Preparation cyclic carbonates from epoxides

Five-membered cyclic carbonates are easily available as a result of the insertion of gaseous carbon dioxide into an oxirane ring (see review [12.]). Recent work in the field of new methods for preparing cyclic carbonates is dedicated primarily to the development of new catalytic systems and the synthesis of monofunctional compounds from epoxides and carbon dioxide (see, for example, reviews [13-16]). Monocyclic carbonates are used in a wide spectrum of applications solvents, components of liquid electrolytes, reactive diluents, chemical intermediates, and so on. It should be noted that this preparation also solves the problem of chemical fixation and utilization of C02. 

Cyclic carbonates are prepared directly from epoxides with LiBr, CO2, NMP (l-methyl-2-pyrrolidinone), 100°. ... 

Easily prepared from the appropriate monosaccharide, a glycal is an unsatu-rated sugar with a C1-C2 double bond. To ready it for use in potysaccharide synthesis, the primary -OH group of the glycal is first protected at its primary -OH group by formation of a silvl ether (Section 17.8) and at its two adjacent secondary - OH groups by formation of a cyclic carbonate ester. Then, the protected glycal is epoxidized. 

Aluminum porphyrins with alkoxide, carboxylate, or enolate can also activate CO2, some catalytically. For example, Al(TPP)OMe (prepared from Al(TPP)Et with methanol) can bring about the catalytic formation of cyclic carbonate or polycarbonate from CO2 and epoxide [Eq. (6)], ° - and Al(TPP)OAc catalyzes the formation of carbamic esters from CO2, dialkylamines, and epoxide. Neither of the reactions requires activation by visible light, in contrast to the reactions involving the alkylaluminum precursors. Another key difference is that the ethyl group in Al(TPP)Et remains in the propionate product after CO2 insertion, whereas the methoxide or acetate precursors in the other reactions do not, indicating that quite different mechanisms are possibly operating in these processes. Most of this chemistry has been followed via spectroscopic (IR and H NMR) observation of the aluminum porphyrin species, and by organic product analysis, and relatively little is known about the details of the CO2 activation steps. 

The use of cyclic sulfates in synthetic applications has been limited in the past because, although cyclic sulfites are easily prepared from diols, a convenient method for oxidation of the cyclic sulfites to cyclic sulfates had not been developed. The experiments of Denmark [70] and of Lowe and co-workers [71 ] with stoichiometric ruthenium tetroxide oxidations and of Brandes and Katzenellenbogen [72a] and Gao and Sharpless [68] with catalytic ruthenium tetroxide and sodium periodate as cooxidant have led to an efficient method for this oxidation step. Examples of the conversion of several diols (67) to cyclic sulfites (68) followed by oxidation to cyclic sulfates (69) are listed in Table 6D.7. The cyclic sulfite/cyclic sulfate sequence has been applied to 1,2-, 1,3-, and 1,4-diols with equal success. Cyclic sulfates, like epoxides, are excellent electrophiles and, as a consequence of their stereoelectronic makeup, are less susceptible to the elimination reactions that usually accompany attack by nucleophiles at a secondary carbon. With the development of convenient methods for their syntheses, the reactions of cyclic sulfates have been explored, Most of the reactions have been nucleophilic displacements with opening of the cyclic sulfate ring. The variety of nucleophiles used in this way is already extensive and includes H [68], [68,73-76], F" [68,72,74], PhCOCT [68,73,74], NOJ [68], SCN [68],... 

Aral and co-workers [432] reported also a direct route for the preparation of cyclic styrene carbonate from styrene, which avoids the preliminary synthesis and isolation of styrene oxide. A catalyst system consisting of Au/Si02, zinc bromide and tetrabutylammonium bromide (Bu4NBr) was applied to the one-pot synthesis of styrene carbonate from styrene, organic peroxide and CO2. Au/Si02 is active for the epoxidation of styrene, and zinc bromide and Bu4NBr cooperatively catalyse the subsequent CO2 cycloaddition to epoxide. 

As an extension of this oxygenative C-C bond fission, cyclic carbonates are prepared in good yield from terminal epoxides and DMF using bismuth bromide as a catalyst and molecular oxygen as an oxidant (Scheme 5.16). This is the first successful use of bismuth bromide as the catalyst for oxidative functionalization [97CC95]. Ph3BiF2 is examined as a catalyst for this reaction [98RCB1607]. 

Carbonates and carbamates. In the presence of phosphine-CBr4 and base, alcohols combine with CO2 to afford carbonates. Mixed carbonates are obtained under different conditions, and with a Mg—A1 mixed oxide as catalyst, epoxides formed cyclic carbonates. The preparation of dimethyl carbonate from acetone dimethyl acetal and supercritical carbon dioxide in the presence of a metal catalyst (e.g., dibutyltin methoxide) is successfully carried out. ... 

The oxidative carboxylation of olefins to cyclic carbonates can proceed through the first step of epoxidation of olefins and the subsequent cycloaddtion of CO2 to epoxides formed (Scheme 18). Thus it is supposed that a system of combining catalysts effective for the first step and for the second one would be effective for the direct synthesis of cyclic carbonates via the oxidative carboxylation of olefins. Indeed the direct preparation of carbonates was successfully achieved with a few catalyst systems including ILs coupled with oxidation catalysts. One patent [66] reported that the cyclic carbonate was formed from an olefin, CO2, and oxygen in the presence of dual catalysts. The catalyst system includes a heavy metal compound and a quaternary ammonium hydroxide or haUde. However, the heavy metal compounds would easily induce the corrosion of equipments and result in the undesired reduction of activity and selectivity. 

Vinyl-functional alkylene carbonates can also be prepared from the corresponding epoxides in a manner similar to the commercial manufacture of ethylene and PCs via CO2 insertion. The most notable examples of this technology are the syntheses of 4-vinyl-1,3-dioxolan-2-one (vinyl ethylene carbonate, VEC) (5, Scheme 24) from 3,4-epoxy-1-butene or 4-phenyl-5-vinyl-l,3-dioxolan-2-one (6, Scheme 24) from analogous aromatic derivative l-phenyl-2-vinyl oxirane. Although the homopolymerization of both vinyl monomers produced polymers in relatively low yield, copolymerizations effectively provided cyclic carbonate-containing copolymers. It was found that VEC can be copolymerized with readily available vinyl monomers, such as styrene, alkyl acrylates and methacrylates, and vinyl esters.With the exception of styrene, the authors found that VEC will undergo free-radical solution or emulsion copolymerization to produce polymeric species with a pendant five-membered alkylene carbonate functionality that can be further cross-linked by reaction with amines. Polymerizations of 4-phenyl-5-vinyl-l,3-dioxolan-2-one also provided cyclic carbonate-containing copolymers. 

The conversion of carbon dioxide into useable substrates for further functionalization remains a current and challenging goal in synthetic chemistry. To this end, the direct conversion of diols into carbonates was achieved using cerium oxide/cyanopyridine as the catalyst system (Scheme 2.25) [30]. Both five- and six-membered carbonates can be generated in excellent yields from the corresponding diols. It should be noted that the preparation of six-membered carbonates through this type of reaction was a rare conversion. In related work, the conversion of epoxides into cyclic carbonates has been accomplished using a porphyrin-based catalyst (Scheme 2.26) [31]. 

At beginning, sihca-supported IL-phase catalyst, which was prepared by treating the silica gel in the acetone solution of IL [BMIm](BF4], was used in the synthesis of cyclic carbonate under supercritical conditions [101]. Epoxides with different structures could be efHciently transferred into cycHc carbonates with up 100% yields. The catalyst could be easily separated by filtration and reused for several runs without obvious deactivation. In order to simplify the separation of catalyst from the reaction mixture, sihca gel-modified magnetite was used as the support for covalent immobilization of IL, and similar results were obtained [102]. 

For example, polymers having hydroxyl end groups can be prepared by reaction of polymer lithium with epoxides, aldehydes, and ketones III-113). Carboxylated polymers result when living polymers are treated with carbon dioxide (///) or anhydrides (114). When sulfur (115, 116), cyclic sulfides (117), or disulfides (118) are added to lithium macromolecules, thiol-substituted polymers are produced. Chlorine-terminus polymers have reportedly been prepared from polymer lithium and chlorine (1/9). Although lithium polymers react with primary and secondary amines to produce unsubstituted polymers (120), tertiary amines can be introduced by use of p-(dimethylamino)benzaldehyde (121). 

Unsymmetrical alkyl phenyl tellurium derivatives were prepared in good yields from phenyl trimethylsilyl tellurium and epoxides, carboxylic acid esters, and linear or cyclic ethers under very mild conditions. In these reactions, which proceed in dichloromethane in the presence of a catalytic amount of zinc iodide, a carbon-oxygen single bond is cleaved. The highly nucleophilic benzenetellurolate binds to the carbon fragment, whereas the trimethylsilyl group becomes linked to the oxygen. 

Because it is often possible to control the stereochemical orientation of substituents on a cyclic array, Baeyer-Villiger cleavages of substituted cyclic ketones have been used extensively in the stereocon-trolled syntheses of substituted carbon chains. An asymmetric synthesis of L-daunosamine intermediate (30) from a noncarbohydrate precursor employed the cyclopentenol (28), prepared in optically pure form (95% ee) from 2-methylcyclopentadiene using asymmetric hydroboration (Scheme 8). Stereoselective epoxidation, conversion to Ae ketone and regioselective Baeyer-Villiger oxidation afforded lactone (29). 

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