chemistry from the viewpoint of developing CO2 as C resource. Organocatalytic reactions employing metal-free organic molecules as catalysts have received unprecedented attention in recent years, with the significant advantages of the catalysts being usually inexpensive and stable, and the reactions can be performed under air. This chapter summarizes and gives an overview of the recent advances in the organocatalytic transformation of CO2 into cyclic carbonates, 2-oxazolidinones, carboxylic derivatives, as well as the synthesis of CO2-adducts and their application as CO2 carriers.
- carbon dioxide
- cyclic carbonates
Carbon dioxide (CO2) exhibits many good qualities as an ideal C resource in organic synthesis such as non-toxicity, natural abundance, and inexpensiveness. Therefore, a variety of efficient catalyst systems have been developed for the transformation of CO2 into the useful and value-added organic compounds, even it is a kinetically and thermodynamically stable final product of all combustion processes of organic matters and some comprehensive reviews have been reported [1–3]. On the other hand, the organocatalytic reactions using simple, cheap, stable, and easily available organic compounds as catalysts for various organic transformations have been widely investigated in the past two decades [4–6] and have also played an important tool for catalytic activation of CO2 and its transformation. In this chapter, we focus on summarizing the representative examples of the recent advancement on the organocatalytic transformation of CO2 into the different types of useful molecules, including cyclic carbonates, 2-oxazolidinones, ureas, and carbamates, as well as the CO2-adducts and their application as CO2 carriers.
2. Transformation of CO2 into cyclic carbonates
The coupling of CO2 with epoxides is an atom-economic transformation for the synthesis of cyclic carbonates, which have high potential application as the aprotic polar solvents , electrolytes for lithium ion batteries , precursors for organic synthesis , and polymers .
Ionic liquids (ILs) have been well applied as the efficient organocatalysts in the coupling of CO2 with terminal epoxides since it was first reported by Deng’s group in 2001 using 1-
Caló  group reported a straightforward method for chemical fixation of CO2 into terminal epoxides by simply dissolving epoxides in molten tetrabutylammonium bromide and iodide (TBAB and TBAI) as solvent under an atmospheric pressure of CO2 (Scheme 1). The cyclic carbonates could be isolated by vacuum distillation or extraction with organic solvents, and the ionic liquid (IL) was insoluble allowing the recycling of the ammonium salt. In addition, polymerization sensitive epoxides also reacted very well to give the corresponding cyclic carbonates, and the reaction rate depended on the nucleophilicity of the halide ion as well as the structure of the cation. TBAI could be also used as the sole solvent, and at 60°C, the reactions gave the cyclic carbonates in the similar yields.
A plausible mechanism was proposed for the formation of cyclic carbonates including the steps of the ring opening of epoxide by a nuclephilic attack of bromide ion, and the reaction of CO2 with the oxy anion species (Scheme 2).
Another IL such as quaternary ammonium-, phosphonium-, imidazolium-, or pyridinium-based cations with inorganic counter anions have been also used as the efficient catalysts in the synthesis of cyclic carbonate
He’s group prepared a series of Lewis basic ILs and examined their catalytic activity in the synthesis of cyclic carbonate from CO2 and epoxides under solvent-free conditions and established an efficient and recoverable catalyst system using [HDBU]Cl (1,8-diazabicyclo[5.4.0]undec-7-enium chloride) as organocatalyst (Scheme 4) . The catalyst system also showed fair catalytic activity to internal cyclohexene oxide.
A further work of the same group designed and synthesized a series of polyethylene glycol (PEG)-functionalized basic ILs, and providing the alternative recoverable and high catalytic activity organocatalysts in the coupling of CO2 with terminal and internal epoxides .
In addition, although PPN salts (Scheme 5, A) with weak nucleophilic anions such as PPN+BF4− and PPN+OTf− were inactive for the coupling of CO2 with epoxides, PPN+Cl− salt was found to be a good organocatalyst for the coupling of CO2 with neat epoxides without the use of organic solvents to afford cyclic carbonates .
Azaphosphatranes as tunable alternative to quaternary ammonium and/or phosphonium catalysts for the synthesis of cyclic carbonates from CO2 and epoxides was also reported by Martinez and Dufaud’s group (Scheme 5, B) . In order to examine the nature of the nanospace of the molecular cavity to affect the stability and reactivity of azaphosphatranes as organocatalyst, the same groups further reported the synthesis of supramolecular azaphosphatranes having cavities of different size and shape, and their excellent catalytic activity in the synthesis of cyclic carbonates from CO2 and epoxides .
Werner’s group synthesized a bifunctional ammonium salt covalently bound to a polystyrene or silica support, which showed efficient catalytic activity under solvent-free conditions for the synthesis of cyclic carbonates, developing an alternative recyclable and reusable organocatalyst for the coupling of CO2 with epoxides (Scheme 5, C) .
In addition, in order to understand the mechanism of the coupling of CO2 with epoxides catalyzed by quaternary ammonium salts, Zhang’s group studied the mechanism by experimental and density functional theory (DFT). The detailed structural and energetic information about each step of the three elementary steps in the catalytic cycle were obtained, and the effects of the chain length and anion on the reaction mechanisms, as well as the outcomes were also reported .
Wong’s group designed and synthesized a new IL (
In recent years, the simple and cheap organic compounds have been also developed as the efficient organocatalysts in the activation of CO2 and its transformation to cyclic carbonates.
Shi’s group studied the catalytic activity of a combination of phenols with organic bases in the coupling of CO2 with terminal epoxides and found that
In addition, Maseras and Kleij’s groups examined the catalytic activation of phenols/
DMF-scCO2 system was reported to be a good solvent system for the coupling of 1,2-epoxystyrene with CO2 affording the corresponding cyclic carbonate . A further investigation by Hua’s group disclosed that under solvent-free conditions, the high yields of cyclic carbonates could be achieved by coupling of CO2 with epoxides in the presence of catalytic amount of DMF , and in some cases, the catalytic activity of DMF could be significantly increased by the addition of catalytic amount of H2O (Scheme 9).
Hua’s group also investigated the catalytic activity of nitrogen-containing organic compounds, such as amines, anilines, amides, and pyridines in the formation of cyclic carbonates
On the other hand, the cycloaddition of propargyl alcohols with CO2 is an efficient and alternative transformation for the formation of α-methylene cyclic carbonates, and Dixneuf’s group first reported a PBu3-catalyzed reaction of tertiary propargyl alcohols with CO2 in an inert autoclave led to the high yield of the cyclic carbonates (Scheme 10) . It was found that in the absence of other solvent, PBu3 showed higher catalytic activity than PPh3 and PCy3.
Minakata’s group developed a strategy to offer an innovative approach to the fixation of CO2 to a wide range of cyclic carbonates
Johnston’s group also reported a three component reaction of CO2, homoallylic alcohol and NIS (NIS =
3. Transformation of CO2 into 2-oxazolidinones
Substituted 2-oxazolidinones are one of the important five-membered heterocyclic compounds, which not only show interesting biological and physiological activities but also have been applied as starting materials in the synthesis of other functional compounds. The coupling of CO2 with aziridine, CO2 with propargylamine, as well as the three-component cycloaddition of CO2, propargyl alcohol, and primary amine is the most interesting and promising synthetic methods.
He’s group designed and synthesized a series of polyethylene glycol (PEG)-functionalized ionic liquids as recyclable and efficient organocatalysts for selective synthesis of 5-substituted-2-oxazolidinones from the coupling of CO2 and aziridines. It was found that PEG6000(NBu3Br)2 (PEG MW6000)  and BrDBN-PEG150-DBNBr (DBN: 1,5-diazabicyclo[4.3.0]non-5-ene; PEG MW150)  were the efficient catalyst not only affording the expected 5-substituted-2-oxazolidinones in good yields, but also showing excellent regioselectivities. The same group also developed a proline-catalyzed synthesis of 5-aryl-2-oxazolidinones from CO2 and aziridines under solvent-free conditions .
Liu’s group developed an efficient catalytic system using DBN as organocatalyst and LiI as an additive under atmospheric pressure of CO2 in toluene to catalyze the coupling of CO2 with aziridines giving 2-oxazolidinones (Scheme 13) . The procedure was tolerated by a number of
Yoshida and Ihara’s groups investigated the reaction of 4-(benzylamino)-2-butynyl carbonates and benzoates with an atmospheric pressure of CO2 in the presence of DBU (1,8-diazabicyclo[5.4.0]undec-7-ene), developing a DBU-mediated synthesis of substituted 5-vinylideneoxazolidin-2-ones, which are attractive and important compounds in both medicinal and synthetic organic chemistry (Scheme 15) .
The three-component cycloaddition of CO2, propargyl alcohol and primary amine catalyzed by organocatalyst affording 4-methylene-2-oxazolidinones was first reported by Dixbeuf’s group with the use of simple and cheap PBu3 as catalyst and use of an excess amount of tertiary propargylic alcohols (Scheme 16) .
Costa’s group studied the synthesis of cyclic carbonates or carbamates and oxalkyl carbonates or carbamates
Hua’s group also investigated the three-component cycloaddition of CO2, propargyl alcohol and primary amine in the presence of pyridine and its derivatives as organocatalysts under solvent-free conditions, and 2,2′,2″-Terpyridine was found to be the efficient organocatalyst to afford 4-methylene-2-oxazolidinones or 2(3
In addition, 2,2′,2″-terpyridine also showed high catalytic activity in the coupling of CO2 with aziridines bearing either electron-donating or electron-withdrawing
In addition, ILs were also reported to be the effective promoter and reaction media for the synthesis of 4-methylene-2-oxazolidinones from CO2, propargyl alcohol, and amines with high yields .
4. Transformation of CO2 into carboxylic derivatives
CO2 is one of the good candidates in the synthesis of carboxylic derivatives. Nitrogen-containing organic bases mediated the formation of diarylureas
Skrydstrup’s group investigated the CO2 trapping with 2-alkynyl indoles in the presence of various organic bases and developed an efficient TBD-catalyzed the cycloaddition of CO2 with a variety of substituted 2-alkynyl indoles to afford tricyclic indole-containing ring compounds, good results were obtained with aromatic, heteroaromatic, and aliphatic 2-alkynyl indoles in terms of both yields and selectivities (Scheme 21) . The new methodology developed a procedure for the formation of C-C bond between CO2 and an indole derivative catalyzed by an organocatalyst.
5. CO2-adduct and its use as precatalyst in CO2 transformation
CO2 is a typical electrophilic reagent, and the synthesis and application of its adduct with nucleophiles have been considered to be an efficient way for CO2 capture, activation, and further transformation.
DBU-CO2 adduct could be prepared and isolated as a white powder in good yield by the reaction of DBU with CO2 in anhydrous acetonitrile at 5°C and was first used as the efficient carrier of CO2 in the synthesis of
On the other hand, unsaturated NHCs with the unique property of the carbon atom having strong basicity, stabilized by the electrondonating heteroatoms on either side, have been applied as versatile ligands in transition metal complexes and organocatalysts . NHCs have been found to react easily with CO2 by its nucleophilic addition to C=O bond as the key step, resulting in the formation of carboxylates, a NHC-CO2 adduct proposed as carriers of NHC as well as CO2 (Scheme 22) [48–50].
Sakai’s group designed and synthesized bifunctional organocatalysts bearing an ammonium betaine framework, which showed high catalytic activation of CO2, and catalyze the coupling of CO2 with terminal epoxides affording cyclic carbonates in good yields (Scheme 23) . Among them, 3-(trimethylammonio)phenolate was found to be one of the most active organocatalysts, and the formation of betaine-CO2 adduct was demonstrated to be the key intermediate (Scheme 24).
Moreover, several thermally stable CO2 adducts of
In addition, Ikariya’s group synthesized several 1,3-dialkylimidazolium-2-carboxylates (NHC-CO2 adduct) and investigated their catalytic activity in the cycloaddition reaction of CO2 with propargyl alcohols affording 4-methylene cyclic carbonates (Scheme 25) . 1,3-di-
The same group also reported the synthesis of various CO2, COS, and CS2 adducts of NHO, and these adducts were found to be efficient in catalyzing the cycloaddition reaction of CO2 with epoxides to selectively afford the corresponding cyclic carbonates. Among them, NHO-CO2 adducts were found to be more active . Furthermore, a variety of CO2 adducts of phosphorus ylides were prepared by the same group, and they were demonstrated to be the highly active organocatalysts for CO2 transformation under mild conditions to cyclic carbonates, oxazolidinone, N-methylated, and N-formylated amines .
The use of CO2 as a C starting material for the synthesis of useful and value-added organic compounds is an important and challenge research topic in the academic and industrial interest. The representative examples summarized in this chapter suggest that the simple, easily available, oxygen- and moisture-tolerated organocatalysts have played an important role in developing the promising and practical catalysis for the transformation of CO2 to various organic compounds. It seems reasonable to expect that the organocatalyzed CO2 transformation to much more different types of functional organic compounds will be greatly developed with the inspiration of the reported innovative progress. For example, Cantat’s group recently developed the novel and interesting organocatalyst systems for the transformation of CO2 to methylene in the synthesis of aminal derivatives, and CO2 as CO source in the formylation of amines using hydrosilanes as reductants catalyzed by nitrogen-containing organic bases [58–59]. The simple accessibility of CO2 and the vast range of possibilities to introduce various functional groups are some of the attractive features of CO2 transformations.
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