Esterification and Transesterification Reactions

Surprisingly, the stereoselectivity of the acyl-transfer reaction was reversed in the presence of a catalytic amount of base such as DMAP. The eifect of DMAP 

In 2003, Ishii and colleagues reported the first example of DKR accompanied by an intramolecular transesterification.This one-pot reaction allowed the synthesis of a chiral 4-hydroxymethyl-2-oxazolidinone with an excellent dia-stereoselectivity, starting from a serinol derivative. It was demonstrated that the 

---

Organotin compounds such as monobutyltin oxide, the main substance used, accounting for 70% of consumption, dibutyltin oxide, monooctyltin oxide, and dioctyltin oxide are used in certain esterification and transesterification reactions, at concentrations between 0.001% and 0.5% by weight. They are used in the production of substances such as phthalates, polyesters, alkyd resins, fatty acid esters, and adipates and in trans-esterifications. These substances are in turn used as plasticizers, synthetic lubricants, and coatings. Organo-tins are used as catalysts to reduce the formation of unwanted by-products and also provide the required colour properties (ETICA, 2002). 

The production of PET is a well-known industrial process. Early patents on PET synthesis refer to the 1940s. Esterification and transesterification reactions have been investigated since the end of the 19th century. PET production plants have been optimized over the last few decades based on well-established production know-how . PET is now a commodity product with unusually rapid growth and further nearly unlimited future growth perspectives. 

There are a few reported cases of esterases that catalyze not only hydrolysis but also the reverse reaction of ester formation, in analogy with the global reaction described for serine peptidases (Fig. 3.4). Thus, cholesterol esterase can catalyze the esterification of oleic acid with cholesterol and, more importantly in our context, that of fatty acids with haloethanols [54], Esterification and transesterification reactions are also mediated by carboxyleste-rases, as discussed in greater detail in Sect. 7.4. 

Catalysts able to promote both transesterification of triglycerides and esterification of FFAs in an oil or fat are still rare and up to now none of them have found a commercial application. Thus far it seems that a moderate to high concentration of strongly acidic sites and a hydrophobic surface are mandatory to achieve good conversions in simultaneous esterification and transesterification reactions, but more research and creative thinking are still needed in this area. 

Many types of solid catalysts have been tested in esterification and transesterification reactions of fatty acids, TG feedstock and simple esters. Nonetheless, it is possible to group most catalysts in three general categories metal, base, and acid catalysts. The following sections deal with these three groups accordingly. 

Perhaps, a more important reason for the little research in this particular area is the slow reaction rate associated with acid catalysis in general. However, the ability of solid acids to catalyze both esterification and transesterification reactions simultaneously and the possibility for employing catalysts that are reusable and green, meaning that they do not pose a great environmental threat, are attractive aspects that make the study of these materials imperative. 

Lipase-catalysed esterification and transesterification reactions have a wide range of applications in the synthesis of aroma compounds. 

A conaparison of the results of potentiometric titration of preparations of the starting cellulose and of the phosphorylated cellulose has shown that the latter contains acidic hydroxyl grou] It has also been found that all of the reaction products obtained contain methoxyl groups. These rraults, together with data frcan chromatographic studies, show that the phosphorylation erf cellulose with monomethylphosphite involves the simultaneous occurrence of the esterification and transesterification reactions. 

Use of Enzymes Lipases are widely used in the processing of fats and oils as catalysts of a number of important lipid reactions, such as hydrolysis, esterification, and transesterification reactions (174). There are a wide number of lipases obtained from different sources, which are available commercially in their free/ crude or immobilized form. However, enzymes with a higher tolerance of pressure would be welcomed, and more research is needed to hopefully develop such enzymes (i.e., genetic engineering or marine sources of the deep ocean). 

Glycerol etherification is carried out at 260°C in a batch reactor at atmospheric pressure under N2 in the presence of 2 wt% of catalyst, water being eliminated and collected using a Dean-Stark system. Reagents and products are analysed with a GPC after silylation [5]. Batch processes are generally used in lipochemistry, especially for the esterification and transesterification reactions (except for the preparation of methyl esters). 

Loupy and co-workers [68] have studied the effectiveness of microwave irradiation in increasing the enzymatic affinity and selectivity of supported lipases in esterification and transesterification reactions under dry media conditions (see Scheme 37). The esterification and transesterifications of racemic 1-phenylethanol 64 were studied in a temperature range of 70-100 °C. The lipases considered were the Pseudomonas cepacia lipase (LP) and Candida Antarctica lipase (SP-435). The initial rates and enantiomeric ratios E were significantly enhanced under microwave irradiation. Even so, in cases where classical conditions showed poor reaction, complete conversion could be achieved with increased reactivity under microwave conditions. This was largely attributed to the exclusion of the volatile by-products from the equilibrium. More importantly, the supported enzymes showed good stability and could be reused three more times in the reactions under study without loss of activity. 

Loupy and co-workers [68] have studied the effectiveness of microwave irradiation in increasing the enzymatic affinity and selectivity of supported lipases in esterification and transesterification reactions under dry media conditions (see Scheme 37). The esterification and transesterifications of racemic... 

Lipases are being used in several reactions of synthesis for the production of valuable compounds. Biodegradable polymers, like butyl oleate and some polyesters, have been synthesized by esterification and transesterification reactions with lipases... 

Yadav GD, Devi KM (2004) Immobilized Upase-catalysed esterification and transesterification reactions in non-aqueous media for the synthesis of tetrahydrofurfuryl butyrate comparison and kinetic modeUng. Chem Eng Sd 59(2) 373-383... 

Esterification and transesterification reactions are commonly used in constructing a synthetic scheme for a product. Because the temperatures employed are usually higher than 200 °C, ion-exchange resins cannot be used, as they are for... 

In this section, the kinetics of the second step, that is, the polycondensation reaction of aliphatic polyesters, is investigated and a simple theoretical model is proposed to simulate both esterification and transesterification reactions taking place during polycondensation. 

IV with temperature was observed with the value at 245 °C almost double that at 210 °C. This is a result of higher esterification and transesterification reaction rates obtained at increased temperatures, as well as higher diffusion rates of byproducts produced (i.e., water and ethylene, propylene, or butylene glycol). The same effect of temperature on the reaction was observed in all polyesters, that is, PESu, PPSu, and PBSu. An increase in polycondensation time increases the IV at each temperature and polyester produced. This increase of IV with time is smoother at low temperatures (e.g., 210 °C), while more abrupt at higher temperatures (e.g., 245 °C). 

Esterification reactions have been described extensively in the literature. Otera (2003), in his study Esterification Methods, Reactions and Applications, has collected approximately 5000 bibhographic references. Despite this huge collection of information, the section regarding industrial applications of esterification (and transesterification) reaction is not complete because some of the industrial processes are not fully disclosed. 

Esterification and transesterification reactions using lipases are well-established processes in the area of foods and flavors. Value-added products are produced by these processes. The lipases are tolerant in organic media. 

Lipases are used to catalyze hydrolytic, esterification and transesterification reactions. These reactions alter the physical properties of fats and oils and thereby produce a wide range of products (Mukherjee, 1990). Lipases have also been used for the kinetic resolution of Isomers of alcohols or fatty acids (Hills, et 1990). The lipases... 

Borges ME, Diaz L Recent developments on heterogeneous catalysts for biodiesel production by oil esterification and transesterification reactions a review. Renew Sustain Energy Rev 16 2839-2849, 2012. http //dx.doi.Org/10.1016/j.rser.2012.01.071. 

Since the beginning of enzyme catalysis in microemulsions in the late 1970s, several biocatalytic transformations of various hydrophilic and hydrophobic substrates have been demonstrated. Examples include reverse hydrolytic reactions such as peptide synthesis [44], synthesis of esters through esterification and transesterification reactions [42,45-48], resolution of racemic amino acids [49], oxidation and reduction of steroids and terpenes [50,51], electron-transfer reactions, [52], production of hydrogen [53], and synthesis of phenolic and aromatic amine polymers [54]. Isolated enzymes including various hydrolytic enzymes (proteases, lipases, esterases, glucosidases), oxidoreductases, as well as multienzyme systems [52], were anployed. 

Acyl Formation Compared with hydrolysis reactions, esterification and transesterification reactions are much slower and require the use of activated esters to facilitate the reaction and to make it kinetically irreversible. These include trichloroethyl esters, trifluoroethyl esters, enol esters, thioesters and vinylcarbamates. Lipases isolated from Pseudomonas species are highly selective for the hydrolysis of esters of secondary alcohols, and therefore also for the corresponding reverse reactions. 

The conventional copolymerization pathways to PDMS and PET copolymers are paved with difficulties due to both physical incompatibility and chemical convertibility issues with regard to the catalysts and temperatures used for esterification and transesterification reactions [22]. In particular, the strong acids typically used in esterification or transesterification reactions will break the siloxane bonds Si-O-Si unless great care is taken. In order to address this problem, a facile enzymatic synthesis of silicone aromatic polyester (SAPE) and silicone aromatic polyamide (SAPA) in toluene under mild reaction conditions has been reported [26, 27], as shown in Schemes 2.4 and 2.5. 

S.V. 330-350 m.p. 55-60 °C There are numerous variations of the above formula possible because the hydroxyl and carboxyl groups of citric acid participate in many esterification and transesterification reactions. 

---