Alcohols from transesterification

One of the best examples of the utility of enzymatic synthesis in catalyzing reactions that cannot be accomplished by any other route is the synthesis of substituted oxazolidine diesters. The oxazolidine ring is extremely water sensitive, the oxazolidine rapidly reverting back to the alkanolamine and aldehyde in the presence of water. Bis-oxazolidines have been used as hardeners for polymer coatings but the diester based on the hydroxyethyl oxazolidine and adipic acid cannot be synthesized directly with chemical catalysis because of the rapid rate of reaction of the oxazolidine ring with either the water from the esterification or the alcohol from transesterification. ... 

We have also investigated the properties of several of our nanostructured catalysts as solid acids in reactions such as the dehydration of alcohols and transesterification reactions [99]. One of the best examples of atomically dispersed solid acid catalysts is aluminosilicates [100]. When aluminium is substituted into silicate frameworks and remains isolated from other A1 centers it can behave as a strong acid site [101]. 

The mathematical apparatus presented in 2.3.3 is restricted to irreversible reactions. Hydrolysis, due to the large excess of one reactant, water, is for practical purposes irreversible. In a transesterification the concentration of the leaving alcohol (from... 

Figure 2.11 Transesterification of a racemic mixture of a secondary alcohol (1 -phenoxy-2-propanol, 1 in Table 2.1) with a butanoic acyl donor follows a ping-pong bi-bi mechanism in which Substrate 1 (acyl donor) enters the enzyme, forms an acyl enzyme expelling Product 1 (the leaving alcohol from the acyl donor). Then another Substrate 2 (the enantiomers of the alcohol to be resolved) reacts with the acyl enzyme to liberate Product 2 (the enantiomers of the produced esters), leaving the enzyme in its original form. In a kinetic resolution one of the enantiomeric alcohols reacts faster than the other to form an excess of one enantiomer of the esters (ideally enantiopure, for 1 the (R)-ester was formed with very high ee). The success of the resolution is expressed by the enantiomeric ratio E, which depends on the difference in free energy of activation of the two diastereomeric transition states. These are in turn related to the two tetrahedral intermediates.

Several processes for the production of biodiesel fuel have been developed by acid-, alkali-, and enzyme-catalyzed transesterification reactions (7-10). Transesterification, called alcoholysis, is the displacement of alcohol from an ester by another alcohol in a process similar to hydrolysis. Transesterification is represented by a number of consecutive and reversible reactions. The reaction step is the conversion of triglycerides to diglycerides, followed by the conversion of diglycerides to monoglycerides and of monoglycerides to glyceride at each step (11,12). 

Higher alkoxides, such as tetra(2-ethylhexyl) titanate, TYZOR TOT [1070-10-6]9 can be prepared by alcohol interchange (transesterification) in a solvent, such as benzene or cyclohexane, to form a volatile azeotrope with the displaced alcohol, or by a solvent-free process involving vacuum removal of the more volatile displaced alcohol. The affinity of an alcohol for titanium decreases in the order primary > secondary > tertiary, and branched > unbranched. Exchange processes are more convenient than direct synthesis of tetraalkoxide from TiCl an alcohol, and a base because a metal chloride need not be handled. However, in general, traces of impurities of mixed tetraalkyl titanates can result. 

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]. 

In the esterification of an acid, an alcohol acts as a nucleophilic reagent in hydrolysis of an ester, an alcohol is displaced by a nucleophilic reagent. Knowing this, we are not surprised to find that one alcohol is capable of displacing another alcohol from an ester. This alcoholysis (cleavage by an alcohol) of an ester is called transesterification. 

The low-frequency Sn shift observed in all cases indicates a coordination expansion of the tin atom of the grafted Cll-SnCls catalyst from a four-coordinate state in its pure, unaltered form, to five- and/or six-coordination in the presence of ester and/or alcohol, from which it can be deduced that both the ester and alcohol coordinate the tin atom, which brings them in close mutual vicinity, favoring the nucleophilic attack of the oxygen of the alcohol onto the carbonyl carbon atom of the ester needed for the transesterification. 

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. 

An intermediate esterification is required to achieve anti-Markovnikov addition of HBr to higher unsaturated alcohols. For instance, even when a peroxide is added, 10-undecen-l-ol and HBr afford 10-bromo-l-undecanol, whereas HBr adds to 10-undecenyl acetate in the presence of peroxides so that the bromine appears in the 11-position whereas in the presence of antioxidants (H2 and diphenylamine) it appears in the 10-position.200 The HBr adducts obtained from the acetates in the presence of dibenzoyl peroxide can be converted into the bromo alcohols by transesterification with methanol and / -toluenesulfonic acid.201 11-Bromo-1-undecanol, 13-bromo-l-tridecanol, and 15-bromo-l-pentadecan ol can also be obtained by intermediate conversion of the corresponding co-unsaturated alcohols into the boric esters. 

Transesterification with other alcohols and phenols (equation 10), particularly if they are high boiling, is another typical reaction of trialkoxy- and triaryloxyboranes. Similar reactions with amines proceed very slowly, and preparation of triaminoboranes by displacement of alcohols from trialkoxyboranes is not a practical synthetic route. 

An interesting side reaction has also been observed in alcohol transesterification of polymer-bound trichlorophenyl esters. Thus, transesterification with secondary alcohols at 50-100 °C is accompanied by the formation of free carboxyl groups on the polymer. Free carboxyl groups may form via elimination of the resulting alkyl esters, or more probably from the corresponding transition states. In the case of tertiary alcohols no transesterification takes place, and only free carboxyl groups are formed. 

Linko et al. 1998). Lipases have been extensively used in the production of surfactants of different chemical nature by esterification of alkyl glycosides and fatty acids, by transesterification of natural oils and alcohols, by transesterification of phospholipids and alcohols, and also by esterification of amino acids and amides (Saxena et al. 1999). Personal care products, hke isopropyl myristate and isopropyl palmitate, have been produced in solvent-free media with immobilized lipase and wax esters (esters from long-chain fatty acids and fatty alcohols) are also being produced with lipases (Hasan et al. 2006). 

It should be pointed out that the side reactions during the transesterification of diphenyl H-phosphonate are strongly depressed. In diphenyl H-phosphonate, there is only one electrophilic center—the phosphorus atom. It is shown that reaction mixtures resulting from transesterification of diphenyl H-phosphonate with alcohols in pyridine were contaminated with triphenyl phosphite and phenyl H-phosphonate, products of the disproportionation of diphenyl H-phosphonate [34]. 

The preparation of glycerol-based Ca-synthons and intermediates represents an interesting example where enzymatic transesterification is widely exploited. Alcoholysis of tributyrin with PPL was already shown to produce a chiral diglyceride (Fig. 3). Alcoholysis of trityl-protected dibutyrin with methanol in organic solvents shows some essential features about regio- and enantioselectivity in bifunctional compounds (Fig. 18) (25). Most lipases, such as lipases PS and AK and Novozym 435, first regioselectively produce the primary alcohol from the diester. With lipase AK, the reaction leads to effective kinetic resolution (A), whereas Novozym 435-catalyzed reaction (B) is not enantioselective. However,... 

After a series of mechanical horror stories, attention was turned from transesterification to direct phosgenation. High molecular weight polymer was produced by passing phosgene into a stirred solution of bisphenol-A in a mixed methylene chloride/pyridine solvent. Excess pyridine and by-product pyridine hydrochloride was removed by water/acid washing. The polymer was recovered by addition of an anti-solvent such as alcohol or aliphatic hydrocarbon. This general process provided initial development quantities of polymer. 

Burton, 2009 Helwani et al., 2009). A typical form of this reaction is shown in Chapter 2. The fatty acid chains present in the feedstock usually range from 12 to 22 carbon atoms. In these types of reactions, the triglycerides are transformed into straight-chain molecules by exchanging the alcohol from an ester with another alcohol in a process similar to hydrolysis, except that an alcohol is used instead of water. The final product is always similar in size to the molecules of the species present in the diesel fuel (Rajan and Senthilkumar, 2009 Sinha et al., 2008). Therefore, such products can be used in normal engines without any modification. By transesterification, oil molecular weight can be reduced to approximately one-third and the viscosity to about one-seventh, and the flash point and in some cases the volatility can be reduced as well (Demirbas, 2009b). 

Alcohols may be released from the esterified form by any of the hydrolytic or transesterification procedures described in Chapter 4. If a pure wax ester fraction is hydrolysed, the alcohols are obtained simply by solvent extraction of the alkaline solution. On the other hand, when other lipids are present, it is advisable to isolate them as a class by adsorption chromatography. TLC on layers of silica gel G with the elution system described for simple lipid separations in Chapter 2, i.e. with hexane-diethyl ether-formic acid (80 20 2 by volume) as the mobile phase, is usually used. With such a system, any secondary alcohols migrate ahead of primary alcohols, which in turn are slightly less polar than cholesterol diols migrate just in front of monoacylglycerols. If cholesterol is present in an extract, it may be necessary to re-run the plate in the same direction to obtain additional resolution and ensure that primary alcohols and cholesterol are fully separated. Procedures of this kind were utilised to isolate trace levels of fatty alcohols from animal tissues, for example [108,662,904]. When wax esters are transesterified, the methyl esters and free alcohols can be separated on a mini-column of... 

Fatty acid alkyl esters (FAAE) can be prepared from transesterification of vegetable oils or animal fats with ahphatic alcohols (Knothe et al., 2005). The transesterification reaction, also known as alcoholysis, is the exchange of alkoxy group of an ester compound (TAG) with an aliphatic alcohol (the acyl acceptor) in the presence of a catalyst. The overall reaction is a sequence of three consecutive and reversible reactions in which DAG and MAG are formed as intermediate products (Ma and Hanna, 1999). The general reaction scheme is given in Figure 14.5. 

Biodiesel has an interesting characteristic its fuel properties depend on the feedstock (Knothe, 2005). Unlike ethanol and hydrogen, biodiesel is not a pure substance but is a mixture of alkyl esters that result from the transesterification reactions between the triglycerides and an alkyl alcohol. The transesterification reactions proceed via the following general reaction. 

The high competitive strength of an alcohol was demonstrated by the transesterification of 1-phenyl ethyl butyrate with octanol as the acyl acceptor in an emulsion system [22]. Addition of a water-miscible solvent, which extracts the alcohol from the interface, can increase the enantiomeric excess of the product. This effect was observed in several different systems [23-25]. 

Most large-scale industrial methacrylate processes are designed to produce methyl methacrylate or methacryhc acid. In some instances, simple alkyl alcohols, eg, ethanol, butanol, and isobutyl alcohol, maybe substituted for methanol to yield the higher alkyl methacrylates. In practice, these higher alkyl methacrylates are usually prepared from methacryhc acid by direct esterification or transesterification of methyl methacrylate with the desired alcohol. 

Apart from the direct action of an alcohol on a chlorosilane or siUcon, the only other commercial method used to prepare alkoxysilanes is transesterification. 

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... 

Transesterification has a number of important commercial uses. Methyl esters of fatty acids are produced from fats and oils. Transesterification is also the basis of recycling technology to break up poly(ethylene terephthalate) [25038-59-9] to monomer for reuse (29) (see Recycling, plastics). Because vinyl alcohol does not exist, poly(vinyl alcohol) [9002-89-5] is produced commercially by base-cataly2ed alcoholysis of poly(vinyl acetate) [9003-20-7] (see Vinyl polymers). An industrial example of acidolysis is the reaction of poly(vinyl acetate) with butyric acid to form poly(vinyl butyrate) [24991-31-9]. 

Chirazymes. These are commercially available enzymes e.g. lipases, esterases, that can be used for the preparation of a variety of optically active carboxylic acids, alcohols and amines. They can cause regio and stereospecific hydrolysis and do not require cofactors. Some can be used also for esterification or transesterification in neat organic solvents. The proteases, amidases and oxidases are obtained from bacteria or fungi, whereas esterases are from pig liver and thermophilic bacteria. For preparative work the enzymes are covalently bound to a carrier and do not therefore contaminate the reaction products. Chirazymes are available form Roche Molecular Biochemicals and are used without further purification. 

The cinnamyl ester can be prepared from an activated carboxylic acid derivative and cinnamyl alcohol or by transesterification with cinnamyl alcohol in the presence of the H-Beta Zeolite (toluene, reflux, 8 h, 59-96% yield). It is cleaved under nearly neutral conditions [Hg(OAc)2, MeOH, 23°, 2-A h KSCN, H2O, 23°, 12-16 h, 90% yield]or by treatment with Sulfated-Sn02, toluene, anisole, reflux. The latter conditions also cleave crotyl and prenyl esters. 

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