Exchange transesterification

Chemistry. Poly(vinyl acetate) can be converted to poly(vinyl alcohol) by transesterification, hydrolysis, or aminolysis. Industrially, the most important reaction is that of transesterification, where a small amount of acid or base is added in catalytic amounts to promote the ester exchange. 

Alkoxides can be formed also by reaction of zirconium dialkylarnines with alcohols, and alkoxides can be exchanged also be transesterification reactions. 

Ester interchange (transesterification) is a reaction between an ester and another compound, characterized by an exchange of alkoxy groups or of acyl groups, and resulting in the formation of a different ester. The process of transesterification is accelerated in the presence of a small amount of an acid or a base. 

Three types of transesterification are known (/) exchange of alcohol groups, commonly known as alcoholysis. In this process the compound with which an ester reacts is an alcohol ... 

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. 

A monoacetate can be isolated by continuous extraction with organic solvents such as cyclohexane/CCI4. Monoacylation can also be achieved by ion exchange resin or acid-catalyzed transesterification. 

Transesterifications, also termed ester exchange or ester interchange reactions, include hydroxy-ester, carboxy-ester, and ester-ester reactions. Hydroxy-ester reaction is the most important one and is used for many aromatic-aliphatic and wholly aromatic polyester syntheses. Carboxy-ester interchange is restricted to the synthesis of wholly aromatic polyesters while the ester-ester route is rarely used for polyester preparation due to slow kinetics. All these reactions may take place in comparable experimental conditions and can be catalyzed by similar classes of compounds. 

Fontes tt al. [224,225 addressed the acid—base effects of the zeolites on enzymes in nonaqueous media by looking at how these materials affected the catalytic activity of cross-linked subtilisin microcrystals in supercritical fluids (C02, ethane) and in polar and nonpolar organic solvents (acetonitrile, hexane) at controlled water activity (aw). They were interested in how immobilization of subtilisin on zeolite could affected its ionization state and hence their catalytic performances. Transesterification activity of substilisin supported on NaA zeolite is improved up to 10-fold and 100-fold when performed under low aw values in supercritical-C02 and supercritical-ethane respectively. The increase is also observed when increasing the amount of zeolite due not only to a dehydrating effect but also to a cation exchange process between the surface proton of the enzyme and the sodium ions of the zeolite. The resulting basic form of the enzyme enhances the catalytic activity. In organic solvent the activity was even more enhanced than in sc-hexane, 10-fold and 20-fold for acetonitrile and hexane, respectively, probably due to a difference in the solubility of the acid byproduct. 

In spite of the above mentioned Co(EII) compounds, kinetically labile metal complexes may provide fast product/substrate exchange and some of these systems show real catalytic activity. In native dinuclear phosphatases Mg(II), Mn(II), Fe(II/III), or Zn(II) ions are present in the active centers. Although the aqua complexes of the weakest Lewis acids, Mg(H) and Mn(II), show measurable acceleration of e.g. the transesterification of 2-hydroxypropyl p-nitrophenyl phosphate HPNP, [Mn(II)] = 0.004 M, kobs/ uncat = 73 at pH 7 and 310 K, [38] or the hydrolysis of S -uridyluridine (UpU) [39], only a few structural [40] but no functional phosphatase-mimicking dinuclear complexes have been reported with these metal ions. 

Covalent polymers with reversible properties arising from dynamic covalent bonds such as disulfide exchange reaction [47 9], transesterification [50,51], transetherification [52], and boronate ester formation [53] were reported without respect to DCC. These studies should involve DCLs in... 

The second factor that additionally effects polycondensations are exchange reactions which can occur between free end groups and junction points in the chain, for example, between OH end groups and ester groups of a polyester (transesterification) ... 

Electrophilic alkenes have been appended to imidazolium-type ILs for use in the Diels-Alder cycloaddition, 1,4-addition, Heck and Stetter reactions.Electrophilic alkenes containing Wang-type linkers were alkylated to imidazole followed by ion exchange and esterification giving the desired TSIL. Diels-Alder cycloaddition was carried out with 2,3-dimethylbutadiene and cyclopentadiene to give corresponding adducts. After washing with ether, transesterification resulted in cyclohexene derivatives. Scheme 29. 

Transesterification of ortho esters using an acid (sulfonated polystyrene) ion-exchange resin [175],... 

Use of acidic ion-exchange resins for the transesterification of ortho esters [175],... 

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. 

The physical meaning of the Langmuir—Hinshelwood model was also examined by means of several transesterification reactions in the vapour phase at 120°C on a macroreticular ion exchanger [439,440,442]. The... 

Values of the parameters of rate equation (27) for vapour phase transesterification catalysed with macroreticular ion exchanger at 120° C [439]... 

A quantitative correlation of structural effects of four esters and four alcohols in the vapour phase transesterification on a macroreticular ion exchanger at 120°C was made using the Taft equation [441]. The authors found that rate coefficients [from eqn. (27)] yielded better correlation with steric (Es) than with polar (a ) parameters, while there was no significant difference between the correlations of the adsorption coefficients of alcohols, Kb, with both parameters. The correlations with Es yielded the slopes 1.4 and 0.6 for the reactivity of the esters and the alcohols, respectively, and —0.4 for the adsorptivity of the alcohols. The observed... 

Ratios of initial transesterification rates on the least and the most cross-linked ion exchanger [437 ]... 

It was found in transesterification of ethyl acrylate in the liquid phase over a non-porous KU-2 catalyst [464], that the structure of the alcohol influenced the value of the limiting sorption of alcohol by the ion exchanger, the logarithm of this value being a linear function of the dielectric constant of the alcohol. As the second-order rate coefficients yielded the same sequence as the limiting sorption values, viz. allyl alcohol > 1-butanol > 3-methyl-l-butanol, Filippov et al. [464] assumed a relation between the dielectric constant and the reactivity of the alcohols. 

Fig. 16. Effect of degree of crosslinking (% DVB) of standard (non-porous) ion exchanger on initial transesterification rate, r° (mol kg-1 h-1), of ethyl acetate with 1-propanol [436]. (1) Liquid phase at 52°C initial composition (mole%), 0.4 ethyl acetate, 0.4 1-propanol, 0.2 dioxan (solvent). (2) Vapour phase at 120°C partial pressure of reactants, 0.5 bar (ester—alcohol ratio 1 1).

In liquid phase reactions, the importance of the swelling properties and the related sorption capacities for the catalytic activity of ion exchangers was demonstrated. The rate coefficient of 1-butanol—acetic acid esterification [431] decreased with the degree of crosslinking in the same manner as did the water sorption capacity and the solvation coefficient of 1-butanol. A similar effect was found for the transesterification of ethyl acrylate with 1-butanol [404]. 

These considerations can be formulated by schemes (e)—(g) below (R1 = H or alkyl). Scheme (e), in which the acid (ester) is protonated and alcohol reacts in non-adsorbed state, corresponds to the mechanisms Aac1 or Aac2 proposed for homogeneous esterification and hydrolysis with ion exchanger catalysts, the mechanism (e) was assumed to be operating in the liquid phase esterification of salicyclic acid with methanol [449] and in the transesterification of ethyl acetate with the same alcohol in dioxan as... 

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