Dialkyl carbonates, synthesis

Scheme 5.8 The one-pot process for dialkyl carbonate synthesis using different alcohols. Reproduced from Ref. [14] by permission of The Royal Society of Chemistry...

Carboxyl and nitrile groups are usually introduced in synthesis with commercial carboxylic acid derivatives, nitriles, or cyanide anion. Carbanions can be carboxylated with carbon dioxide (H.F. Ebel, 1970) or dialkyl carbonate (J. Schmidlin, 1957). 

Both, the Kolbe-Schmitt reaction and the dialkyl carbonate route may be beneficial, notably when taking into account the former syntheses used to produce these carboxylato derivatives. Prior to the study of Tommasi et al., the formation of imi-dazolium carboxylates via highly reactive N-heterocydic carbenes (NHC) had been reported, although this rather challenging synthesis could not be extended to an industrial scale. The synthesis of l,3-diisopropyl-4,5-dimethyl-imidazohum-... 

Although the direct C02-based route to dialkyl carbonates offers selectivity improvement (almost 100% for DMC synthesis), limited yields may result due to unfavorable thermodynamics and catalyst deactivation by water. Until now, mechanistic studies have been fruitful in identifying some of the key steps of the... 

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. 

In the synthesis of carbamates, R NH.C02R, from A iV -dialkylureas, (R NH)2CO, and dialkyl carbonates, (RO)2CO, dibutyltin oxide, Bu2SnO, acted as an efficient catalyst. The proposed mechanism (Scheme 13) involves addition of the dialkyl carbonate to Bu2SnO to give an adduct (43), which is attacked by the urea to yield a new tin complex (44) and one molecule of carbamate. Attack by dialkyl carbonate upon this complex (44) yields a further molecule of carbamate and regenerates the original tin complex (43), which can continue the catalytic cycle.42... 

This method has been applied to a monomer synthesis (2.20).61 The quaternary ammonium salt is used as a phase transfer catalyst. The potassium iodide converts the starting chloride to a more reactive iodide in situ. The alcohol traps the intermediate isocyanate before it can react with the small amount of water that has to be present. If no methanol is present, the corresponding urea is formed in up to 87% yield. If only 0.3 equivalent of water is used, the product is the iso-cyanurate, the cyclic trimer of the isocyanate. Where n = 1, the isocyanatoethyl methacrylate is a useful monomer now made with phosgene (2.21).62 Dow has patented a route from ethanolamine plus a dialkyl carbonate, followed transesterification to a methacrylate carbamate, which is py-rolyzed to the isocyanate (2.22) in 50% yield.63... 

Oxidative carbonylation of alcohols in the presence of CO provides an economically viable route to dialkyl carbonates and/or oxalates (Eqs. (8.4) and (8.5)), both of which have important industrial applications. Dialkyl carbonates (e.g., dimethyl carbonate, propylene carbonate) are excellent solvents for a variety of organic substances [14]. Dialkyl oxalates have utility as solvents, C2 building blocks in fine chemicals synthesis, and intermediates in the manufacture of oxamide (as a fertilizer) [15]. Hydrogenation of dialkyl oxalates provides an alternative route to ethylene glycol that is independent of oil-derived resources [15,16]. 

The formation of dialkyl carbonates was observed in every case of the dialky oxalate synthesis that we discussed in the previous section. Generally, it is accepted that higher CO pressure favors dialkyl oxalate formation and lower pressure favors dialkyl carbonate formation. But in the palladium-alkyl nitrite system, we confirmed the oxidation state of... 

Another route to the synthesis of dialkyl carbonates from CO2 is the reaction with alcohols and tertiary amines mediated by acetylene. As alcohols, ethanol, isopropanol, and allyl alcohol have been employed. The resulting dialkyl carbonates are obtained in yields of 12-30% [707]. 

An approach for the synthesis of mixed dialkyl carbonates 989 employs the above three-component coupling system of aliphatic alcohol/C02/alkyl halide in the presence of CS2CO3, but without CO2 pressure. This method shows great versatility in terms of the alcohols 985 and alkyl halides 988 that can be used (see Table 4.27), reaction times are 2.5-23 h, and the yields of the resulting mixed dialkyl carbonates 989 are 91-98% [711]. 

A method for the synthesis of dialkyl carbonates makes use of l,l -carbonyl-bis(4-benzylidene-l,4-dihydropyridine) 993 as a reagent. The required activation energy for this reaction is provided by the aromatization energy of the 1,4-dihydropyridine system forming the 4-substituted pyridine 995. Di-tert-butyl carbonate 994 can be obtained in 66% yield [715]. 

An alternative technical method of BPA synthesis is transesterification of diphenyl carbonates and dialkyl carbonates with bisphe-nols (Figure 7.14). 

The reaction of the carbonato complex with C02 has allowed the demonstration of a facile insertion-deinsertion equilibrium. The study of the deinsertion reaction has allowed estimation of the activation parameters as being = 130 4.0 kJ mor and AS = 121.6 11.9 J moP K . From the above values the authors have calculated an approximate value of the equiUbrium cmistant for the carboxylation reaction equal to 3 x 10 M at 195 K (or a AG value for the same reaction of < 50 kJ moP ), showing that the insertion of CO2 into the M-O-alkyl bond is both kineticaUy and thermodynamically very favored. This trend has also been confirmed for the insertion of CO2 into the Nb-OR bond in [Nb(OR)5]2 (R = methyl, ethyl, aUyl) (see Sect. 6.2.2.1), a catalyst for the synthesis of dialkyl carbonates [67]. Very recently, the facile insertion of CO2 into metal-phenoxide bonds has been reported [68] for cobalt and zinc complexes (Fig. 4.2). It should be noted that such metal systems are used as catalysts in the copolymerization of CO2 with epoxides. 

Honda M, Kuno S, Begum N, Fujimoto K-I, Suzuki K, Nakagawa Y, Tomishige K (2010) Catalytic synthesis of dialkyl carbonate from low pressure CO2 and alcohols combined with acetonitrile hydration catalyzed by Ce02. App Catal A Gen 384(1-2) 165-170... 

Zhao W, Peng W, Wang D, Zhao N, Li J, Xiao F, Wei W, Sun Y (2009) Zinc oxide as the precursor of homogenous catalyst for synthesis of dialkyl carbonate from urea and alcohols. Catal Commun 10 655-658... 

After considering those previous reviews and other chapters of this book, we will concentrate our attention on the application of ILs to the synthesis of carbonate and related compoimds using carbon dioxide. Those compounds are of practical importance as solvents, reagents, fuel additives, and intermediates in the production of pharmaceuticals and fine chemicals [6,7]. The authors previously reviewed the potential application of ILs for the synthesis of cyclic carbonates from carbon dioxide [8]. The present chapter will deal with the synthesis of cyclic carbonates, dialkyl carbonates, other related compounds, and polycarbonates using ILs as solvents and catalysts. 

Scheme 30. Electrochemical synthesis of dialkyl carbonates from CO2 and alcohols in CO2-saturated [BMIm]BF4 in the presence of alkylating agent.

Relatively nontoxic dialkyl carbonates are very effective alkylating agents that promise to provide a safer alternative to alkyl halides or sulfates for some kinds of alkylation reactions. The promise of dimethyl carbonate for methylation (alkylation in which the methyl group, -CH3, is attached) has been enhanced by the straightforward synthesis of this compound from methanol and carbon monoxide in the presence of a copper salt ... 

What is the potential use of dialkyl carbonates, such as dimethyl carbonate, in green ehemical synthesis ... 

Synthesis of carbonates (Homogeneous catalyst) alkyl phenyl carbonate from dialkyl carbonate and phenol 2-hydroxyethyl methyl carbonate from dimethyl carbonate... 

Cyclopropanes are now readily available and have become useful, through hydrogenolysis, for synthesis of compounds containing quaternary carbons, em-dialkyl, r-butyl, and angular-methyl substituents (779), compounds often available only with difficulty otherwise (.77,5i,55,750,756), Cyclopropanes can be formed in good yields by hydrogenation of cyclopropenes (26). 

Notable examples of general synthetic procedures in Volume 47 include the synthesis of aromatic aldehydes (from dichloro-methyl methyl ether), aliphatic aldehydes (from alkyl halides and trimethylamine oxide and by oxidation of alcohols using dimethyl sulfoxide, dicyclohexylcarbodiimide, and pyridinum trifluoro-acetate the latter method is particularly useful since the conditions are so mild), carbethoxycycloalkanones (from sodium hydride, diethyl carbonate, and the cycloalkanone), m-dialkylbenzenes (from the />-isomer by isomerization with hydrogen fluoride and boron trifluoride), and the deamination of amines (by conversion to the nitrosoamide and thermolysis to the ester). Other general methods are represented by the synthesis of 1 J-difluoroolefins (from sodium chlorodifluoroacetate, triphenyl phosphine, and an aldehyde or ketone), the nitration of aromatic rings (with ni-tronium tetrafluoroborate), the reductive methylation of aromatic nitro compounds (with formaldehyde and hydrogen), the synthesis of dialkyl ketones (from carboxylic acids and iron powder), and the preparation of 1-substituted cyclopropanols (from the condensation of a 1,3-dichloro-2-propanol derivative and ethyl-... 

Reductive and oxidative transformations of small ring compounds form the basis of a variety of versatile synthetic methods which include functionalization and carbon skeleton construction. Redox mechanisms of organotransition metal compounds play an important role in inducing or catalyzing specific reactions. Another useful route in this area is based on one-electron redox reactions. The redox tautomerism of dialkyl phosphonate also contributes to the efficiency of the reductive transformation of small ring compounds. This review summarizes selective transformations which have a high potential for chemical synthesis. 

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