Cyclooctatetraene Diels-Alder addition

The 18 n-electron system furo[3,4-c]octalene (356) is available by a Wittig reaction starting from cyclooctatetraene-1,2-dicarboxaldehyde it is obtained as an unstable pale yellow liquid, which is rapidly oxidized in air. The electronic spectrum resembles that of 353, showing little extended conjugation. Compound 356 underwent Diels-Alder addition at the terminal cyclooctatetraene ring thus treatment with dimethyl fumarate leads to monoadduct 357. ... 

Exercise 22-40 The rate of the Diels-Alder addition between cyclooctatetraene and tetracyanoethene is proportional to the tetracyanoethene concentration, [C2 (CN)4], at low concentrations of the addends but becomes independent of [C2(CN)4] at high concentrations. Write a mechanism that accounts for this behavior. 

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. 

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