Synthesis of Cyclic Carbonates

Alkylene CCs have been prepared through the transesterification of appropriate glycols with dialkyl carbonates (usually diethyl or dimethyl carbonate) in the presence of a suitable catalyst. One of the first such examples was the synthesis of six-membered CCs by the transesterification of propane-1,3-diols with DEC catalyzed by sodium ethanolate (Equation 7.31) [289], The reaction was carried out at temperatures between 293 and 333 K, and a conversion yield of 40% was obtained. 

A similar method has been reported by Albertsson et al. [290], in which equimolar amounts of propane-1,3-diol and DEC were used, with stannous 2-ethylhexanoate as the transesterification catalyst, affording a yield of 53%. 

Other examples [291, 292] have included the use of propane-1,3-diols which had been differently substituted and treated with DEC in the presence of catalytic amounts of sodium methoxide. Then, depending on the reaction conditions, either polycarbonates or CCs were produced in high yield. 

Six-membered alkylene carbonates have also been synthesized by reacting several 1,3-diols with a 15% excess of EC in the presence of titanium(IV) isopro-poxide at 293-323 K and 15-30mmHg [293], In this case, the high-purity (99%) alkylene carbonate was obtained via a short-path distillation at 323-373 K at reduced pressure. Alkylene carbonates have also been obtained by the disproportionation of l,3-bis(alkoxycarbonyloxy)propanes, using colloidal silica or Sn(II) stearate [294]. 

The transesterification of 1,2-diols by reaction with carbonates, both cyclic and linear, produces five-membered alkylene carbonates almost exclusively. A well-known example of this is the reaction of DMC with propene glycol to yield PC [295], 

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Ni(cyclam)]2+ was shown to be an efficient electrocatalyst for the intramolecular cyclization-carboxylation of allyl or propargyl-2-haloaryl ethers,200 and for the synthesis of cyclic carbonates from epoxides and carbon dioxide.201... 

Silica gel-based catalytic systems have been described as efficient promoters for a number of organic reactions.28 Illustrative examples include the oxidative cleavage of double bonds catalyzed by silica-supported KM11O4,29 reaction of epoxides with lithium halides to give /i-halohydrins performed on silica gel,30 selective deprotection of terf-butyldimethylsilyl ethers catalyzed by silica gel-supported phosphomolybdic acid (PMA),31 and synthesis of cyclic carbonates from epoxides and carbon dioxide over silica-supported quaternary ammonium salts.32... 

J.-Q. Wang, D.-L. Kong, J.-Y. Chen, F. Cai, and L.-N. He, Synthesis of cyclic carbonates from epoxides and carbon dioxide over silica-supported quaternary ammonium salts under supercritical conditions, /. Mol. Cat. A Chem., 249 (2006) 143-148. 

Synthesis of cyclic carbonates from alkenes epoxidation-cum-cycloaddition... 

Han, L., et al., Ionic liquids grafted on carbon nanotubes as highly efficient heterogeneous catalysts for the synthesis of cyclic carbonates. Applied Catalysis A General, 2012. 429-430(0) p. 67-72. 

The synthesis of cyclic carbonates, starting from olefins, can be also carried out via a multistep method based on two separate reactions. To this end, C02 and the carboxylation catalyst have been added to the same reactor in which a preliminary epoxidation process had been carried out. 

Synthesis of Cyclic Carbonates from Propargylic Alcohols... 

Dunach, Inesi and others also investigated the electrochemical synthesis of cyclic carbonates from C02 with epoxides, alcohols and glycols [66]. In this regard, Yang et al. [67] reported the use of pure room temperature ionic liquids (ILs) as reaction media in the electrochemical activation of C02 for the synthesis of cyclic carbonate from epoxide, under mild conditions. C02-saturated IL (BMIMBF4) solutions were also used for the electrochemical carboxylation of activated olefins [68]. Monocarboxylic acids were obtained in moderate yield (35-55%), and the IL was recycled five times. 

Dharman MM, Choi H-J, Kim D-W et al (2011) Synthesis of cyclic carbonate through microwave irradiation using silica-supported ionic liquids Effect of variation in the silica support. Catal Today 164(1) 544—547... 

Guanidinium salt has special properties of high thermal and chemical stability, tunable groups attached on the three N atoms in the molecular structure, and excellent catalytic activity for cycloaddition of C02 and epoxides to produce cyclic carbonate [11, 12]. A functionalized-PEG, hexaalkylguanidinium bromide (Scheme 5.3) being covalently tethered to PEG (MW = 6000) is utilized as an active homogeneous catalyst, which includes the benefits of recyclability and high catalytic activity, for the synthesis of cyclic carbonates from C02 and epoxides with almost quantitative yield and excellent selectivity [13]. 

Scheme 5.5 Structures of PEG-functionalized ionic liquids for the synthesis of cyclic carbonates. Reproduced from Ref. [15] by permission of The Royal Society of Chemistry...

Scheme 5.9 Synthesis of cyclic carbonates from halohydrin with C02 catalyzed by PEG/K2C03 system. Reprinted with the permission from Ref. [42].

Dou X, Wang J, Du Y et al (2007) Guanidinium salt functionalized PEG an effective and recyclable homo-geneous catalyst for the synthesis of cyclic carbonates from C02 and epoxides under solvent-free conditions. Synlett 18(19) 3058—3062... 

Wang JL, He LN, Dou XY et al (2009) Poly (ethylene glycol) an alternative solvent for the synthesis of cyclic carbonate from vicinal halohydrin and carbon dioxide. Aust J Chem 62(8) 917-920... 

The synthesis of cyclic carbonates, which start from COj and epoxides, is possible even with nonmetal catalysts and examples of the catalysts which have been used are ammonium salts [193 196], amines [197-199], halides... 

Synthesis of Cyclic Carbonates 4.6.1.5.1. Synthesis of Cyclic Carbonates Mediated by Electrophiles... 

Synthesis of Cyclic Carbonates via Radical Attack to a Double Bond... 

Figure 8.8 Functionalized PEG in solventless catalytic synthesis of cyclic carbonates from carbon dioxide and epoxides.

These easily-made carbonates have become useful intermediates for (i) the direct synthesis of cyclic carbonates via the Heck reaction [82] (ii) optically active carbonates by enantioselective hydrogenation [83] and (iii) to oxazolidinones [84, 85] as an alternative route to the Evans reagent [86-88]. 

Scheme 6. Key steps of the catalytic synthesis of cyclic carbonates involving CO2.

Figure 2.45 Acid-base bifunctional reaction mechanism in the synthesis of cyclic carbonates over mixed oxides obtained from a Mg-Al hydrotalcite. Source Kaneda et al. [28].

Tetrahaloindate(iii)-based ILs, prepared using a similar MW protocol, have been used as catalysts for the synthesis of cyclic carbonates. CO2 reacted with various epoxides in the presence of catalytic amounts of tetrahaloindate(iii)-based ILs, producing important line chemicals in good to excellent yields in benign fashion (Scheme 8.29). ... 

Scheme 8.29 Tetrahaloindate(iii)-based IL-catalyzed synthesis of cyclic carbonates.

Scheme 16.16 Electrochemically activated carbon dioxide in the synthesis of cyclic carbonates and unsymmetiical carbonates and carbamates (Reprinted from Ref. [132] and [133] with kind permission of Elsevier)...

The synthesis of cyclic carbonates in RTILs, via cycloaddition of cathodically activated carbon dioxide to epoxide, has been reported by Deng et al. [139]. Ionic liquids, saturated with CO by bubbling at normal pressure and containing the epoxidic substrate, were electrolyzed in an undivided cell (Cu as cathode. Mg or Al as sacrificial anode). The electrolyses were carried out under potentiostatic conditions at a potential negative enough to the selective reduction of CO to CO (E=-2.4 V vs. 

Yang H, Gu Y, Deng Y, Shi F (2002) Electrochemical activation of carbon dioxide in ionic liquid synthesis of cyclic carbonates at mild reaction conditions. Chem Commun 3 274-275... 

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