Tetracyanoethylene Diels-Alder reaction

Benzo[Z)]furans and indoles do not take part in Diels-Alder reactions but 2-vinyl-benzo[Z)]furan and 2- and 3-vinylindoles give adducts involving the exocyclic double bond. In contrast, the benzo[c]-fused heterocycles function as highly reactive dienes in [4 + 2] cycloaddition reactions. Thus benzo[c]furan, isoindole (benzo[c]pyrrole) and benzo[c]thiophene all yield Diels-Alder adducts (137) with maleic anhydride. Adducts of this type are used to characterize these unstable molecules and in a similar way benzo[c]selenophene, which polymerizes on attempted isolation, was characterized by formation of an adduct with tetracyanoethylene (76JA867). 

Experimentally, the rates of Diels-Alder reactions between electron-rich dienes and electron-poor dienophiles generally increase with increased alkyl substitution on the diene. This is because alkyl groups act as electron donors and lead to buildup of electron density on the diene. An exception to this is the reaction of Z,Z-hexa-2,4-diene with tetracyanoethylene (TCNE), which is actually slower than the corresponding addition involving E-penta-1,3-diene. 

In another aspect of the mechanism, the effects of electron-donating and electron-withdrawing substituents (p. 1065) indicate that the diene is behaving as a nucleophile and the dienophile as an electrophile. However, this can be reversed. Perchlorocyclopentadiene reacts better with cyclopentene than with maleic anhydride and not at all with tetracyanoethylene, though the latter is normally the most reactive dienophile known. It is apparent, then, that this diene is the electrophile in its Diels-Alder reactions. Reactions of this type are said to proceed with inverse electron demand ... 

Thermal cracking at 150°C of the dimer (mixed isomers) of the silole in presence of reactive dienophiles (maleic anhydride, tetracyanoethylene or dimethyl acetylenedicarboxylate) inevitably produced violent explosions arising from exothermic Diels-Alder reactions. 

The thermal Diels-Alder reactions of anthracene with electron-poor olefinic acceptors such as tetracyanoethylene, maleic anhydride, maleimides, etc. have been studied extensively. It is noteworthy that these reactions are often accelerated in the presence of light. Since photoinduced [4 + 2] cycloadditions are symmetry-forbidden according to the Woodward-Hoffman rules, an electron-transfer mechanism has been suggested to reconcile experiment and theory.212 For example, photocycloaddition of anthracene to maleic anhydride and various maleimides occurs in high yield (> 90%) under conditions in which the thermal reaction is completely suppressed (equation 75). 

Chemical separation of conjugated dienes and other polyunsaturated hydrocarbons is based on the availability of tt delocalized electrons. The use of a strong dienophile (e.g. tetracyanoethylene, TCNE) will derivatize only conjugated dienes, thus separating the polyunsaturated compounds into two groups. However, such derivatization is not always reversible since a retro-Diels-Alder reaction may require a high temperature. Hence, the retrieved compounds may be the thermostable ones and not those present in the initially analysed mixture. 

Furthermore, the synthetic utility of 2,6-divinyl-l,4-dithiin 68 as a reactive diene in Diels-Alder reactions was reported with tetracyanoethylene, maleic anhydride, A -phenylmaleimide, and dimethyl acetylenedicarboxylate (DMAD) and allowed the preparation of various dihydrothianthrene derivatives (Equation 9) <2003S849>. 

The cycloaddition reactions of 368 (X = CO) with 7V-phenylmaleimide, p-benzoquinone, dimethyl acetylenedicarboxylate, and tetracyanoethylene afforded [4 + 2]-cycloadducts with endo-stereochemistry ethyl acrylate gave two isomers (probably endo and exo). l,4-Dihydro-l,4-oxidonaph-thalene (19) yielded the endo-exo isomer. The Diels-Alder reaction of 368 (X = SO2) with ethyl acrylate gave a mixture of isomers, from which the higher melting product could be isolated in pure form. ... 

Otto and Ruf [96] have incorporated the 3-(prop-2-enylidene)azetidin-2-one derivatives 85 for the synthesis of spiro-(3-lactams 86 via Diels-Alder reactions (Scheme 23). The reaction of (Z)-85 with highly reactive tetracyanoethylene (TCNE) was performed in THF at refluxing temperature, which afforded spiro-(3-lactams 86. 

More recently, however, it has been suggested that carbocation intermediates might result from the addition of an amminium ion (through the aromatic ring) to a jr-bond114. Recently, this controversy seems to have been definitively settled Most hole-catalyzed Diels-Alder reactions actually do involve cation radicals115, except for the addition of tetracyanoethylene to electron-rich alkenes116. 

The Diels-Alder reaction with /V-phenylmaleimidc has frequently been used for the separation, purification, and structure determination of ortho photocycloadducts [12,47,86,90,108,116,126,132,133,138], Other dienophiles that have been successfully employed in Diels-Alder reactions with ortho adducts are A-(para-bromophenyl)maleimide [116,120], maleimide [116,118,127], maleic anhydride [127,191], tetracyanoethylene [11], and dimethyl acetylenedicarboxy-late [73,127], The Diels-Alder product of A-(para-bromophenyl)maleimide with the exo-ortho adduct formed from 1,4-dioxene and benzene [120] and the Diels-Alder product of maleimide with the endo-ortho adduct from cis-cy-clooctene and benzene [118] were obtained in crystalline form and their structures could be determined by means of x-ray diffraction. 

This is an example of a Diels-Alder reaction in which the typical electronic characteristics have been reversed. Now suggest what would be the relative rates of reaction between the perchloro compound and (a) tetracyanoethylene, (b) maleic anhydride and (c) cyclopentene. 

The perchloro compound reacts the fastest with cyclopentene, and only slowly with maleic anhydride, and not at all with tetracyanoethylene. The last is usually a very reactive reagent in Diels-Alder reactions operating under the normal electronic basis. However, under the reverse basis, what was favourable is now unfavourable, to such an extent that the reaction does not proceed to a noticeable extent. 

We can also, now, see rather clearly what makes a good dienophile in normal Diels-Alder reactions the most important factor is a low-lying LUMO. Thus, the more electron-withdrawing groups we have on the double bond, the lower the energy of the LUMO, the smaller the separation of the HOMO(diene) and the LUMO(dienophile), and hence the faster the reaction. Tetracyanoethylene is a very good dienophile. 

There are numerous attempts to correlate solvent parameters with the reaction rate of Diels-Alder reactions. Examples are the Brownstein Polarity Parameter the Solvophobicity Parameter 5 p 24,i25 D-jt parameter (based on the solvent effect on the reaction of tetracyanoethylene and diazodiphenylmethane with benzene as the reference solvent) or the Acceptor Number aa 2V.128 parameter which describes the ability of a solvent to act as an electron pair acccplor). These examples included either reactions that were next to insensitive to solvent effects (like that in Table 9) or reactions in which the reactants mainly interact with the electron pair on the donor atom of the solvent ". 

The N=C—C 0 system of a I-phenothiazinone undergoes an intramolecular Diels-Alder reaction [B-42, 2455, 3067a, 3880] with diethyl acetylenedicar-boxylate, but with tetracyanoethylene (reviews (3146, 3244]), a mixture of products is obtained. 

A beautiful illustration of a delicate balance between a stepwise and a concerted reaction has been found in the reactions of 1,1-dimethylbutadiene 6.133.716 This diene rarely adopts the s-cis conformation necessary for the Diels-Alder reaction with tetracyanoethylene giving the cyclohexene 6.136. However, it can react in the more abundant s-trans conformation in a stepwise manner, leading to a moderately well stabilised zwitterion 6.134. The intermediate allyl cation is configurationally stable, and a ring cannot form to C-l, because that would give a trans double bond between C-2 and C-3 in the cyclohexene 6.137. Instead a cyclobutane 6.135 is formed. All this is revealed by the solvent effect. In the polar solvent acetonitrile the stepwise ionic pathway is favoured, and the major product (9 1) is the cyclobutane 6.135. In the nonpolar solvent hexane, the major product (4 1) is the cyclohexene 6.136 with the Diels-Alder reaction favoured. 

More readily identifiable geometrical factors probably outweigh the contribution of the frontier orbitals in the remarkable reaction 6.47 between tetracyanoethylene and heptafulvalene giving the adduct 6.49 (see p. 261). The HOMO coefficients for heptafulvalene 6.420 (see p. 347) are highest at the central double bond, but a Diels-Alder reaction, with one bond forming at this site is impossible. The best reasonable possibility for a pericyclic cycloaddition, from the frontier orbital point of view, would be a Diels-Alder reaction across the 1,4-positions (HOMO coefficients of -0.199 and 0.253), but this evidently does not occur, probably because the carbon atoms are held too far apart. This is well-known to influence the rates of Diels-Alder reactions cyclopentadiene reacts much faster than cyclohexadiene, which reacts much faster than cycloheptatriene (see p. 302). The only remaining reaction is at the site which actually has the lowest frontier-orbital electron population, the antarafacial reaction across the 1, f-positions, which have HOMO coefficients of —0.199. 

Nitrile, azo, and nitroso groups, and even the oxygen molecule, take part in such reactions, and acetylenic triple bonds in particular confer reactivity as philodiene. As for dienes, so for philodienes the reactivity depends on the constitution. Activating groups particularly favor addition. The most reactive components include <%,/ -unsaturated carbonyl compounds such as acrolein, acrylic acid, maleic acid and its anhydride, acetylenedicarboxylic acid, p-benzo-quinone and cinnamaldehyde, as well as saturated nitriles and <%,/ -unsaturated nitro compounds. Tetracyanoethylene also reacts with dienes.41,42 Conjugation of the double bond to an active group is not absolutely essential for a philodiene, for dienes add under certain conditions also to philodienes with isolated double bonds examples of the latter type are vinyl esters and vinyl-acetic acid. Ketenes do not undergo the Diels-Alder reaction with dienes, but instead yield cyclobutanone derivatives 43,44... 

In contrast to ethylene and tetracyanoethylene, tetrafluorQethylene does not undergo a Diels-Alder reaction with 1,3-butadiene instead of this 1,4-addition, it reacts by 1,2-addition to give a cyclobutane derivative 54... 

Substituents on the diene can increase the energy barrier to the cisoid form and retard the Diels-Alder reaction. Attempts to react 2-methyl-2,4-pentadiene (23) with maleic anhydride at ambient temperature gave no reaction. The more reactive alkene tetracyanoethylene (TCNE, 24) gave 11% of 25 in 20 h, but the major product was the [2+2]-cycloaddition product (26) in 69%. This poor reactivity was attributed to the energy barrier for 23 to assume a cisoid conformation. 

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