Bicyclo octatrienes

Alkynes undergo cycloaddition on irradiation with benzene or naphthalene derivatives or with other aromatic compounds. With a benzene derivative the product is usually a cyclo-octatetraene which results from thermal electrocyclic ring-opening of the bicyclo-octatriene formed initially by 1,2-addition of the alkyne to the benzene ring (equation 61) . The intermediate can be trapped using a dienophile such as tetracyanoethylene (equation 62) ". The first step of the photoaddition process involves excitation of the alkyne , and orbital symmetry considerations suggest that concerted 1,2-addition is allowed if the alkyne is excited but not if the benzene is excited ... 

However, perfluoropropyl arsenicals seem to be difficult to isolate (32). Perhaps the most unusual product of this type of reaction is a bicyclo-octatriene (33). 

Besides simple condensation reactions, valence isomerization reactions, in particular, arc used for the synthesis of unsaturated, eight-membered-ring azaheterocycles. These isomeriz-ations mainly involve rearrangements of nitrogen-containing bicyclo[4.2.0]octatriene or semi-bullvalene systems. 

The tendency to undergo valence isomerization is generally of fundamental importance regarding the stability of the compounds. In the case where an equilibrium exists between the eight-membered ring and the bicyclo[4.2.0]octatriene, decomposition may readily occur by a [2 + 2] cycloreversion process, particularly if molecular nitrogen or a cyano compound can be eliminated. 

As for compounds having modified bridges in a bicyclo[2.2.2]octatriene system, 1 -rm-butyl-1,4-dihydronaphthalene 1,4-endoxides (109) and 9-rm-butyl-l,2,3,4-tetrachloro-9,10-dihydro-9,10-ethenoanthracene (110) were prepared. The... 

Indicate the steps involved in the following synthesis of bicyclo[4.2.0]-2,4,6-octatriene (benzocyclobutene) ... 

Cyclooctatetraene itself does not undergo 4 + 2 cycloadditions it is its valence isomer bicyclo[4.2.0]octatriene which reacts with dienophiles.36 Use Dewar s PMO method to explain why the direct reaction with cyclooctatetraene is disfavored. 

The intermediate bicyclo[5.1.0]octatriene (225) may be trapped as a Diels-Alder adduct, while at elevated reaction temperature, the heptafulvene (224) may likewise be trapped. The chloride (223, X = Cl, Y = H) may similarly be converted to the parent heptafulvene by reaction with the same base in tetraglyme at 90 °C and low pressure 152). 

Tetrachlor- 1,2-phenylen dioxy ]-4,8,13-trimethyl- tribenzo-l -aza-4Xs-phospha-bicyclo 2.2.2]octatrien)50 45°/o... 

Electrolytic reduction of 4-cyclooctenone leads smoothly in 64 % yield to cis-bicyclo[3.3.0]octan-l-ol.106) The thermolysis of octachlorocyclooctatetraene produces perchlorinated bicyclo[3.3.0]octatrienes,107,108) several of which have been examined in detail by X-ray structural methods.109) The 1,3,5,7-tetramethyl-cyclooctatetraene dication (57), generated by oxidation of the neutral hydrocarbon with antimony pentafluoride in S02C1F at —78 °C and below, is subject to dis-rotatory cyclization above — 60°.1 °) The latter step does not follow orbital sym-... 

Bicyclo(2.2.2 octatrien 2-Methyl-E19b, 241 (Carben-Umlagerung)... 

Example 6.3. The thermal cyclization of cyclooctatetraene to bicyclo[4.2.0l-octatriene. 

Both trans and cis adducts can be formed in reactions of bicyclo[ 2.2.2]octene ° and related bicyclooct-2-enes °, bicyclo[2.2.2]octadiene °, homobarrelenes or dibenzobicyclo[2.2.2]octatrienes ... 

Skeletal rearrangements are rare, but are found for strained bi- and polycyclic alkenes, e.g., bullvalene , bicyclo[4.2.2]deca-2,4,7,9-tetraene , 4,5-exo-trimethylene-2-norbornene , 6-methylenebicyclo[3.1.1]heptane ", 5-methylbicyclo[2.1.1] hexene N-benzyl-9-aza-bicyclo[4.2.2]deca-2,4,7-triene , dibenzobicyclo[2.2.2]-octatriene derivatives and bornylene ... 

Reaction with cyclooctatetraene.6 With most dienophiles, cyclooctatetraene (I) reacts through the quasiplanar diene system of the valence tautomer bicyclo[4.2.0]-octatriene (2). For example, tetracyanoethylene reacts in this way to give (3). However,... 

Dicyanoacetylene reacts with A - -bicyclo[2.2.2]octatriene (1, barrelene) at room temperature to give the adduct in 95% yield dimethyl acetylenedicarboxylate reacts with the hydrocarbon at 100° and the yield of adduct is only 29%. When adduct lib was heated for 100° for 3.5 hrs. it afforded 1,2-dicyanonaphthalene in... 

Cyclooctatetraene can undergo three different isomerizations ring inversion la lb due to simple rotation about the C-C single bonds, reversible bond migration 1 a 1 b involving all four double bonds and dynamic equilibration with bicyclo[4.2.0]octatriene 2. ... 

Barton has suggested 35> that in photochemical reactions of cyclic conjugated olefins, ring fission will predominate in rings of n annular atoms containing (n/2)-l double bonds and valence tautomerization will occur in other systems. Consequently, it was anticipated that 1,3,5-cyclo-octatriene upon photolysis should give rise to an acyclic tetraene in the same manner that 1,3-cyclohexadiene opens 35> 36> to 1,3,5-hexatriene. When 1,3,5-cyclooctatriene (55) was irradiated in solution (ether or pentane) 37> 39> two isomerization products were isolated, bicyclo[4.2.0]-octa-2,7-diene (56) and tricyclo[5.1.0.0 4> 8]oct-2-ene (57). The formation of 56 is not exceptional. The formation of 57 has been visualized by... 

Cyclooctatetraen geht photochemisch bei niedrigen Temperaturen in das valenz-isomere Bicyclo[4.2.0]octatrien- 2 4,7) iiber, das weiter zu Benzol und Acctylcn photo-fragmentiert wcrdenkann4. Mit Aceton als Sensibilisator entsteht Semibullvalen (12 % d. Th. )5 ... 

Das vvahrschcinlich erste Beispiel einer als solclie erkannten Di-TZ-methari-Umlagerung7 war die Aeeton-sensibilisierte Isomerisierung von Bicyclo[2.2.2]octatrien-(2,5,7)... 

Dagegen isomerisiert Naphtho-[2,3-b]-bicyclo[2.2.2]octatrien (XVIII) in Ge-genwart odcr in Abwcsenheit eines Sensibilisators zu Naphtho-[2,3-c]-tricyclo [3.3.0.02 8] octadien- 3,6) (XIX F 139-140°) bei fast gleicben Quantenausbeuten1 (s. Tab. 65) ... 

Die Belichtung von Acetylenen in Benzol fiihrt in vielen Fallen zu Cyclooctatetraon-Derivaten2-6. Als Zwischenprodukt darf mit groBer Sicherheit das durch 1,2-Addition an den Aromaten entstandene Bicyclo [4.2.0]octatrien-(2,4,7)-Derivat angesehen werdcn, welches sich ansclilieBend in das Cyclooctatetraen isomerisiert. 

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