Diene unstable

Liquid products from coking are very unstable (high diene contents), very olefinic, and highly contaminated by sulfur and nitrogen. The production of gas is considerable. 

The o-quinodimethanes are very reactive, unstable dienes, which are usually prepared in situ. The cycloaddition under high pressure of the dibromo-o-quinodimethane 91, generated in situ from a,a,a, a -tetrabromo-o-xylene. 

The Diels-Alder addition of unstable thiadiazoline 498 to 1,4,5,5-tetrafluorocyclo-penta-l,3-diene provided the corresponding 4-thia-2,6-diazatricyclo[5.2.1.0 2,6 ]dec-8-ene-3,5-dione, which upon hydrogenation and following irradiation gave l,4,7,7-tetrafluoro-2,3-diazabicyclo[2.2.1]hepta-2-ene (see Scheme 85) <2006JFC(127)688>. 

Benzo[c]furans (isobenzofurans) are very reactive but generally unstable dienes which are prepared in situ and trapped. The in ihu-generated isobenzo-furan 33 was trapped by cycloaddition reaction with bis(methyl (S)-lactyl) ester 34 to afford [32] optically active naphthols (Equation 2.12). The cycloaddition was carried out in the presence of a catalytic amount of glacial acetic acid and represents a facile one-pot procedure to synthesize substituted naphthols. 

Cyclobutadiene complexes are also well established (hough (hey must be synthesized by indirect routes since the parent dienes are either unstable or non-existent. Four geneial routes are available  

Because of its great reactivity PTAD has found wide use in the interception of reactive, unstable dienes. For example, unstable isoindenes,226 3a//-indenes,146 1,3-divinylallenes,227 and benzene oxides228 have all been successfully trapped with PTAD. 4-(4-Bromophenyl)-l,2,4-triazole-3,5-dione (5, R = 4-Br—C6H4) is often used if the derivatives ai e required for X-ray structure determination.229 Azodicarboxylic esters have been used to trap tetra(trifluoromethyl)cyclobutadiene,230 and spiro[4.4]nonatetraene.231 

The Diels-Alder reaction of morphinan-6,8-dienes with nitroethene affords a novel type of opium alkaloids (Eq. 8.3).10a High reactivity of nitroethylene is demonstrated for the Diels-Alder reaction with thermally unstable dienes, and this is used for synthesis of polycyclic kopsane-like alkaloids.1013 

The fluoboric acid-catalyzed aza-Diels-Alder reaction of aldimine and Danishefsky s diene proceeds smoothly to afford dihydro-4-pyridones in high yields [90] (Equation 4.16). Unstable aldimines generated from aliphatic aldehydes can be prepared in situ and allowed to react under one-pot reaction conditions. This one-pot Bronsted acid-catalyzed three-component aza-Diels-Alder reaction affords the adducts in good to high yields. 

Since the corresponding endoperoxide precursors are all too unstable for isolation, the diimide reduction constitutes an important chemical structure confirmation of these elusive intermediates that are obtained in the singlet oxygenation of the respective 1,3-dienes. However, the aza-derivative 14 and the keto-derivative 15 could not be prepared,17> because the respective endoperoxides of the pyrroles 18) and cyclopentadienones suffered complex transformations even at —50 °C, so that the trapping by the diimide reagent was ineffective. 

The bicyclo[2.2.0]hexa-2,5-diene ring system is a valence isomer of the benzene ring and is often referred to as Dewar benzene. After many attempts to prepare Dewar benzene derivatives failed, a pessimistic opinion existed that all such efforts would be finitless because Dewar benzene would be so unstable as to immediately revert to benzene. Then, in 1962, van Tamelen and Pappas isolated a stable Dewar benzene derivative by photolysis of 1,2,4-tri(/-butyl)benzene.  

In a study of the relative rates of addition of 20 dienophiles to cyclopentadiene, TCNE was at the head of the Hst, eg, it added 7700 times as rapidly as maleic anhydride (15). Reaction with most 1,3-dienes takes place rapidly and in high yield at room temperature. TCNE has often been used to characterize 1,3-dienes, including those that are unstable and difficult to isolate (16). 

Thiopyranopyrrolizines can be prepared readily from the enamine 170 upon treatment with DMAD. Alternatively, heating of the thiacyclooctadiene derivative 171 in methanol gives the same tricycle 172, but this time as a 5 2 mixture with the (Z,E)-isomer of the precursor 171. These reactions probably involve the the intermediacy of an unstable cyclobutene and/or a zwitterionic diene, as shown in Scheme 51 <1984JA1341>. 

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). 

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