Dialkyl carbonates

One example of a new class of compound in this area is dioctyl carbonate (1, Table 4.9). The product is synthesized by the trans-esterification reaction of octa-nol and dimethyl carbonate in the presence of alkali catalyst (Fig. 4.20). Dioctyl carbonate (Cetiol CC) is a dry emollient with excellent dermatological compatibility and a comprehensive and convincing performance profile for various appli- 

Spreading value Sensory feeling Skin compatibility Stability Origin 

Dicaprylyl carbonate 1600 mm2 in 10 min Dry, volatile silicon oil like Excellent Hydrolysis stable Vegetable oil (fatty alcohol) Yes 

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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). 

CycHc carbonates are made by treating 1,2-diols with dialkyl carbonates using an alkyl ammonium and tertiary amine catalyst. The combination of propylene glycol and dimethyl carbonate has been reported to result in a 98% yield of propylene carbonate (21). 

In general the -alkylation can be suppressed by using excess malonic ester 1. Another side-reaction is the decarbalkoxylation, whereby dialkyl carbonates 6 are formed ... 

The trialkyltin alkoxides can often be prepared more conveniently by azeotropic dehydration of the appropriate bis(trialkyltin) oxide and alcohols, or by heating together the bis(trialkyltin) oxide and dialkyl carbonate (192). The latter reaction involves formation, and then decarboxylation, of the alkyl trialkyltin carbonate, e.g.,... 

A carboxylic acid (not the salt) can be the nucleophile if F is present. Mesylates are readily displaced, for example, by benzoic acid/CsF. Dihalides have been converted to diesters by this method. A COOH group can be conveniently protected by reaction of its ion with a phenacyl bromide (ArCOCH2Br). The resulting ester is easily cleaved when desired with zinc and acetic acid. Dialkyl carbonates can be prepared without phosgene (see 10-21) by phase-transfer catalyzed treatment of primary alkyl halides with dry KHCO3 and K2C03- ... 

Cobalt(II) alkoxides are known and monomeric forms are part of a wider review.413 The interest in these compounds pertains to a potential role in catalysis. For example, a discrete cobalt(II) alkoxide is believed to form in situ from a chloro precursor during reaction and performs the catalytic role in the decomposition of dialkyl pyrocarbonates to dialkyl carbonates and carbon dioxide.414 A number of mononuclear alkoxide complexes of cobalt(II) have been characterized by crystal structures, as exemplified by [CoCl(OC(t-Bu)3)2 Li(THF)].415 The Co ion in this structure and close relatives has a rare distorted trigonal-planar coordination geometry due to the extreme steric crowding around the metal. 

Ito and co-workers observed the formation of zinc bound alkyl carbonates on reaction of carbon dioxide with tetraaza macrocycle zinc complexes in alcohol solvents.456 This reversible reaction was studied by NMR and IR, and proceeds by initial attack of a metal-bound alkoxide species. The metal-bound alkyl carbonate species can be converted into dialkyl carbonate. Spectroscopic studies suggested that some complexes showed monodentate alkyl carbonates, and varying the macrocycle gave a bidentate or bridging carbonate. Darensbourg isolated arylcarbonate compounds from zinc alkoxides as a by-product from work on polycarbonate formation catalysis.343... 

CO3 species was formed and the X-ray structure solved. It is thought that the carbonate species forms on reaction with water, which was problematic in the selected strategy, as water was produced in the formation of the dialkyl carbonates. Other problems included compound solubility and the stability of the monoalkyl carbonate complex. Van Eldik and co-workers also carried out a detailed kinetic study of the hydration of carbon dioxide and the dehydration of bicarbonate both in the presence and absence of the zinc complex of 1,5,9-triazacyclododecane (12[ane]N3). The zinc hydroxo form is shown to catalyze the hydration reaction and only the aquo complex catalyzes the dehydration of bicarbonate. Kinetic data including second order rate constants were discussed in reference to other model systems and the enzyme carbonic anhy-drase.459 The zinc complex of the tetraamine 1,4,7,10-tetraazacyclododecane (cyclen) was also studied as a catalyst for these reactions in aqueous solution and comparison of activity suggests formation of a bidentate bicarbonate intermediate inhibits the catalytic activity. Van Eldik concludes that a unidentate bicarbonate intermediate is most likely to the active species in the enzyme carbonic anhydrase.460... 

There are relatively few reports of phase-transfer catalysed syntheses of phenols from activated haloarenes using quaternary ammonium salts, presumably because of the instability of the ammonium salts under the reaction conditions. A patented procedure for the conversion of, for example, 2,6-dichloropyridine into 6-chloropyrid-2-one (98%) using aqueous sodium hydroxide in the presence of benzyl-triethylammonium chloride at 120-150°C has been filed [32], A possible route to the phenols, however, comes from the observed reaction of phenols with potassium carbonateipotassium hydrogen carbonate to yield the aryl carbonates (80-85%) using the procedure described for the preparation of dialkyl carbonates (3.3.13) [50]. 

The phase-transfer catalysed reaction of alkyl halides with potassium carbonate in dimethylacetamide, or a potassium carbonate/potassium hydrogen carbonate mixture in toluene, provides an excellent route to dialkyl carbonates without recourse to the use of phosgene [55, 56], An analogous reaction of acid chlorides with sodium hydrogen carbonate in benzene, or acetonitrile, produces anhydrides (3.3.29.B, >80%), although there is a tendency in acetonitrile for aliphatic acid chlorides to hydrolyse yielding the acids [57]. 

DMC and other dialkyl carbonates offer powerful perspectives for the development of aUcylation/carboxyalkylation methods with low environmental impact. Moreover, these reactions are catalytic processes whose high selectivity allows minimization of the production of waste and of unwanted by-products as well. 

Figure 23. Anodic stability of linear dialkyl carbonates on GC and a composite cathode surface. Electrolytes 1.0 LiPFe in DMC or EMC, respectively. (Reproduced with permission from ref 76 (Figure 2). Copyright 1999 The Electrochemical Society.)...

Carbonic acid and dialkyl carbonates are also carbonyl-protonated in HSO3 F—SbFj —SO2 low temperature (Olah and White, 1968). 

Arylsulfonyl methyl isocyanides (3) and isocyano esters (1) can also be prepared by the deprotonation of alkylisocyanides (5) by strong bases (nBuLi or NaH) followed by the addition of TosF and dialkyl carbonates or ethyl chloroformate, respectively (pathB) [37, 38]. However, the use of small and foul-smelling alkyl isocyanides makes this route less attractive. 

The replacement of both -OH groups with chlorine produces Ccirbonyl dichloride, also known as phosgene, a useful reactant. For example, phosgene reacts with two moles of alcohol to form a dialkyl carbonate. The reaction of phosgene with one mole of alcohol produces an alkyl chloroformate, which is a useful intermediate in organic syntheses. The reaction of phosgene with four moles of ammonia yields urea and two moles of ammonium chloride, NH Cl. Figure 12-40 shows the structures of some of these compounds. 

Reaction with alcohol can produce two different types of products. While two molar equivalent of alcohol yields dialkyl carbonate, with one molar equivalent of alcohol the product is an alkyl chloroformate ... 

The reaction of aliphatic and aromatic ketone oximes 97 with a dialkyl carbonate 98 in the presence of K2CO3 at 180-190 °C yields 3-alkyl-4,5-disubstituted-2(3//)-oxazolones 104 in 22-48% yields. Mechanistically, it is proposed that N-alkylation of the initially formed oxime O-carbonate 99 yields 100, which affords the enamine 101 in the presence of base. A [3,3] sigmatropic rearrangement ensues to produce 102, which then cyclizes to 104. In cases where 97 contains two methylene groups in proximity to the C=N bond, one of which is benzylic, the above reaction sequence is regioselective for the benzylic methylene group (Fig. 5.25 Table 5.6, Fig. 5.26). ... 

The sodium methoxide was obtained from Matheson, Coleman and Bell Co. The submitters carried out all operations with this reagent in a dry-box under a stream of dry nitrogen. Sodium ethoxide and potassium /eri-butoxide have been successfully substituted for sodium methoxide 2 the choice of sodium methoxide is here principally one of convenience. With other olefins, the choice of alkoxide depends upon the boiling points of the dichlorocarbene adduct and the corresponding dialkyl carbonates. 

The value 42 kcal/mole foi fhe resonance energy is given by the thermochemical data for the dialkyl carbonates. This value for resonance of the double bond among three positions is not unreasonable when it is compared with the corresponding value of 24 kcal/mole for esters of the fatty acids, in which the bond resonates between two positions. 

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