Cyclooctatetraene tautomerism

The addition of benzonitrile oxide to cyclooctatetraene produced a monoadduct which was induced to undergo valence tautomerism to produce a tricycloisoxazoline (Scheme 104). A similar reaction with tropone gave a minimum of eight adducts from which two monoadducts were isolated (Scheme 104) (70T5113). 

Although there is little recorded evidence that silver ion acts as a catalyst for the reactions of olefins, it has been reported that it decreases the rate of tautomerism of bullvalene (10) and catalyzes the isomerization of tricyclooctadienes to cyclooctatetraenes (410). 

In the first step, cyclooctatetraene undergoes slow valence tautomerism to form a bicyclooctatriene. Due to similar thermodynamic stability, these two isomers will exist in equilibrium. In step 2, in the presence of Cz (CN) t, the bicyclooctatriene undergoes addition to yield a three-ringed product. This type of addition, involving the 1,4 addition of an alkene to a conjugated diene, is the Diels-Alder reaction. A simple scheme for the Diels-Alder mechanism is given below ... 

At 100° the mixture contains but 0.01% of the bicyclic isomer [5] but this isomer is far more Teactive in, for example, Diels-Alder reactions, so that the products of such reactions come from this isomer rather than from cyclooctatetraene itself. However in the halogenation reactions valence tautomerism only takes place in the last step, after addition of bromine or chlorine. In these cases the first reaction appears to be formation of an 8-halogenohomo-tropylium ion, which is then attacked by a halide ion to give a 7,8-dihalogenocyclooctatriene which in turn isomerises to give the bicyclic product. 

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 ... 

Cyclooctatetraene 83 has also been shown to yield an addition product of the diene formed from the valence bond tautomerization. However,... 

The complex CgHgMo(CO)3 also displays this phenomenon (379a) NMR of this in toluene solution shows a sharp singlet at 70° C, which becomes broadened (half-width 25 cps) at room temperature and freezes to a complex pattern at — 30°C. This is therefore the first known complex of cyclooctatetraene whose proton magnetic resonance shows observable change from rapid to arrested valency tautomerism. 

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