Transesterification

Transesterification (also known as ester exchange, ester interchange, or ester alcoholysis) is the most important of the esterification reactions. Polyesters of relatively low molecular weight (around 2000) prepared from aliphatic dicarboxylic acids and glycols are most commonly made by direct esterification (Section 5.2) with or without addition of an external acidic catalyst. Phosphoric acid, p-toluenesulphonic acid, and antimony pentafluoride have been used as catalysts. The preparation of PET by the direct esterification of TA with ethylene glycol was not practical until the 

The transesterification of a diester and a glycol is customarily carried out using an excess of glycol to drive the equilibrium 

After the more volatile alcohol ROH has been eliminated, further reaction proceeds by elimination of glycol, usually at higher temperatures and subsequently under vacuum, viz. 

It is convenient to consider the preparation of polyesters by transesterification as taking place in two stages (Sections 5.3.1 and 5.3.2) as shown by the last two reactions. The last, of course, also applies to the process of direct esterification when a two-fold or greater excess of volatile glycol is used. The glycolysis of DMT (Section 5.3.1) md the polycondensation stage (Section 5.3.2) are fundamentally the same, especially in regard to catalysis [56]. 

Most PET is prepared by catalysed ester-interchange between DMT and glycol, followed by catalysed polycondensation. In the case of PET prepared directly from TA, catalysts are customarily used only in the polycondensation stage. The choice of the best catalyst or catalysts is obviously of extreme importance in the commercial preparation of PET. Rates of reaction must be as fast as can practically be achieved, and colour-forming side reactions, degradations, or reactions that lead to the copolymerization of excessive amounts of diethylene glycol must be minimized. 

The equilibrium for the transesterification of isoamyl acetate by ethanol can be affected by the solvent system. This reaction was studied in SCCO2, the IL [BMIMjfPF j and the scC02-[BMIM][PF6] biphasic system at 65 °C using p-xylenesulfonic acid (p-TSA) as a catalyst [Eq. (12)] [60]. 

Transesterification has also been used to demonstrate that enzyme catalysis can be carried out in SCF-IL biphasic systems. ILs can stabilize enzymes, thereby enabling their use at higher temperatures [61, 62]. SCCO2, on the other hand, may cause reductions in activity for enzymes either because of changes in pH, as a result of the acidic CO2 dissolving in water, or as a result of conformational changes 

Lozano et al. [64—67], and Reetz, Leitner and co-workers [68, 69], simultaneously reported SCCO2-IL biphasic, biocatalytic systems for lipase-catalyzed transesterifications using vinyl esters as the transesterification agent [Eq. (13)]. Vinyl butyrate was used because the product vinyl alcohol tautomerizes to acetaldehyde and hence the reaction is irreversible. 

A two-step extraction procedure, typically 9-10.5 MPa CO2 at 60 °C, dropping to 8 MPa in the collector, over 90 h to extract the alcohol followed by 20 MPa and 45 °C also worked well for other secondary alcohol substrates so that yields of each of the unreacted alcohol and the recovered ester can be 90% with ee values 90%. The ee is generally lower for the ester than for the recovered alcohol, the exception being PhCH2CH2CH(OH)CH3, where only 72.5% of the ester was recovered ee 88.2%). The recovered alcohol (92.4%) had an ee of only 65% [69]. 

Finally, the teams of Reetz and Leitner operated a reaction using 2-phenylethanol and vinyl dodecanoate continuously in a reactor consisting of two consecutive autoclaves containing the enzyme-IL suspension, to ensure high conversion, and two decompression chambers with a liquid recycling system and the ability to add further CO2 independently [69], The process was performed at 50 °C and 2 MPa with a mixed substrate flow rate (33% alcohol) of 0.6 cm min . The CO2 stream leaving the reactor was decompressed to 1.3 MPa at 50 °C in a first chamber, where extra CO2 was added to help remove the alcohol into the second chamber (100 °C, 

The mechanism and thermodynamics of transesterification of acetate-ester enolates in the gas phase have been investigated. The catalytic effect of alkali-metal t-butoxide clusters on the rate of ester interchange for several pairs of esters has been determined in non-polar and weakly polar solvents. Reactivities increase in the order (Li+ Na+ K+ Rb+ Cs+) with the fastest rates reaching lO catalytic 

It has been disclosed that 1,3-disubstituted tetraalkyldistannoxanes are extremely effective catalysts for transesterifications of carboxylic esters in which secondary 

It has been reported that the distannoxanes can also promote transformation of the carbonyl function, e.g. esterification [333b], lactonization [340a], polymerization [340 b, c], acetahzation [341], deacetalization [342], and desilylation [342] as well as transesterification, and in each reaction the mild conditions enable survival of a variety of acid-labile functions. With recourse to organotin-catalyzed transesterification, a variant of deacylation can be performed under mild conditions while conventional deacylation demands acidic or basic conditions. When l,n-diol diacetate was treated 

At the ripe old age of 31, Otto Bayer, who just happened to have the same surname as the famous German company that he worked for, Farbenfabriken Bayer A.G., became the head of their Central Scientific Laboratory in Leverkusen. This was in 1934 and Otto Bayer s initial focus was on dyestuff chemistry, a field in which German chemists excelled. But he realized that the future of the company required diversification into new areas of industrial chemistry. Crop protection and the emerging field of macromolecu-lar science appeared to offer considerable potential. 

Otto Bayer was aware of the work of Staudinger and Carothers. He was particularly impressed with the latter s discovery of polyamides and its implications for the fiber and textile industries. He knew that in the condensation (linear step-growth) polymerization used to prepare nylon and polyesters a small molecule, usually water, is formed and has to be removed. 

The hydrophobic ionic liquid [BMIMJPFg has been consistently shown to provide the desired conformational flexibility to enzymes without drastically altering their catalytically active conformations. Therefore, this ionic liquid has been widely studied as a suitable medium for enzymatic transformations. 

The hydrophobicity of ionic liquids was found to be particularly beneficial for lipase PS-C-catalyzed transesterification of 2-hydroxymethyl-1,4-benzodioxane in the presence of vinyl acetate (277). The hydrophobic [BMIMJPFg functioned as a better promotional medium than methylene chloride and hydrophilic [BMIM]BF4, with either supported or unsupported enzyme for the catalytic transesterifications. The ionic liquid not only acted as a medium but also as a permanent host for the enzymes, so that the enzyme-ionic liquid system could be recycled several times without substantial diminution in lipase activity. 

In a similar investigation, transesterification reactions of vinyl acetate with alcohols in [BMIM]BF4 and [BMIM]PF6 in the presence of immobilized lipases CALB and PS-C were found to proceed with higher enantioselectivities than in THF or toluene, with the best result again being observed with [BMIM]PF6 (280). 

In a recent study of protease oc-chymotrypsin transesterification reactions, several ionic liquids ([EMIM]BF4, [EMIM]Tf2N, [BMIM]BF4, [BMIM]PF6, and [MTOA]Tf2N) were used as the isolation media for the enzyme. Among these ionic liquids, [EMIM]BF4 showed the least activity, and [BMIMJPF offered the best stabilization of the enzyme (281). 

Although [BMIM]BF4 has been evaluated as an isolation medium for lipase-catalyzed biotransformations, the general experience with it has not been favorable, relative to that with other ionic liquids, such as [BMIM]PF6. However, excellent performance was recently reported for [BDMIM]BF4 when it was used to host Candida antarctica (Novozym 435) for the enantioselective transesterification of 5-phenyl-l-penten-3-ol (+ and -) with vinyl acetate. The working hypothesis was that the oligomerization of acetaldehyde may be caused by the C2 proton of the [BMIM] ion because of the unfavorable acidity of this group (226). In contrast, the cation in [BDMIM]BF4 lacks this acidity. 

Ester-to-ester interchange has been applied successfully when the corresponding acids are difficult to process due either to their insolubility or lability. The reaction is equilibrium limited, which implies a need to have either of the reactants in excess, usually the alcohol, to shift the equilibrium 

Monoglycerides, the glycerol monoesters of fatty acids, are biodegradable and non-toxic molecules with a hydrophilic head and a hydrophobic tail, having surfactant and emulsifying properties. They can be used in foods, detergents, plasticizers, and cosmetic and pharmaceutical formulations.  

Oxide catalyst BET surface area Total acidic sites (pmol NHs-g ) Total basic sites (pmol COj-g ) Conversion (%) Monoglyceride selectivity (%) 

The selective hydrogenation of a,P-unsaturated aldehydes to unsaturated alcohol is an important reaction in the production of many pharmaceutical, agrochemical, and fragrance compounds. The hydrogenation of the C=C bond is thermodynamically more favorable than C=0 hydrogenation, and low yields of the desired product are obtained with conventional hydrogenation catalysts. 

Cerium-based platinum catalysts have been extensively studied for the hydrogenation of crotonaldehyde (CH3-CH=CH-CHO) or citral ((CH3)2C=CH-(CH2)2-C(CHs)=CH-CHO). 9 o The activation of the carbonyl bond is induced by the presence of oxygen vacancy sites located at the interface between ceria and the platinum particles. 

The selective hydrogenation of a,P-unsaturated aldehydes is used as a probe reaction in studying the strong metal/support interaction (SMSI). Ceria is able to form oxygen vacancies and interme-tallic compounds after reduction treatment at relatively high temperatures. 

---

Some esters of substituted alcohols have been synthesized by transesterification. Treatment of 4-methyl-5-thiazolecarboxylic acid (14) with 3-chloroethyldiethylamine in acetone in the presence of anhydrous potassium carbonate gives the desired ester (15) in good vield (60%) (Scheme 10) (163). 

Rea.ctlons, The chemistry of butanediol is deterrnined by the two primary hydroxyls. Esterification is normal. It is advisable to use nonacidic catalysts for esterification and transesterification (122) to avoid cycHc dehydration. When carbonate esters are prepared at high dilutions, some cycHc ester is formed more concentrated solutions give a polymeric product (123). With excess phosgene the usefiil bischloroformate can be prepared (124). 

Acryhc esters may be saponified, converted to other esters (particularly of higher alcohols by acid catalyzed alcohol interchange), or converted to amides by aminolysis. Transesterification is comphcated by the azeotropic behavior of lower acrylates and alcohols but is useful in preparation of higher alkyl acrylates. 

Higher alkyl acrylates and alkyl-functional esters are important in copolymer products, in conventional emulsion appHcations for coatings and adhesives, and as reactants in radiation-cured coatings and inks. In general, they are produced in direct or transesterification batch processes (17,101,102) because of their relatively low volume. 

Transesterification of a lower acrylate ester and a higher alcohol (102,103) can be performed using a variety of catalysts and conditions chosen to provide acceptable reaction rates and to minimize by-product formation and polymerization. 

Dialkylaminoethyl acryhc esters are readily prepared by transesterification of the corresponding dialkylaminoethanol (102,103). Catalysts include strong acids and tetraalkyl titanates for higher alkyl esters and titanates, sodium phenoxides, magnesium alkoxides, and dialkyitin oxides, as well as titanium and zirconium chelates, for the preparation of functional esters. Because of loss of catalyst activity during the reaction, incremental or continuous additions may be required to maintain an adequate reaction rate. 

RandomiZation/Interesterification. Transesterification occurs when a carboxyUc acid (acidolysis) or alcohol (alcoholysis) reacts with an ester to produce a different ester (20). Ester—ester interchange is also a form of transesterification. If completely unsaturated triglyceride oil (UUU) reacts with a totally saturated fat (SSS) in the presence of an active catalyst such as sodium, potassium, or sodium alkoxide, triglycerides of intermediate composition may be formed. 

Alternative technology for modifying a poly(aLkylene terephthalate) by incorporation of a phosphinate stmcture has been developed by Enichem. Phosphinate units of the stmcture —P(CgH5) (0)CH20— are introduced into a polyester such as PET or PBT by transesterification with an oligomer comprised of the aforementioned units (136). 

PET) is produced by esterification of terephthahc acid [100-21 -0] (1) to form bishydroxyethyl terephthalate [959-26-2] (BHET) (2). BHET polymerizes in a transesterification reaction catalyzed by antimony oxide to form PET (3). 

Glycols may undergo intramolecular cyclization or cycHcaHy condense with other molecules to form a number of ring stmctures. Transesterification of carbonates with ethylene glycol produces ethylene carbonate [96-49-1] (eq. 4). Numerous materials catalyze carbonate transesterifications. 

Esters. Neopentyl glycol diesters are usually Hquids or low melting soflds. Polyesters of neopentyl glycol, and in particular unsaturated polyesters, are prepared by reaction with polybasic acids at atmospheric pressure. High molecular weight linear polyesters (qv) are prepared by the reaction of neopentyl glycol and the ester (usually the methyl ester) of a dibasic acid through transesterification (37—38). The reaction is usually performed at elevated temperatures, in vacuo, in the presence of a metallic catalyst. 

Cychc carbonates are prepared in satisfactory quaUty for anionic polymerization by catalyzed transesterification of neopentyl glycol with diaryl carbonates, followed by tempering and depolymerization. Neopentyl carbonate (5,5-dimethyl-1,3-dioxan-2-one) (6) prepared in this manner has high purity (99.5%) and can be anionically polymerized to polycarbonates with mol wt of 35,000 (39). 

Manufacture. Cyanoacetic acid and cyanoacetates are iadustrially produced by the same route as the malonates starting from a sodium chloroacetate solution via a sodium cyanoacetate solution. Cyanoacetic acid is obtained by acidification of the sodium cyanoacetate solution followed by organic solvent extraction and evaporation. Cyanoacetates are obtained by acidification of the sodium cyanoacetate solution and subsequent esterification with the water formed being distilled off. Other processes reported ia the Hterature iavolve the oxidation of partially oxidized propionittile [107-12-0] (59). Higher esters of cyanoacetic acid are usually made through transesterification of methyl cyanoacetate ia the presence of alumiaiumisopropoxide [555-31-7] as a catalyst (60). 

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. 

Transesterification of methyl methacrylate with the appropriate alcohol is often the preferred method of preparing higher alkyl and functional methacrylates. The reaction is driven to completion by the use of excess methyl methacrylate and by removal of the methyl methacrylate—methanol a2eotrope. A variety of catalysts have been used, including acids and bases and transition-metal compounds such as dialkjitin oxides (57), titanium(IV) alkoxides (58), and zirconium acetoacetate (59). The use of the transition-metal catalysts allows reaction under nearly neutral conditions and is therefore more tolerant of sensitive functionality in the ester alcohol moiety. In addition, transition-metal catalysts often exhibit higher selectivities than acidic catalysts, particularly with respect to by-product ether formation. 

Polycarbonates are prepared commercially by two processes Schotten-Baumaim reaction of phosgene (qv) and an aromatic diol in an amine-cataly2ed interfacial condensation reaction or via base-cataly2ed transesterification of a bisphenol with a monomeric carbonate. Important products are also based on polycarbonate in blends with other materials, copolymers, branched resins, flame-retardant compositions, foams (qv), and other materials (see Flame retardants). Polycarbonate is produced globally by several companies. Total manufacture is over 1 million tons aimuaHy. Polycarbonate is also the object of academic research studies, owing to its widespread utiUty and unusual properties. Interest in polycarbonates has steadily increased since 1984. Over 4500 pubflcations and over 9000 patents have appeared on polycarbonate. Japan has issued 5654 polycarbonate patents since 1984 Europe, 1348 United States, 777 Germany, 623 France, 30 and other countries, 231. 

The historical direct reaction route, which utilised phosgenation of a solution of BPA in pyridine, proved inefficient commercially because of the need for massive pyridine recycle. Calcium hydroxide was used as an HCl scavenger for a period of time. In the historical transesterification process, BPA and diphenyl carbonate are heated in the melt in the presence of a catalyst, driving off by-product phenol, which is recycled to diphenyl carbonate. Using a series of reactors providing higher heat and vacuum, the product polymer was eventually produced as a neat melt. 

Transesterification. There has been renewed interest in the transesterification process for preparation of polycarbonate because of the desire to transition technology to environmentally friendly processes. The transesterification process utilizes no solvent during polymerization, producing neat polymer direcdy and thus chlorinated solvents may be entirely eliminated. General Electric operates a polycarbonate plant in Chiba, Japan which produces BPA polycarbonate via this melt process. 

The polymer is exposed to an extensive heat history in this process. Early work on transesterification technology was troubled by thermal—oxidative limitations of the polymer, especially in the presence of the catalyst. More recent work on catalyst systems, more reactive carbonates, and modified processes have improved the process to the point where color and decomposition can be suppressed. One of the key requirements for the transesterification process is the use of clean starting materials. Methods for purification of both BPA and diphenyl carbonate have been developed. 

An analogue of the transesterification process has also been demonstrated, in which the diacetate of BPA is transesterified with dimethyl carbonate, producing polycarbonate and methyl acetate (33). Removal of the methyl acetate from the equihbrium drives the reaction to completion. Methanol carbonylation, transesterification using phenol to diphenyl carbonate, and polymerization using BPA is commercially viable. The GE plant is the first to produce polycarbonate via a solventiess and phosgene-free process. 

---