Esters thermal elimination reactions

The kinetics of concerted thermal elimination reactions of a series of ethyl (hetero) arylcarboxylate esters (2-thienyl-, 3-thienyl-, 2-furyl, 3-furyl, 4-pyridyl-, 3-pyridyl-, and 2 -pyridylcarbo x y I ate) in the gas phase seem to indicate that there is tittle charge separation in the transition state (83) this is in contrast with the behaviour of the corresponding /-butyl and isopropyl esters for which a semi-concerted transition state (82) was proposed previously.49 Results of a kinetic study of the gas-phase elimination reactions of methylbenzoyl fonnate (84) and 3-hydroxy-3-methylbutan-2-one (85) have been compared with those for pyruvic acid (87) and benzoylformic acid (86).50 The relative rates of reaction [(86)/(87) 46, (87)/(85) = 1.1 x 105 and (86)/(82) = 1 x 106] reveal that the acidity of the hydrogen atom involved in the elimination process, rather than the initial polarization of the C—C bond which undergoes cleavage, is the important rate-controlling factor. 

Special Topic Thermal Elimination Reactions of Esters... 

FIGURE 18.59 The thermal elimination reaction of esters containing a hydrogen in the P position. 

For example, the sulfur-containing xanthate esters undergo elimination especially easily (Fig. 18.62). Xanthate esters can he prepared as shown in the figure by addition of alkoxide to CS2 (recall the addition of nucleophiles to carbon dioxide, p. 840). These intramolecular thermal elimination reactions provide a new route to alkenes, so update your file cards. 

There is a class of intramolecular thermal elimination reactions that provides a new route to alkenes. Esters, xanthates, and amine oxides are commonly used in this reaction. The reactions are concerted (one-step), and steiic requirements dictate that a syn elimination must occur in the reaction, as the carbonyl group cannot reach a hydrogen in an anti position (Rg. 18.61). 

Controversy continues over the precise nature of the transition state in the cis thermolytic ester elimination reaction. Whereas one group interpret their data as indicative of a symmetrical, non-heterolytic, non-planar transition state (12) analogous to other pericyclic fragmentations, another author proposes charge development (13), defining the order of extent of movement of electrons as 1 > 2 > 3. An unsymmetrical concerted transition state (14) has been assigned to the thermal / -elimination reaction of alkyl... 

E Tertiary alcohol group, neutral, prone to elimination by dehydration at high temperatures. Where the hydroxyl group at E is converted to an ester such as valerate, e.g. betamethasone valerate, thermal elimination of the ester can occur quite readily. Another decomposition reaction of the valerate esters is the... 

Di-tert-butyl methylenemalonate was originally prepared by phenyl-sulfenylation of di-tert-butyl methylmalonate and thermal elimination of the related sulfoxide.8 Because methylenemalonate esters are customarily prepared by Knoevenagel-type condensation of malonic esters with formaldehyde equivalents, the considerably more convenient procedure described herein was subsequently adapted from Bachman and Tanner s study using paraformaldehyde under metal ion catalysis.39 The approximately 6% di-tert-butyl malonate accompanying the product has presented no interference in the aforementioned reactions with nucleophilic alkenes under neutral or acidic conditions, but its presence should be taken into consideration in other applications. 

The synthesis of the [2.3.4]cyclazine derivatives (301) and (186) has been described in Section 3.08.6.3.2 (Scheme 24). It has been shown in Section 3.08.6.1.3 that attempts to synthesize 9-aza[2.3.4]cyclazine (302) yielded the 8,9-dihydro derivatives (146), which may have been formed by reduction of the intermediate cyclazine. Attempts to remove the ester groups of (301) and (186) by hydrolysis of the ethyl ester and subsequent thermal decarboxylation or by thermolysis of the f-butyl ester failed. The reaction of (301) with LAH led to decomposition. The preparation of a dihydro[2.3.4]cyclazine derivative (89) has been described in Section 3.08.3.5. It has been pointed out that an attempted elimination of HCN, to give the fully conjugated cyclazine, failed. 

Alkenes are formed by the thermal decomposition of esters, xanthates, amine oxides, sulfoxides, and selenoxides that contain at least one (3-hydrogen atom. These elimination reactions require a cw-configuration of the eliminated group and hydrogen and proceed by a concerted process. If more than one (3-hydrogen is present, mixtures of alkenes are generally formed. Since these reactions proceed via cyclic transition states, conformational effects play an important role in determining the composition of the alkene product. 

In addition to the depolymerization reaction discussed earlier, other reactions may be favoured. These are elimination from a side chain and cyclization. For example, propylene is eliminated from the side chain of poly (isopropyl acrylate) as shown in Scheme 1.51(a), leaving poly(acrylic acid). This occurs in all polymers having ester side groups with P-hydrogens available to form a six-membered transition state, as shown. Thus both acrylate and methacrylate polymers will undergo this reaction and, since depolymerization is the dominant thermal-degradation reaction in methacrylates, elimination of alkenes is more important in the poly(acrylates). 

We have shown that f-butyl esters of polyamic acids can be prepared in good yield and with acceptable molecular weights. The reaction has also been extended to other esters. Moreover, the f-butyl esters can be prepared as pure meta or para polymers, which show distinct differences in solubility. The curing behavior of these t-butyl esters has been shown to be similar to that of the parent polyamic acid and much faster than that for analogous linear esters. The cure of t-butyl ester has been shown to proceed by a more facile mechanism involving the liberation of a free polyamic acid rather than by a thermal elimination of alcohol. In future studies we will examine other bulky protecting groups capable of thermal removal. 

To illustrate the recently discovered pathways to functional monomers, Meier and colleagues studied the synthesis of a long-chain diester from a (o-hydroxy fatty acid derived from palmitic acid. The idea was to transform the (0-hydroxyl function into a mesylate, followed by an elimination reaction to prepare the (O-unsaturated fatty acid methyl ester (FAME), which was dimerised by a SM coupling to obtain the desired C30 diester (Scheme 5.5) [14]. This macromonomer was then polymerised with diols and diamines to prepare long-chain polyesters and polyamides (PA) with interesting thermal properties, such as a melting temperature (T ) of 109 °C for the polyester and 166 °C for the PA. 

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