Cyclic carbonates synthesis mechanism

Figure 4.8 Possible reaction mechanism for cyclic carbonate synthesis from C02 and epoxides on Cu aza complexes [74].

North M, Pasquale R (2009) Mechanism of cyclic carbonate synthesis from epoxides and C02. Angew Chem Int Ed 48(16) 2946-2948... 

The above studies [8-10] strongly indicate the participation of silanol groups as acid sites in the amine-catalyzed reactions. The synergistic catalysis of immobilized organic bases and silanol groups has been highlighted by the cyclic carbonate synthesis from carbon dioxide and epoxide. Acid-base dual activation mechanism of the cyclic carbonate synthesis is widely accepted. Sakakura and coworkers [11] reported silica-supported phosphonium salts as highly active catalysts for the cyclic carbonate synthesis. The results for propylene carbonate synthesis are summarized... 

The preparation of a number of medium ring benzoic acid lactones was achieved by treatment of compounds such as VIII/176 with an excess of meta-chloroperoxybenzoic acid in dichloromethane, Scheme VIII/33 [103]. However, this oxidation reaction is not general for the synthesis of aromatic lactones. If the same reaction conditions are used as in the conversion of VIII/176 to VIII/177, the methoxy derivative VIII/178 is not transformed into the corresponding lactone. Instead the cyclic carbonate VIII/183 was isolated in a yield of 50 %. The proposed mechanism of this abnormal reaction is shown in Scheme VIII/33. From model compounds, the methoxyl group in the para-position to the center of oxidation seems to be important for the formation of VIII/183 [103]. The carbonate VIII/183 is unstable in aqueous alkaline medium and decomposes to the spiro compound, VIII/185, Scheme VIII/33 [103]. For an analogous reaction, see ref. [104]. 

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].

Most importantly, layered materials are currently of particular interest as supports for the immobihzation and/or intercalation of various ILs in order to prepare polymer nanocomposites [83, 84] with improved thermal and mechanical properties, nanohybrid materials for electrochemical sensors [85, 86], and efficient catalysts for the synthesis of cyclic carbonate by the cycloaddition of CO2 to allyl glycidyl ether [87] and propylene glycol methyl ether (PGME) from propylene oxide and methanol [88]. A detailed list of applications involving layered materials and ILs can be found in a recent review [16]. 

An example of its exploitation by more conventional mechanisms relates to the synthesis of bis-cyclic carbonates [129]. Methyl 10-undecenoate was reacted with various diols and diamines to open the pathway to the synthesis of the corresponding bis-carbonates macromonomers bearing an ester or an amide core, as shown in Scheme 4.29 for the case of butane-l,4-diamine. These reactive intermediates were then used to prepare polyhydroxyurethanes (PHU) using a wide selection of diamines following the coupling reactions shown in Scheme 4.30. 

Other researchers [69, 70] have shown that polymer-immobilized nano-gold particles had unprecedented catalytic activity for activation of carbon dioxide and high turnover frequency (TOF) for the synthesis of cyclic carbonates. Moreover, Xiang et al. [69] have reported a novel and convenient route for the direct synthesis of cyclic carbonates that avoids the preliminary synthesis and isolation of intermediate alkene oxide, coupling the two sequential reactions of epoxidation of alkene and cycloaddition of CO to epoxide into one pot. It is still not clear at this stage about the reaction mechanism. Shi et al. [70] proposed that the activation of... 

Figure 8.18 Possible mechanism for (a) cycli2ation reactions of epoxides and (b) one-pot synthesis of cyclic carbonate from epoxide and CO over polymer-supported nano-gold...

As described in this subsection, metal halides, metal complexes and water can improve the activities of ILs for the synthesis of cyclic carbonates from epoxides and CO2. A generally accepted reaction mechanism for those bi-functional catalyst systems can be drawn as Scheme 5. Because these additives have Lewis acidic nature, they would interact with the oxygen atom of the epoxide. On the other hand, the basic anion of IL would attack the less hindered carbon atom of the epoxide ring. Such cooperative activation of the epoxide should make the ring opening easier, being the reason of the promotional effects of the additives. In some cases, the additives may have a role in the activation of CO2. 

The above mentioned direct synthesis of DMC requires organic or inorganic dehydration reagents to get high DMC yields because of the reaction equilibrium. On the other hand, the transesterification of cyclic carbonate with methanol relatively easily gives high DMC yields. ILs can also be used as the catalysts for this reaction. Scheme 26 illustrates the generally accepted reaction mechanism for the base catalyzed transesterification of... 

A much more convenient method of obtaining five-membered cyclic carbonates is the insertion of gaseous carbon dioxide into the appropriate oxirane ring (Scheme 7). The catalyst development and the reaction mechanisms over alkali metal halides, " organic bases, " metal oxides, zeolite,titanosilicates, " and metal com-plexes " have been reported for the synthesis of cyclic carbonates from CO2 and epoxides. 

Figure 9-5 The mechanism of cyclic ether synthesis from a bromoalcohol and hydroxide ion upper reactions). A competing but slower side reaction, direct displacement of bromide by hydroxide, is also shown jower reaction). The curved lines denote a chain of carbon atoms.

When an a-chloroaldehyde or an a-chloroketone is condensed with a /3-ketoester, in the presence of aqueous base, a furan is produced bearing an ester substituent at the /3-position. It is thought that the reaction is of the aldol type intermediate dihydrofurans (256) have been isolated in certain cases (Scheme 70) (74BSF519). The condensation of ethyl bromopyru-vate and sodium oxaloacetate follows a similar mechanism (54JOC1671). The one-pot synthesis of 2,4,5-trisubstituted furans (257) from ketones and ethyl 3,4-dibromo-2-butenoate is a useful addition to a well known route (80S52). The analogous reaction of cyclic /3-diketones, i.e. cyclohexane-1,3-dione and 5,5-dimethylcyclohexane-l,3-dione, results in the formation of the condensed furans (258) and (259). These reactions are preformed either in ethanol with sodium ethoxide or in DMF with potassium carbonate. 

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