Transesterification base-induced

Alcohols can also be prepared from support-bound carbon nucleophiles and carbonyl compounds (Table 7.4). Few examples have been reported of the a-alkylation of resin-bound esters with aldehydes or ketones. This reaction is complicated by the thermal instability of some ester enolates, which can undergo elimination of alkoxide to yield ketenes. Traces of water or alcohols can, furthermore, lead to saponification or transesterification and release of the substrate into solution. Less prone to base-induced cleavage are support-bound imides (Entry 2, Table 7.4 see also Entry 3, Table 13.8 [42]). Alternatively, support-bound thiol esters can be converted into stable silyl ketene acetals, which react with aldehydes under Lewis-acid catalysis (Entries 3 and 4, Table 7.4). 

PROBLEM 18.9 Write mechanisms for acid- and base-induced transesterification. 

Anhydrides react with water to generate carboxylic acids, with alcohols to give esters, and with amines to form amides. Esters behave similarly. There are both acid-catalyzed and base-induced reactions of esters with water (ester hydrolysis) to give carboxylic acids. There are both acid- and base-catalyzed versions of the reaction of esters with alcohols (transesterification) that generate new esters. Esters also react with amines to form amides. 

DCP as a Chiral Controller in Oxidative Free Radical Cyclizations. As a chiral auxiliary, DCP (1) is also reported to induce modest diastereoselection (60% de) in Mn(III)-based oxidative free-radical cyclizations of p-keto esters (eq 12). Chiral p-keto ester 25 was prepared by transesterification reaction with methyl ester 23, 1, and 0.3 equiv of DMAP (catalyst) in anhydrous toluene at reflux for 3-5 d as described by Taber. Oxidative cyclization of a 0.1 M solution of 24 in AcOH with 2 equiv of Mn(OAc)3-2HzO and 1 equiv of Cu(OAc)3 HzO provided bicyclo[3.2.1]octan-2-one (25). 

Transesterification is catalyzed by acids or bases, or performed under neutral conditions. It is an equilibrium reaction and must be shifted in the desired direction. In many cases low-boiling esters can be converted to higher boiling ones by the distillation of the lower boiling alcohol as fast as it is formed. Reagents used to catalyze transesterification include Montmorillonite KIO and various Lewis acids. A polymer-bound siloxane has been used to induce transesterification. This reaction has been used as a method for the acylation of a primary OH in the presence of a secondary OH. Regioselectivity has... 

The key structural feature of POST-1 - the presence of dangling pyridine groups in the channels - affords a unique opportunity to perform asymmetric heterogeneous catalysis. Thus, potentially, any base catalyzed reactions (e.g., esterification or hydrolysis) can be performed with POST-1. Moreover, chiral pores should induce a degree of enantioselectivity in the final product mixture. The catalytic activity of POST-1 in the transesterification reaction was examined. Although the reaction of 16 and ethanol in the presence of POST-1 in carbon tetrachloride produced ethyl acetate in 11% yield, little or no transesterification occured without POST-1 or with the iV-methylated POST-1 (Sect. 2.2). The post chemical modification of the pyridine groups in POST-1 proves the role of free pyridine moiety in transesterification reaction. Transesterification of ester 16 with bulkier alcohols such as isobutanol, neopentanol, and 3,3,3-triphenyl-l-propanol occurs at a much slower rate under otherwise identical reaction conditions. Such size selectivity suggests that catalysis mainly occurs in the channels. 

The principles of biodiesel synthesis are relatively simple oleochemical reactions (see Chapter 9.1) and have been known and applied for many decades (Figure 8.5)." The basic technology consists in a catalyst induced transesterification of a vegetable oil in a batch, semi-batch or continuous process to create a fatty acid methyl ester (FAME). The catalyst used is a strong base,... 

The earliest studied main-chain polymers consisted of chromophores with their dipole moments oriented head-to-tail along the polymer chain [127-129]. It was expected that this head-to-tail arrangement of the molecular dipole moments might result in a coherent enhancement of the second-order nonlinear properties [130]. Williams et al. [131,132] reported a series of head-to-tail materials based on 2-cyano-3-(4-hydroxyphenyl)-2-propenoate ester random copolymers [Structure (15)] prepared by transesterification. The homopolymers are insoluble, intractable materials, whereas copolymers are soluble and can be spun to form thin films. In those polymers, in which the dipoles of the individual chromophores point in one direction, large dipole enhancement will be observed. By electric field-induced second-harmonic generation (EFISH) measurements in solution, enhancement was observed but when extended to the polymer films, the expected enhancement in dipole moments and stability have been not realized. 

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