Transesterification of cyclic carbonates

The transesterifications of chloropropene carbonate and propene carbonate with methanol and phenol catalyzed by TS-1, Ti-MCM-41, and Ti02 (Table XLI) have been reported (248). Neither Ti02 nor TS-1 showed any activity in the transesterification reactions. Ti-MCM-41 catalyzed the reaction with a high selectivity for DMC (86%). Ti-MCM-41 also catalyzes the transesterification of cyclic carbonates with phenols (Table XLI). 

Transesterification of cyclic carbonates with CH3OH and phenol catalyzed by Ti-MCM-41... 

Figure 2.27 Sequential coupling ofthe transesterification of cyclic carbonates with the selective N-methylation of anilines catalyzed by faujasites [220],...

Scheme 24 Synthesis of DMC (A) from CO2 and methanol, and (B) by the transesterification of cyclic carbonate with methanol.

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... 

Aromatic polycarbonates are currently manufactured either by the interfacial polycondensation of the sodium salt of diphenols such as bisphenol A with phosgene (Reaction 1, Scheme 22) or by transesterification of diphenyl carbonate (DPC) with diphenols in the presence of homogeneous catalysts (Reaction 2, Scheme 22). DPC is made by the oxidative carbonylation of dimethyl carbonate. If DPC can be made from cyclic carbonates by transesterification with solid catalysts, then an environmentally friendlier route to polycarbonates using C02 (instead of COCl2/CO) can be established. Transesterifications are catalyzed by a variety of materials K2C03, KOH, Mg-containing smectites, and oxides supported on silica (250). Recently, Ma et al. (251) reported the transesterification of dimethyl oxalate with phenol catalyzed by Sn-TS-1 samples calcined at various temperatures. The activity was related to the weak Lewis acidity of Sn-TS-1 (251). 

Polycarbonates (. Polycarbonates may be prepared by a variety of synthetic methods. Procedures most commonly used include reaction of diols with phosgene (Reaction 2A), transesterification of diesters of carbonic acid with dihydroxy compounds (Reaction 25), and polymerization of cyclic carbonates (Reaction 26). 

Several decades later, in the early 1930s, work (and interest) once more revived on aliphatic polycarbonates when Carothers and van Natta were able to produce low-melting, low-molecular-weight, microcrystalline aliphatic polycarbonates via two different synthetic pathways [27]. The first method was the transesterification of aliphatic dihydroxylic compounds with diethyl carbonate. The second approach was via ring-opening polymerization of cyclic carbonates of aliphatic dihydroxylic... 

The alkylene carbonates are easily available through transesterification of dialkyl carbonates (usually dimethyl or diethyl carbonate) or diphenyl carbonate with appropriate 1,2-diols in the presence of alkaline catalysts.This approach for the preparation of five-membered cyclic carbonates also was described in the 1950s by Ludwig and Piech ° and Sarel et al ... 

The ROP of cyclic carbonates with nucleophihc initiators is a chain reaction in which, besides initiation and propagation reactions, transesterification reactions must also be considered, with termination and transfer reactions being absent (Scheme 12.1) [3]. 

In our previous works[8,9] on the synthesis of various 5-membered cyclic carbonate, quaternary ammonium salts such as tetrabutylammonium halides showed excellent catalytic activities in relatively mild reaction conditions, under atmospheric pressure and below 140 U. hi this work, several kinds of quaternary ammonium salts have been used for the transesterification reactions of the ethylaie carbonate with methanol to DMC and ethylene glycol. 

Komura, H., Yoshino, T. and Ishido, Y., Synthetic studies by the use of carbonates. II. Easy method of preparing cyclic carbonates of polyhydroxy compounds by transesterification with ethylene carbonate. Bull. Chem. Soc., 1973, 46, 550-553. 

Cyclic carbonates result from polyols by transesterification using organic carbonates (115). Thus sorbitol and diphenylcarbonate in the presence of dibutyl tin oxide at 140—150°C form sorbitol tricarbonate in quantitative yield (116). 

Glycols may undergo intramolecular cydization or cyclically condense with other molecules to form a number of ring structures. Transesterification of carbonates with ethylene glycol produces ethylene carbonate [96 49-1] (eq. 4). Numerous materials catalyze carbonate transesterifications. 

Cyclic carbonates are prepared in satisfactory quality for anionic polymerization by catalyzed transesterification of neopentyl glycol with diaryl carbonates, followed by tempering and depolymerization. Neopentyl carbonate (5,5-dimethyl-l,3-dioxan-2-one) (6) prepared in this manner has high purity (99.5%) and can be anionically polymerized to polycarbonates with mol wt of 35,000 (39). 

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],... 

R,R-diphenyl ethylene carbonate CR,R-DPEC)) with a racemic zirconaaziridine. (C2-symmetric, cyclic carbonates are attractive as optically active synthons for C02 because optically active diols are readily available through Sharpless asymmetric dihydroxylations [67].) Reaction through diastereomeric transition states affords the two diastereomers of the spirocyclic insertion product protonolysis and Zr-mediated transesterification in methanol yield a-amino acid esters. As above, the stereochemistry of the new chiral center is determined by the competition between the rate of interconversion of the zirconaaziridine enantiomers and the rate of insertion of the carbonate. As the ratio of zirconaaziridine enantiomers (S)-2/(R)-2 is initially 1 1, a kinetic quench of their equilibrium will result in no selectivity (see Eq. 32). Maximum diastereoselec-tivity (and, therefore, maximum enantioselectivity for the preparation of the... 

Common synthetic methods for the preparation of cyclic 8-enamino esters are the condensation between a lactim ether and benzyl cyanoacetate followed by hydrogenolytic decarboxylation, or the imino ester carbon-carbon condensation with tert-butyl cyanoacetate followed by a trifluoroacetic acid treatment. The use of a thiolactam condensed with ethyl bromoacetate gives, after sulfur extrusion by triphenylphosphine, cyclic 8-enamino esters. Compared with these methods, the Meldrum s acid condensation followed by the monodecarboxylating transesterification described here is more convenient and practical. An extension of this procedure permits preparation of smaller... 

Other step-polymerization reactions of large cyclic monomers by entropically driven ring-opening polymerization have been reviewed (Hall and Hodge, 1999). An important example is the heating of cyclic bisphenol-A carbonates over a titanium isopropoxide transesterification catalyst to yield a polycarbonate with a relative molar mass of about 250000. This has also been performed under conditions of reaction injection moulding (Hall and Hodge, 1999). 

An interesting feature that made the commercialization of the synthesis of linear carbonates possible is that both linear and cyclic carbonates easily undergo transesterification with alcohols (Scheme 6.9). This process is used industrially for the synthesis of diphenylcarbonate from DMC.72 The coproduct methanol is recycled to DMC. Nevertheless, once a highly efficient direct C02 process for an organic carbonate is found, transesterification makes possible the production of any other carbonates. 

Resolution by transesterification. Using vinylic acetates to esterify allyl alcohols, propargyl alcohols, 2-phenylthiocycloalkanols, a-hydroxy esters," methyl 5-hydroxy-2-hexenoates, and 2-substituted 1,3-propanediols, the enantioselective esterification provides a means of separation of optical isomers. Vinyl carbonates are also resolved by lipase-mediated enantioselective conversion to benzyl carbonates. Other esters that have also been used in the kinetic resolution include 2,2,2-tri-fluoroethyl propionate. There is a report on a double enantioselective transesterification" of racemic trifluoroethyl esters and cyclic meso-diols by lipase catalysis. 

The polycondensation of a diol and the diester of a dicarboxylic acid (e.g., the dimethyl ester) can be carried out in the melt at a considerably lower temperature than for the corresponding reaction of the free acid. Under the influence of acidic or basic catalysts a transesterification occurs with the elimination of the readily volatile alcohol (see Example 4.3). Instead of diesters of carboxylic acids one can also use their dicarboxylic acid chlorides, for example, in the synthesis of high-melting aromatic polyesters from terephthaloyl dichloride and bisphenols. The commercially very important polycarbonates are obtained from bisphenols and phosgene, although the use of diphenyl carbonate as an alternative component is of increasing interest (see Example 4.4). Instead of free acids, cyclic carboxylic... 

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