Diels-Alder reactions, thermally reversible

Diels-Alder reactions with butadiene are generally thermally reversible and can proceed in both gas and Hquid phases. The reactions are exothermic and foUow second-order kinetics first-order with respect to each reactant. 

Literature articles, which report the formation and evaluation of difunctional cyanoacrylate monomers, have been published. The preparation of the difunctional monomers required an alternative synthetic method than the standard Knoevenagel reaction for the monofunctional monomers, because the crosslinked polymer thermally decomposes before it can revert back to the free monomer. The earliest report for the preparation of a difunctional cyanoacrylate monomer involved a reverse Diels-Alder reaction of a dicyanoacrylate precursor [16,17]. Later reports described a transesterification with a dicyanoacrylic acid [18] or their formation from the oxidation of a diphenylselenide precursor, seen in Eq. 3 for the dicyanoacrylate ester of butanediol, 7 [6]. 

The reversal of the thermal decomposition of 6 to ethylene and vinylacetylene cannot be utilized to generate 6, since, according to a quantum-chemical analysis, the reaction is slightly endergonic and requires a large free activation enthalpy (0.9 and 42 kcal mol-1, respectively) [59]. The intramolecular variant of this process as well as the addition of typical dienophiles of the normal Diels-Alder reaction to divinylace-tylenes will be discussed at the end of Section 6.3.3. 

With one exception, naphthalen-l,4-imines with a double bond between C-2 and C-3 are not known to dissociate thermally by either possible retro-Diels-Alder pathway (the reverse of reactions described in Section III, A, 1 and 2), and the enthalpy requirements for the formation of a benzyne or an acylic acetylene are doubtless unfavorable. However, the mass spectra of compounds 93-99 reveal one important fragmentation of the molecular ions to be loss of dimethyl acetylene-dicarboxylate, and another fragmentation pathway involves the formation of nitrilium ions MeC=NR and PhC=NR from 93-95 and 96-99, respectively. ... 

Diels-Alder reactions with p-quinones (6. 65 66). The orientation of Diels-Alder reactions of 6-meihoxy-l-vinyl-3,4-dihydronaphthalene (1) with p-quinones is subject to reversal by addition of BF, etherate (1.3 equivalent). Thus the thermal reaction with 2,6-dimethyl-/>-bcnzoquinone (2) results in exclusive formation of 3, whereas the catalyzed reaction leads predominately to the isomer 4. The adduct 3 is stable to base, but the syn, m-isomer 4 on treatment with NaX O, is converted to the more stable anti, frau.s-isomer 5. 

This orientation reversal is noted also with 2,5-dimethyl-p-benzoquinone and 2-acetyl-p-benzoquinone, but not with 6-methoxy-p-toluquinone. The orientation observed in the thermal reactions is unexpected, since Diels-Alder reactions with simple ilicnophilcs give the two possible adducts in approximately equal amounts. 

Disubstituted cyclohexadienones undergo Diels-Alder reactions more slowly than the unsubstituted counterparts. Thus 12 does not react with pipcrylene (13) at 180°, but in the presence of SnCl4 the reaction proceeds in 85% yield at 25°. Moreover a complete reversal of face selectivity can be achieved by use of a Lewis acid catalyst. Thus 15 reacts thermally with 13 to give 16, whereas the catalyzed reaction results in 17. Thus the stereochemistry of four asymmetric centers can be controlled.5... 

The Diels-Alder reaction is one of the most important carbon-carbon bond forming reactions,521 522 which is particularly useful in the synthesis of natural products. Examples of practical significance of the cycloaddition of hydrocarbons, however, are also known. Discovered in 1928 by Diels and Alder,523 it is a reaction between a conjugated diene and a dienophile (alkene, alkyne) to form a six-membered carbo-cyclic ring. The Diels-Alder reaction is a reversible, thermally allowed pericyclic transformation or, according to the Woodward-Hoffmann nomenclature,524 a [4 + 2]-cycloaddition. The prototype reaction is the transformation between 1,3-butadiene and ethylene to give cyclohexene ... 

Thermal cycloreversion of the adducts can be accomplished at a convenient rate when heated in toluene under reflux. If a new diene is present in the reaction mixture, the thioaldehyde thus generated in the retro-Diels-Alder reaction may give a new adduct. Therefore, adducts 81 and 82 act as thioaldehyde or thioketone transfer reagents. These adducts dissociate reversibly on heating, thus ensuring that the concentration of the labile species remains very low. For this reason, polymerization is not a serious problem especially in the case of thioaldehydes224. The transient thiocarbonyl compounds can be trapped not only by dienes but also by 1,3-dipolar cycloadditions332 (equation 85). 

Dicyclopentadiene can be converted back to cyclopentadiene by thermally reversing the Diels-Alder reaction. Cyclopentadiene also undergoes the Diels-Alder reaction with other olefins, and this chemistry has been used to make highly chlorinated, polycyclic hydrocarbon pesticides. These pesticides are so resistant to degradation in the biosphere, however, that they are now largely banned from use. It is also used as a monomer and a chemical intermediate. 

Biemann (1962a) noted that his type-D reaction was the mass-spectrometric equivalent of the reverse Diels-Alder reaction. Several examples of this type of fragmentation are now known (Budzikiewicz et al., 1965 Maccoll, 1968a). Dougherty (1968b), assumed that both the thermal and electron impact-induced processes were electrocyclic in his explanation of the apparent similarity between the type D process and the reverse Diels-Alder reaction in terms of a simple perturbation molecular orbital approach (Section VIIID). 

Jen [2] prepared thermally reversibly electrooptic polymers, (V), via a Diels-Alder reaction, as illustrated below, that were used in second-order nonlinear optical devices. 

Application of Hint 2.15 may help to organize your thoughts about how the products might form. Formation of the first product involves the loss of one carbon. It appears most likely that this would be CO2 or CO. It is probably CO2, because CO usually is lost only in strong acids or, under thermal conditions, in reverse Diels-Alder reactions or free radical decar-bonylation of aldehydes. 

Diels-Alder reactions of oxazoles have proven to be quite versatile and continue to attract attention. Oxazoles have traditionally been used as the diene component and react with alkyne dienophiles to give furan products after extrusion of a nitrile molecule via a reverse-cycloaddition process. This method has been used to access highly substituted furans and has been utilized in numerous natural product syntheses. The reaction typically requires the use of high temperatures for efficient conversion. The furan intermediate 67 was obtained by a thermal intermole-cular Diels-Alder reaction between oxazole 66 and an acetylene. Furan 67 was a key intermediate for the synthesis of (—)-teubrevin G (Scheme 10) <2000JA9324>. Similarly, furan 68, obtained from a Diels-Alder reaction between 4-phenyloxazole and an acetylene, served as an intermediate in the total synthesis of the natural product cornexistin (Scheme 10) <20030L89>. 

In all of the above discussion we have assumed that a given molecule forms both the new ct bonds from the same face of the n system. This manner of bond formation, called suprafacial, is certainly most reasonable and almost always takes place. The subscript s is used to designate this geometry, and a normal Diels-Alder reaction would be called a [ 2s + 4J-cycloaddition (the subscript 71 indicates that n electrons are involved in the cycloaddition). However, we can conceive of another approach in which the newly forming bonds of the diene lie on opposite faces of the n system, that is, they point in opposite directions. This type of orientation of the newly formed bonds is called antarafacial, and the reaction would be a [ 2 + 4a]-cycloaddition (a stands for antarafacial). We can easily show by the frontier-orbital method that this reaction (and consequently the reverse ring-opening reactions) are thermally forbidden and photoche-mically allowed. Thus in order for a [fZs + -reaction to proceed, overlap between the highest occupied n orbital of the alkene and the lowest unoccupied 71 orbital of the diene would have to occur as shown in Fig. 15.10, with a + lobe... 

The homolog of prismane, quadricyclane (29) was obtained through the homo-Diels-Alder reaction between diethyl azodicarboxylate and norborna-2,5-diene, ° although the deazetization has to be carried out photochemically. Thermal deazetization of the diazene 28 (and of closely related diazenes ) results in a reversion to the homodiene. 

Diels-Alder reactions can sometimes reverse themselves through Retro-Diels-Alder reactions. For example, dicyclopentadiene can be cracked to form 1,3-cyclopentadiene by thermal dissociation. Retro reactions occur under situations where the fragments are stable by themselves. 

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