Base Catalyzed Transesterification

The cleavage of peptides from the support by transesterification of the benzyl ester type linkage to the polymer yields the end product entirely protected, if side functions are shielded by residues stable against the basic detachment conditions. This is best fulfilled by protecting groups derived from sterically hindered tert. butanol. 

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The alternative method for making activated esters is base-catalyzed transesterification. Fmoc-amino acids are esterified in excellent yields by reaction with pentafluorophenyl trifluoroacetate at 40°C in the presence of pyridine (Figure 7.13). A mixed anhydride is formed initially, and the anhydride is then attacked by the pentafluorophenoxy anion that is generated by the pyridine. Succinimido, chlorophe-nyl, and nitrophenyl esters were made by this method when it was introduced decades ago. A unique variant of this approach is the use of mixed carbonates that contain an isopropenyl group [Cf C CfyO-COjR]. These react with hydroxy compounds in the presence of triethylamine or 4-dimethylaminopyridine (see Section 4.19) to give the esters and acetone.30 35... 

Dialkyl 4-pyrimidinylaminomethylenemalonates (1426, n = 2,3,5) were prepared in 70-80% yields in the base-catalyzed transesterification of diethyl 4-pyrimidinylaminomethylenemalonates (1426, n = 1) with the appropriate alkanol at ambient temperature for 120 hr in the presence of sodium hydride. Acid-catalyzed transesterification was unsuccessful (84JHC247). 

The methanolysis catalyst is generally a base such as potassium carbonate, since the base catalyzed transesterification is generally lower in energy(5). For the transesterification of the hydroxymethylated fatty esters, however, a Lewis acid (stannous 2-ethylhexanoate) is employed. Although this catalyst requires higher temperatures to achieve rapid equilibrium, it has the benefit of not requiring removal... 

Polycarbonates are prepared commercially by two processes Schotten-Baumann reaction of phosgene (qv) and an aromatic diol in an amine-catalyzed interfacial condensation reaction or via base-catalyzed 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 annually. Polycarbonate is also the object of academic research studies, owing to its widespread utility and unusual properties. Interest in polycarbonates has steadily increased since 1984. Over 4500 publications 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. 

Figu re 14.8 Mechanism of base-catalyzed transesterification of triglicerides [30]. 

EXAMPLE Base-catalyzed transesterification of an ester, cyclopentyl benzoate. Step 1 Addition of the nucleophile gives a tetrahedral intermediate. 

Rather than just showing the mechanisms for acid-catalyzed and base-catalyzed transesterification, let s consider how one might work out these mechanisms as in a problem. 

First consider the base-catalyzed transesterification of ethyl benzoate with methanol. This is a classic example of nucleophilic acyl substitution by the addition-elimination mechanism. Methoxide ion is sufficiently nucleophilic to attack the ester carbonyl group. Ethoxide ion serves as a leaving group in a strongly exothermic second step. 

Base-catalyzed transesterification is the process that converts waste cooking oil to biodiesel fuel. 

Base-catalyzed transesterification is a simple two-step nucleophilic acyl substitution ... 

All these plastics are essentially the same compound, composed of terephthalic acid (para-phthalic acid) esterified with ethylene glycol. This polyester is made by a base-catalyzed transesterification of dimethyl terephthalate with ethylene glycol at a temperature around 150 °C. At this temperature, methanol escapes as a gas, driving the reaction to completion. We will study polyesters and other polymers in more detail in Chapter 26. 

Scheme 7.1. Base-catalyzed transesterification of triacylglycerols (TAGs) to produce fatty acid esters (biodiesel). Methyl esters (shown) are the most common but others, such as ethyl esters, can be produced depending on the alcohol used in the reaction. Ri, R2 and R3 represent unique fatty acids attached to the glycerol backbone of the TAG.

Poly(vinyl alcohol) is made by free-radical polymerization of vinyl acetate and subsequent base-catalyzed transesterification with methanol to yield the alcohol polymer. 

Vicinal dioxalates are cleaved electrochemically to alkenes in DMF at a less negative potential ( —1.2 V vs. Ag/AgI) than vicinal diacetates or diols [72,73]. Oxalic esters of monohydric alcohols give quasi-reversible peaks in cyclic voltammetry, whereas oxalates of vicinal diols give irreversible peaks. Oxalates of the diols may be formed through a rapid, base-catalyzed transesterification using diethyl oxalate, for example [74]. 

The melt polymerization process involves the base-catalyzed transesterification reaction of BPA with diphenyl carbonate (Fig. 8). A small amount (less than 0.01% molar) of basic catalyst such as Na, Li, K, or tetralkylammonium hydroxide or carbonate is used during the initial stages of the reaction. The reaction is performed under vacuum at 180-300°C. At later stages of the reaction, the temperature and the vacuum are increased (less than ImmHg) to remove phenol and drive the product to high molecular weight. Subsequently, the polymer becomes very viscous, and special devices, such as devolatilizing extruders, are required to ensure complete removal of phenol. 

Poly(vinyl acetate) of number-average molecular weight 250,000 is hydrolyzed by base-catalyzed transesterification with methanol to yield poly(vinyl alcohol). Oxidation of the latter with periodic acid yields a poly(vinyl alcohol) with number-average degree of polymerization 485. Calculate the percentages of head-to-tail and head-to-head liiikages in poly(vinyl acetate). 

In the United States, the most utilized process is the base catalyzed transesterification of oil with alcohol. The base catalysis is popular because of its... 

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