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Fu et al. In situ Catalytic Transformation To address the energy penalties associated with CCS strategy and realize the direct fixation of CO2 from the atmosphere or industrial exhaust, the CO2 capture and utilization (CCU) strategy, whereby the captured CO2 is used as a non-toxic, abundant, and sustainable feedstock to produce valuable organic compounds via chemical, electrochemical or photochemical reactions, was proposed and now is flourishing (Scheme 1). By now, both organic compounds and functional materials containing the bridging-carbonato metal complexes can be obtained from atmospheric CO2 using the CCU strategy (Yang et al., 2011; Liu et al., 2012; Massoud et al., 2015). Although realizing the attractive prospect of the CO2 capture and in situ conversion in the industry scale remains a challenge (Zhang and Lim, 2015), the emergence of efficient absorbents and the development of CO2 transformation will cast light on it. In continuation of our work on the conversion of the captured CO2 into value-added organic chemicals, this review summarized the recent progress on CO2 capture and in situ conversion into organic products. To realize the carbon capture and in situ conversion strategy, effective absorbents are always necessary. Ideally, the absorbents for CCU strategy should not only capture CO2, but also activate CO2 and even the substrate. Thus, the chemical transformation can proceed under mild conditions. Up to now, organic and inorganic bases, N -heterocyclic carbenes (NHCs) and N - heterocyclic olefins (NHOs), ionic liquids (ILs) and frustrated Lewis pairs (FLPs) have already been applied to CO2 capture and in situ conversion. A plethora of valuable organic chemicals have been obtained through the CCU strategy as shown in Scheme 2. INORGANIC/ORGANIC BASES Due to the electrophilicity of carbon atom in CO2, the organic and inorganic bases containing strong nucleophilic atom have been widely used in CO2 trapping, where the base can interact with CO2 directly or function as a proton acceptor. The resulting CO2 capture products i.e., CO2 adducts have been employed for subsequent synthesis of various valuable chemicals. Considering the transformations of the captured CO2 derived from primary and secondary amines and amino alcohols to isocyanates, carbamates, ureas, and oxazolidinones have been concerned by several excellent review papers (Hampe and Rudkevich, 2003; Chaturvedi and Ray, 2006; Yang et al., 2012; Tamura et al., 2014; Wang et al., 2017a,b), here we focus on the transcarboxylation effect and other transformations of the captured CO2, namely CO2 derivatives. Synthesis of Carbamates and Ureas In the synthesis of carbamates, the aprotic organic bases can function as CO2 absorbents and transcarboxylation agents. The initial attempt was made by Rossi group, in which CO2 is trapped by a methanol solution of commercially available tetraethylammonium hydroxide. The resulting tetraethylammonium hydrogen carbonate can be used as a surrogate of CO2 in the synthesis of carbamate. Meanwhile, the presence of tetraethylammonium ion as counterion increases the nucleophilicity of carbamate anion (Inesi et al., 1998). Soon after, Franco group has successfully identified the DBU-CO2 complex via reacting CO2 with DBU (1,8- Diazabicyclo[5.4.0]undec-7-ene) in anhydrous acetonitrile, implying that DBU can be used as CO2 trap reagent (Pérez et al., 2002). Moreover, the resulting reactive DBU-CO2 adduct can be utilized as transcarboxylating reagent for synthesis of N-alkyl carbamates. Later, the same group revealed the activation capacity of CO2 by other bicyclic amidines and observed the inverse relation between the thermal stability and the transcarboxylating activity for the amidine-CO2 adducts (Scheme3) (Pérez et al., 2004), which is the first time to investigate the activation ability of organic bases to CO2. The combination of organic base and alcohol is an efficient CO2 capture system and the absorbed CO2 can be in situ transformed. The prototypical example is the polyethylene glycol (PEG)/superbase system developed by our group in 2011 (Yang et al., 2011). In the capture step, the superbase is used as a proton acceptor and almost equimolar CO2 per mole superbase can be absorbed (Scheme 4). The resulting liquid amidinium carbonate can directly react with n-butylamine at 110◦C to afford dibutyl urea in almost quantitative yield (96%) without any other additives. This protocol can be used in the synthesis of other symmetrical urea derivatives. In the above examples, the captured CO2 in the transcarboxylating agents can be regarded as the activated CO2 because the linear structure of CO2 is converted to bent structure, which is more liable to nucleophilic attack. Synthesis of Oxazolidinones The “CO2 absorption and subsequent transcarboxylation” triggers the research on CO2 capture and in situ transformation. Several years later, M. Yoshida and coworkers use DBU to enrich and activate CO2 in air and perform the first example of directly transforming atmospheric CO2 into the substituted 5-vinylideneoxazolidin-2-ones using propargylic substrate 4- (benzylamino)-2-butynyl carbonates or benzoates as a substrate (Scheme4) (Yoshida et al., 2008). In their follow-up work, they further improve the reaction efficiency by utilizing AgNO3 as catalyst and propargylic amines as substrates (Scheme4) (Yoshida et al., 2012). Inspired by these works, our group designs a series of novel CO2 capture and activation systems. For example, by employing ammonium iodide as catalyst, the cycloaddition reaction of various aziridines with the captured CO2 by NH2 PEG150 NH2 gives rise to oxazolidinones at 40◦C in >94% yield and selectivity (Scheme 4) (Yang et al., 2011). Soon after, we report the first example of steric-hindrance- controlled CO2 absorption, where the sodium N-alkylglycinates and N-alkylalaninates dissolved in PEG150 are used to capture CO2, generating the carbamic acid rather than the ammonium carbamate (Liu et al., 2012). N-isopropylglycinate is found to be the best absorbent for the rapid and reversible capture of almost equimolar CO2. Crucially, the captured CO2 can be activated simultaneously and the resulting carbamic acid can react with either aziridine or propargyl amine to afford oxazolidinones in the presence of NH4I and AgOAc as a catalyst, respectively (Scheme 4). Frontiers in Chemistry | www.frontiersin.org 2 July 2019 | Volume 7 | Article 525PDF Image | CO2 Capture and in situ Catalytic Transformation
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