Bicyclo octatriene

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. 

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. 

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,... 

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. ... 

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 ... 

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. 

When the starting material 71 was unsubstituted, this reaction route is unavailable. Reaction of 75 proceeds via 76, a similar intermediate to 73, but then adds another molecule of PhC=CCH3 to give the bicyclo[4.2.0]octatriene 77. 

Beyond the few simple substitution reactions summarized below, the dominant chemistry reported for the fully-unsaturated diazocines (15) and (16) involves their thermal and photochemical ring contraction. Both the thermal and photochemical reactions appear to proceed via intermediate formation of bicyclo[4.2.0]octatriene valence tautomers. However, the product distribution differs considerably between the thermal and photochemical reactions. Thus, thermolysis of (15) in dode-cane solution at 140°C affords benzene (43.2%) and pyridine (56.8%) at 175°C, benzene becomes the major product (55.7% vs. 44.3% pyridine) (Scheme 3). Pyridazine, however, is not formed. In contrast, photolysis in tetrahydrofuran at >300 nm affords only benzene neither pyridine nor pyridazine are detected <79JOCl264>. 

It is evident even from this small selection of reactions that loss of the monocyclic eight-membered ring type of structure figures largely in the reactions of cyclooctatetraene. In particular products may be bicyclo[4.2.0]diene derivatives. In fact cyclooctatetraene is in valence tautomeric equilibrium with bicyclo[4.2.0]octatriene ... 

Cyclooctatetraene is involved in two other dynamic processes in addition to. the valence tautomerism with bicyclo[4.2.0]octatriene. They are ring inversion (a) and exchange of the positions of the single and double bonds (p) which may be depicted as follows ... 

Here apparently the two fully aromatic benzene rings in the tetracyclic compound make it more stable than the less aromatic benzene rings of dibenzocyclooctatetraene. The activation free energy is 29.5 kcal/mol which is roughly 10 kcal/mol higher than that for conversion of the parent bicyclo[4.2.0]-octatriene to cyclooctatetraene reflecting the differential stabilization (see Chapter 9, Section 2.1). 

Dicyclopentadiene has two cyclopentene rings with different reactivities. In the reaction with nickel complexes and carbon dioxide only the more reactive norbornene ring couples with CO2, whereas the unstrained ring proved to be unreactive [14,16]. The last example in Figure 6 is cyclooctatetraene, which stands in a temperature-dependent equilibrium with bicyclo[4.2.0]octatriene. Both isomers undergo an oxidative coupling with nickel and CO2. Decomposition by hydrochloric acid leads to cycloocta-2,4,6-triene-carboxylic acid and bicyclo-[4.2.0]octa-2,4-diene-7-carboxylic acid [14]. 

Bicyclo[4,2,0]octatrienes have been routinely trapped by Diels-Alder reactions with maleic anhydride, triazolenediones, and TCNE. ... 

Thermal rearrangement of the iron tetracarbonyl complex of syn-tricyclo-[4,2,0,0 ]octa-3,7-diene to a bicyclo[4,2,0]octatriene complex is believed to proceed by loss of CO followed by a concerted disrotatory ring-opening (Scheme 59) in which the metal plays an important role in overcoming the forbidden nature of... 

The effects of molybdenum carbonyls on the rearrangements of bicyclo[4,2,0]-octatrienes (see p. 118) have been examined. ... 

A stable crystalline bicyclo[4,2,0]octatriene has been prepared.Photodecar-bonylation of the dione (288) gave 2,3,4,5-tetrachlorobicyclo[4,2,0]octatriene (289) as crystals m.p. 38 °C. Isomerization to the tetrachlorocyclo-octatetraene (290) occurred at 60 °C, = 15 min, and the conversion was essentially quantitative. Bond... 

The chemistry of cyclo-octatetraene has been reviewed. Calculations using the MIN DO method suggest that the isomerization of cyclo-octatetraenes may proceed via tetracyclo[4,2,0,0 ,0 ]octene (510) at high temperatures. The interconversion of (510) and bicyclo[4,2,0]octatriene is symmetry allowed and (510) is claimed... 

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