Alcoholysis/transesterification reactions

As expected, such reactions are susceptible to steric factors, as exemplified by the formation of Al(O-i-Pr) (0-r-Bu)2 2 and Ln Al(0-/ -Pr)(0-r-Bu)3 3 as the final products in the alcoholysis/transesterification reactions of relevant binary as well as heterometallic isopropoxides. 

Ester alcoholysis (transesterification) in organic media is an equilibrium reaction and must be shifted in the desired direction. For example, Bornscheuer and coworkers [61] reported the resolution of ibuprofen vinyl ester by transesterification tvith n-hexanol in the presence of CAL-B. The vinyl alcohol generated during the reaction tautomerizes to acetaldehyde, thus making the reaction irreversible, as illustrated in Figure 6.14. 

Alkaline earth metal oxides and hydroxides have also been tested in transesterification reactions. Ca(OH)2 did not show significant catalytic activity in the transesterification of rapeseed oil with methanol at conditions normally used to prepare biodiesel.Peterson et al. reported relative alcoholysis activities of a series of supported CaO catalysts under near reflux conditions of methanol-rapeseed oil mixtures at 6 1 molar ratios.Among the catalysts tested, the most active was CaO (9.2 wt% CaO) on MgO. For instance, in a 12 h reaction the total oil conversion using this catalyst was over 95%, similar to... 

Transesterification reactions have been extensively used for the preparation of further alkoxides.158-165 Mixed alkoxides [M(OR)5. r(OR )j ] were obtained from the same alcoholysis reactions.166,167 Exchange reactions between [Nb(OEt) ] or [Nb(OPr )s] and organic acetates have also been exploited for the preparation of higher alkoxides. 

Based on the very different behaviors of lipases A (CAL-A) and B (CAL-B) from Candida antarctica towards polyfunctional compounds in non-aqueous media, Liljeblad et al82 reported a novel lipase-catalyzed method for the resolution of A -heterocyclic amino esters using methyl pipecolinate 69 as a model compound. For this purpose, the chemo-and enantioselective alcoholysis and transesterification reaction of 69 in the presence of CAL-B and the A-acylations using CAL-A were studied, (cf. fig. 37 and 38). 

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). 

Esters such as di(2-ethylhexyl) adipate and an oligomeric ester of neopentyl glycol find application as lubricants, and it is suggested that they can be used as environmentally friendly substitutes for petroleum-derived lubricants. They have been synthesized recently by alcoholysis of dimethyl adipate ester and the corresponding alcohols, with alkaline earth metal compounds as the catalysts.[75] MgO does not show any activity for either transesterification reaction, whereas CaO gives yields of both diesters near 100 % after 4h in a batch reactor. 

Metal-oxides of the type (Al203)x(Sn0)Y(Zn0)z were also studied as heterogeneous catalysts for the transesterification reaction of soybean oil (Macedo et al., 2006). It was observed that these materials are active for soybean oil alcoholysis with different alkyl-chain alcohols using several alcohols, including branched ones. The best result was achieved using methanol, with conversion yields up to 80% in 4 h. As observed for the complex 1 in homogenous conditions, the catalytic activities are strongly dependent on the nature of the alcohol. For alcohols with a linear chain, the reaction activities decrease with... 

Metal alkoxides (homo as well as hetero) undergo facile and reversible alcoholysis and transesterification reactions that can be represented by the following equations ... 

The hydrolase-catalyzed reactions utilized most for the selective transformation of such substrates are hydrolysis (Schemes 11.1-1, 11.1-2, 11.1-4, 11.1-5 and 11.1-11), acylation (transesterification) (Schemes 11.1-3, 11.1-6 and 11.1-11) and alcoholysis (transesterification) (Schemes 11.1-7,11.1-8 and 11.1-15). Hydrolase-catalyzed esterification of an alcohol with a carboxylic acid, although highly useful in some casesl6Z, has been utilized to a lesser extent. Catalysis of formation and cleavage of the C-O bond of an ester or lactone by pig liver esterase, most lipases, a-chymotrypsin and subtilisin, which are all serine hydrolases, involves the following steps (Scheme... 

An ester reacts with an alcohol to form a new ester and a new alcohol. This is an example of an alcoholysis reaction. This particular alcoholysis reaction is also called a transesterification reaction because one ester is converted to another ester. 

SEC was also used to study the transesterification reaction by following the rate of equilibration of PET samples with non-equilibrium molecular mass distributions. It was found that the cyclic trimer was more susceptable to alcoholysis than were linear oligomers. SEC was particularly useful in determining the cyclic trimer content in PET. 

An alcoholysis reaction, in which an ester reacts with an alcohol to form a new ester and a new alcohol, is called a transesterification reaction. An example of this alcoholysis reaction is given in Figure 19.5. Transesteiification reactions are slow reactions because alcohols are poor nucleophiles and esters have very basic leaving groups hence, they occur under support of acid catalysis (Bmice, 2004). Similar to esterifications, transesteiification reactions are equihbiium reactions (Hoydonckx, Vos, Chavan, Jacobs, 2004) hence, an excess of alcohol and a direct extraction of products is needed to shift the equilibrium. 

In addition to the above, alcoholysis or transesterification reactions of metal alkoxides themselves have been widely used for obtaining the targeted homo- and heteroleptic alkoxide derivatives of the same metal. Since the 1960s, the replacement reactions of metal dialkylamides with alcohols has provided a highly convenient and versatile route (Section 2.9) for the synthesis of homoleptic alkoxides of a number of metals, particularly in their lower valency states. 

Transesterification reactions have the following advantages over alcoholysis reactions ... 

Many other processes and reactions of the monomer synthesis are described extensively in literature [12-14]. For different acrylic esters, especially on a laboratory scale, the alcoholysis of the corresponding acid chlorides as well as direct esterification reactions of methacrylic acid, but also transesterification reactions of MMA, are often preferred [13-15]. The physical properties of various monomers are well summarized in literature [16,17]. 

As shown in Figure 9, the first alcoholysis reaction for 4 h at 40°C with methanol using chlorogenate hydrolase produced methyl caffeate from 5-caffeoylquinic add. Next, the unreacted methanol was removed under reduced pressure (14 hPa) for 1 h at 80°C, and chlorogenate hydrolase was deactivated at 80°C. In the subsequent transesterification reaction by Novozyme 435 with 3-cyclohexyl-l-propanol, the methyl caffeate produced was converted to 3-cyclohexylpropyl caffeate under reduced pressure (845 hPa) to remove the by-product methanol. 

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. 

Step-growth (co)polymers bearing ester or amide bonds are synthesized via reversible reactions (see also Chapters 2, 3, and 10). It is relatively easy to convert them back to their monomers or oligomers (or value-added chemicals) by solvolytic reactions, such as hydrolysis, alcoholysis (including glycolysis), and other transesterification reactions, as well as by ammonolysis, aminolysis, acidolysis, or transamidation [1]. 

SchBine 20 Mechanism of sequential lipase-catalyzed alcoholysis-esterification. The alcoholysis reaction (A) was followed by a transesterification reaction (B) by the subsequent addition of 3 molar equivalents of vinyl acetate. 

Applications. Transesterifications via alcoholysis play a significant role in industry as well as in laboratory and in analytical chemistry. The reaction can be used to reduce the boiling point of esters by exchanging a long-chain alcohol group with a short one, eg, methanol, in the analysis of fats, oils, and waxes. For more details see References 7 and 68. A few examples are given below. 

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]. 

In 1965, Breslow and Chipman discovered that zinc or nickel ion complexes of (E)-2-pyridinecarbaldehyde oxime (5) are remarkably active catalyst for the hydrolysis of 8-acetoxyquinoline 5-sulfonate l2). Some years later, Sigman and Jorgensen showed that the zinc ion complex of N-(2-hydroxyethyl)ethylenediamine (3) is very active in the transesterification from p-nitrophenyl picolinate (7)13). In the latter case, noteworthy is a change of the reaction mode at the aminolysis in the absence of zinc ion to the alcoholysis in the presence of zinc ion. Thus, the zinc ion in the complex greatly enhances the nucleophilic activity of the hydroxy group of 3. In search for more powerful complexes for the release of p-nitrophenol from 7, we examined the activities of the metal ion complexes of ligand 2-72 14,15). 

Chemoenzymatic synthesis of alkyds (oil-based polyester resins) was demonstrated. PPL-catalyzed transesterification of triglycerides with an excess of 1,4-cyclohexanedimethanol mainly produced 2-monoglycerides, followed by thermal polymerization with phthalic anhydride to give the alkyd resins with molecular weight of several thousands. The reaction of the enzymatically obtained alcoholysis product with toluene diisocyanate produced the alkyd-urethane. 

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