Esters from transesterification

Figure 3. 3,5-Dinitrobenzoic acid esters from transesterification of some plasticizers Kieselgel G Eluent benzene,methyl acetate (150 1 pt. by vol.)...

Acrylic Esters. A procedure has been described for preparation of higher esters from methyl acrylate that illustrates the use of an acid catalyst together with the removal of one of the products by azeotropic distillation (112). Another procedure for the preparation of butyl acrylate, secondary alkyl acrylates, and hydroxyalkyl acrylates using -toluenesulfonic acid as a catalyst has been described (113). Alurninumisopropoxide catalyzes the reaction of amino alcohols with methyl acrylate and methyl methacrylate. A review of the synthesis of acryhc esters by transesterification is given in Reference 114 (see... 

The process of transesterification is an important way to prepare a large number of esters from more complex or more simple esters without passing through the carboxylic acid. Transesterification can be used to convert one type of ester to another type that can then be removed under a different set of conditions. This section describes many of the methods that have been found effective for ester metathesis. ... 

Biodiesel is a fuel derived from renewable natural resources such as soybean and rapeseed and consists of alkyl esters derived from transesterification of triglycerides with methanol. In spite of all the advantages of biodiesel, such as low emissiotts, biodegradability, non-toxicity, and lubricity, the major hurdle in penetration of biodiesel is its high cost because of the expensive food grade refined vegetable oil feedstock. 

Dimethylaminoethane-2-ol (20) is a compound that, by virtue of its nucleophilic center (Me2NH+C2H40), is employed to convert protected segments bound to supports as benzyl esters into acids by transesterification into dimethylaminoethyl esters [C(=0)0C2H4NMe2] that are hydrolyzable by a dimethylformamide-water (1 1) mixture. Compound 20 readily forms esters from acid chlorides. The hydrolysis and esterification are facilitated by anchimeric assistance by the adjacent nitrogen atom (see Section 2.10). The amino alcohol also reacts with dichloromethane. 

It, thus, appears that the capacity to catalyze reactions of transesterification and esterification is a characteristic of various hydrolases (Chapt. 3). Apart from the carboxylesterases discussed here, lipoprotein lipase has the capacity to synthesize fatty acid ethyl esters from ethanol and triglycerides, or even fatty acids [127]. Ethanol, 2-chloroethanol, and other primary alcohols serve to esterify endogenous fatty acids and a number of xenobiotic acids [128-130]. In this context, it is interesting to note that the same human liver carboxylesterase was able to catalyze the hydrolysis of cocaine to benzoylecgonine, the transesterification of cocaine, and the ethyl esterification of fatty acids [131]. 

The next step is not immediately obvious. The generation of an ethyl ester from a lactone can be accommodated by transesterification (we might alternatively consider esterification of the free hydroxyacid). The incorporation of chlorine where we effectively had the alcohol part of the lactone leads us to nucleophilic substitution. That it can be SnI is a consequence of the tertiary site. Cyclopropane ring formation from an Sn2 reaction in which an enolate anion displaces a halide should be deducible from the structural relationships and basic conditions. 

If a diastereomeric mixture of (S,S)-2,5-dimethyl-3.4-hexanediol esters 2 and 3 reacts with methylmagnesium bromide, the result is kinetic resolution, as verified for R1 = Bn. The diastereomeric mixture was prepared from the racemic ethylene glycol a-chloro boronic ester via transesterification with chiral diol. The products isolated were (S,S)-2,5-dimethyl-3,4-hexanediol [(/ )-2-phcnyl-l-methylethyl]boronate, diastereomeric ratio >95 5 as indicated by the rotation of the (/ )- -phenyl-2-propanol derived by deboronation with hydrogen peroxide, and (S,S)-2,5-dimethyl-3.4-hexanediol methylboronate. Phenylacetaldehyde was identified by 1H NMR4. 

Esters are produced by acid-catalysed reaction of carboxylic acids with alcohols, known as Fischer esterification. They are also obtained from acid chlorides, acid anhydrides and other esters. The preparation of esters from other esters in the presence of an acid or a base catalyst is called transesterification. All these conversions involve nucleophilic acyl suhstitu-tions (see Section 5.5.5). 

Natural Ethoxylated Fats, Oils, and Waxes. Castor oil (qv) is a triglyceride high in ricinoleic esters. Ethoxylation in the presence of an alkaline catalyst to a polyoxyethylene content of 60—70 wt % yields water-soluble surfactants (Table 20). Because alkaline catalysts also effect transesterification, ethoxylated castor oil surfactants are complex mixtures with components resulting from transesterification and subsequent ethoxylation at the available hydroxyl groups. The ethoxylates are pale amber liquids of specific gravity just above 1.0 at room temperature. They are hydrophilic emulsifiers, dispersants, lubricants, and solubilizers used as textile additives and finishing agents, as well as in paper (qv) and leather (qv) manufacture. 

In most homogeneously catalysed esterifications, the protonated species is considered to be formed from the acid (or from the ester in transesterification) [397,398,467]. Starting with this view, Bochner et al. [449] assumed a mechanism for heterogeneous esterification [see eqn. 

Keto acids and 2-alkyl- or 2-arylmalonic acids readily decarboxylate, and acylations with these proceed satisfactorily only at low temperatures. An alternative to direct acylation with /3-keto acids is thermolysis of acyl Meldrum s add (to generate an acylketene) in the presence of a nucleophile (Scheme 7.7). Esters of /J-keto acids can also be conveniently prepared from other keto esters by transesterification [27, 28],... 

The molar ratio of methanol to fatty acids is also an important parameter that controls the reaction. Figure 6 shows the obtained yields of methyl esters from oleic acid, a model of fatty acids, treated at various molar ratios of methanol to fatty acid. Interestingly, compared with the transesterification reaction shownby the dashed line (13), methyl esterification proceeded more at the lower molar ratio, and it is apparent that at a molar ratio of 3, oleic acid was mostly converted to its methyl ester. This result is important in designing the production process, since a reaction with a low molar ratio requires less energy for the process. 

Stern, R., Hillion, G, Rouxel, X-X, and Leporq, S. 1999. Process for the production of esters from vegetable oils or animal oils alcohols. United States Patent 5908946. Suppes, G. J., Dasari, M. A., Doskocil, E. X, Mankidy, P. J., and Goff, M. X 2004. Transesterification of soybean oil with zeolite and metal catalysts. Appl. Catal. A Gen., 257(2), 213-223. 

Shieh et al. (2003) indicated a biodiesel transesterification using soybean oil and methanol and commercial immobilized lipase from R. miehei (Lipozyme IM-77). The response surface analysis showed that the following variables were important reaction time, temperature, enzyme amount, molar ratio of methanol to soybean oil, and added water content on percentage weight conversion to soybean oil methyl ester by transesterification. The optimum yield based on ridge max analysis gave a 92.2% weight conversion. 

Alcoholysis. Methanolysis of triacylglycerols is used to prepare methyl esters for fatty acid analysis, a process frequently referred to as transesterification. This can be acid-or base-catalyzed, the method being chosen to avoid modifying acid-or base-sensitive fatty acids and to minimize reaction times. Sterol esters of fatty acids react more slowly than triacylglycerols, and samples containing them require more vigorous reaction conditions. The preparation of methyl esters from oils and fats for GC and GC-MS analysis has been extensively reviewed (19, 22, 23). 

The macrocyclization has been accelerated by heating the reaction mixture in a flask fitted with a reflux condenser. With alkoxy-containing complexes, care must be taken since upon inefficient cooling low-boiling boric acid esters can be distilled off, whereby the product yield is drastically reduced. In some cases, an appreciable effect has been observed when the water from the reaction mixture has been distilled off as azeotrope [47]. The preformed FeDm3(BOH)2 complex has undergone esterification in alcohol medium to give alkoxyboron-capped macrobicyclic compounds. Compounds of this type can also arise from transesterification of preformed complexes (Scheme 6) [47]. 

Most of the reported work on the kinetics of catalysed polycondensation by transesterification is quite recent and in the main deals with the preparation of PET, though related work on other esters from ethylene glycol has been reported. Reinisch et al. [34] found that the melt polycondensation leading to poly(ethylene sebacate) in 3—4 mm films under vacuum, with manganous acetate as catalyst, was second order from p = 0.70 to p = 0.97. Rate coefficients are listed in Table 5 for this reaction, for the formation of poly(ethylene sebacate-co-terephthalate) and for PET. 

The product this time is a 3-ketoester. A suitable base would be the alkoxide that is derived from the parent alcohol of the ester, because it does not matter if the ester undergoes transesterification that is, where one alkoxy moiety is exchanged for another. Note that, instead of the anionic adduct just picking up a proton, a alkoxide anion is eliminated on the formation of the carbonyl group. If both ester groups are in the same molecule, then an internal condensation reaction is possible. This is called the Dieckmann cyclisation, and works best when the ring formed contains five, six or seven members. 

Finally, the enzymatic nature of CPIA-cholesterol ester formation will be briefly mentioned. None of the enzyme preparations of three known biosynthetic pathways for cholesterol esters, namely, acyl-CoA cholesterol Q-acyltransferase (ACAT), lecithin cholesterol 0-acyltransferase (LCAT), nor cholesterol esterase, was effective in producing CPIA-cholesterol ester from the Ba isomer or CPIA. In contrast, the 9,000 g supernatant or microsomal fractions from liver or kidney homogenate were found to be capable of producing CPIA-cholesterol ester without the addition of any cofactors. As substrate, only the Ba isomer was effective, and none of the 3 other fenvalerate isomers nor free CPIA was effective. The hepatic enzyme preparation also catalyzed hydrolysis of fenvalerate, and in this case all the 4 isomers were utilized as substrates. These facts imply that CPIA-cholesterol ester is formed from the Ba isomer through a transesterification reaction via intermediary acyl-enzyme complex. 

Although the reaction of dialkyl phosphites with orthoformic esters has been extended to triethyl trithioorthoformate," the preferred synthesis of formylphosphonate dithioacetals is usually the high-yield Michaelis-Arbuzov reaction of trivalent phosphorus compounds with the appropriate chlo-rodithioacetals.""" For the corresponding hemithioacetals, a Pummerer-type reaction of a-phos-phoryl sulfoxides with alcohols in the presence of iodine is usually the method of choice (Scheme 5.3). " ° However, hemithioacetal formation is solvent dependent and generally gives a moderate yield of product in a mixture with several other byproducts arising from transesterification reactions. 

The preparation of esters can be classified into two main categories (1) carboxylate activation with a good leaving group and (2) nucleophilic displacement of a carboxylate on an alkyl halide or sulfonate. For simple esters, acid-catalyzed esterification with azeotropic removal of water is also very effective, but limited to simple systems for the most part. The nucleophilic approach is generally not suitable for the preparation of esters if the halide or tosylate is sterically hindered, but there has been some success with simple secondary halides and tosylates (ROTs, DMF, K2CO3, 69-93% yield). The section on transesterification should also be consulted, since this technology can be quite useful for the preparation of esters from other esters. 

The section on transesterification should be consulted since this method is applicable to the preparation of f-Bu esters from other esters. For example by transesterification of a methyl ester with f-BuOH and sulfated Si02. ... 

Fig. 7.2 Stoichiometric scheme of kinetically controlled synthesis of esters by transesterification from vinyl esters and alcohols catalysed by CALB...

Transesterification. Ethyl esters from carboxylic acids and acetic esters of benzylic alcohols are obtained in the catalyzed transesterification processes, using ethyl acetate as the donor of ethoxy or acetyl group. 

Transesterifications of (Reversion of hydrolytic reactions) (racemic) alcohols with nonactivated esters (racemic) alcohols with activated esters irreversible transesterification with vinyl-carboxyl ates esterases, lipases, proteases CCL, PPL, lipase from Pc PPL, pancreatin CCL, PFL, PPL, lipase PS and SAM II, lipase from Pf... 

Hu and Lambla [1995] have blended EM Ac (90-65 parts) with mono-hydroxy-terminated PS (10-35 parts) in an internal mixer at 180-220°C in the presence of dibutyltin dilaurate or dibutyl-tin oxide catalyst. A compatibilizing copolymer arises from transesterification between pendent ester groups of EMAc and terminal hydroxy groups of PS. The effects on blend properties of PS molecular weight were reported. The effects of processing conditions and addition of solvent on conversion kinetics were studied. 

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