Symmetric esters

Experiment IV represents another example of a symmetric ester, R Cq, in its parent acid solvent apart from n/i-isomer formation, no complications arise. 

The data in Table 2 show the potential of the Na2B407 based catalyst system tested over large number of representative alcohols. The primary alcohols were oxidized to the corresponding aldehydes at complete conversion of the alcohol and at 90-93% selectivity. The only by-products observed were the corresponding acid and minor amounts of the symmetrical ester (Entry 2, 3). Benzyl alcohol was quantitatively converted to benzaldehyde. The secondary alcohols, 4-methyl cyclohexanol and 4-methylpentanol were converted to the corresponding ketones at room temperature. 

A sulfoxide alkylatioii formed the key step of a synthesis of the important vitamin biotin. Biotin contains a five-membered heterocyclic sulfide fused to a second five-membered ring, and the bicyclic skeleton was easy to make from a simple symmetrical ester. The vital step is a double S 2 reaction on primary carbon atoms. 

The tautomerism of phosphites has been proved by the preparation of two triethyl phosphites.3 The one, prepared by the action of phosphorus trichloride on sodium ethoxide, was probably the symmetrical ester, being formed according to the equation... 

As shown below, there are essentially four types of symmetrical, ester triad mesogenic structures, and three types of ester dyad mesogenic structures ... 

Carbon disulfide is a valuable synthon (see Chapter 9, p. 147) which can be used for the synthesis of thiocarbonic acid derivatives. Thus, carbon disulfide reacts with ammonium polysulfide or hydrogen sulfide to give trithiocarbonic acid (70) or symmetrical esters (73) after reaction with an alkyl halide. With alkoxides or thiolates, carbon disulfide forms xanthates (77) or S-alkyl trithiocarbonates the latter by further treatment with alkyl, acyl or diazonium halides affords the derivatives (74)-(76) (Scheme 39). 

The transesterification reaction is a useful transformation that converts esters to other esters upon reaction with alcohols, as a result of exchange of alkoxy groups, avoiding the use of air- and moisture-sensitive reagents, such as acid chlorides. Further, ester-to-ester transformation is particularly useful when the carboxylic acid and their activated derivatives are not readily available [44]. In general, transesterification of secondary alcohols is slower than that of primary alcohols. The reaction is not atom-economical, as it produces, in addition to the desired ester, an equivalent of alcohol. Circumventing this undesired path, we have devised a novel method for transesterification in which dihydrogen is formed as the only byproduct, rather than alcohols, upon reaction of symmetrical esters with secondary alcohols. 

Synthesis of amides from the coupling reactions of esters and amines is a potentially attractive method however, stoichiometric amounts of promoters or metal mediators are normally required [52], Complex 8 also catalyzes the amide synthesis from esters and amines and the reaction is general and efficient, and proceeds under neutral conditions, thus providing an environmentally benign method [15]. As demonstrated in the alcohol acylation process (Section 1.3.3, Table 1.6), when symmetrical esters are used, acylation of amines occurs by both acyl and alkoxo parts of the esters and they are incorporated in the products, and the only byproduct generated from this reaction is H2. 

GC chromatography has the advantage of providing the identification, with practically equal ease, of secondary plasticisers with a concentration of possibly only 1 % of that of the principal plasticiser. At the same time traces of acid esters or heavy alcohols may easily be detected in commercial plasticisers, as well as symmetrical esters that are usually found in unsymmetric plasticisers. 

The triesters of glycerol are called glycerides. Many glycerides are symmetrical esters in which all three R groups are identical. Two examples are... 

Iodine triacylates (29) are readily obtained by ozonization of solutions of iodine in aliphatic carboxylic acid anhydrides thermolysis with mercuric oxide affords the symmetrical esters (30) in good yield. Mercuric iodate can perform the same transformation by direct reaction with the anhydride. 

The direct conversimi of esters into amides is a synthetically useful transformation. However, most of the methodologies developed till now usually require harsh reaction conditions, are poorly compatible with sensitive substrates, and present a low atom economy [136, and references cited therein]. Very recently, MUstein and co-workers demonstrated that esters can be selectively converted into amides generating molecular hydrogen as the only by-product (Scheme 31) [137]. The catalytic reactions were carried out with 2 equiv. of amine per ester in toluene or benzene at reflux in the presence of 0.1 mol% of the dearomatized PNN-pincer ruthenium complex [RuH(CO)(PNN)] (28) (see Scheme 21). Strikingly, both the acyl and the aUcoxo units of the starting ester are involved in the amide production. Hence, to avoid mixtures of products, the process was only applied to symmetrical esters. The catalytic protocol was effective for both primary and secondary cyclic amines. In addition, the coupling of piperazine and butylbutyrate provided compound 46, which results from the bis-acylation of the diamine (Scheme 31). 

In the light of the above a suitable system would be the phenyl diacrylates. This has the additional advantage that, as has previously been observed, in the symmetric esters there is an attractive intermolecular interaction between the phenyl and a carbonyl group [39,40] which stabilizes structures suited to topochemical reaction (e.g. Scheme 8). 

Scheme 7 Photodegradation of biobased model compounds a symmetrical ester, b unsymmet-rically ester [39] (reproduced with kind permission from John Wiley and Sons— License no. 3879230249556)...

The fully intermolecular variant of this concept has also been realized. A variety of propiolates were coupled with allenes to afford a regioselective aromatic cycloadduct (Scheme 2.12) [9]. A symmetric ester bearing an alkyne was coupled successfully with butyl allene to afford arene 46 in high yields. Unsymmetrical internal alkynes (i.e., methyl- and phenyl-substituted propiolates) were also coupled with butyl-, cyclohexyl-, and phenyl-substituted allenes in high yields (47 to 50, Scheme 2.12). The current scope of the reaction is confined to the use of activated alkyne substrates in this cycloaddition. 

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