Cyclooctatetraene, chlorination

Dry chlorine gas is admitted into a solution of 104 g. (1.0 mole) of cyclooctatetraene and 150 ml. of dry carbon tetrachloride contained in a tared 500-ml. three-necked flask equipped with a gas inlet tube, a low-temperature thermometer, and a calcium chloride drying tube. The reaction mixture is maintained between —28° and —30° throughout the addition, which is then terminated after 71 g. (1.0 mole) of chlorine has been added. After the addition, which takes approximately 1 hour, the reaction mixture is allowed to warm to 0° and 50 g. of powdered sodium carbonate is added and the contents then shaken gently for several minutes. This treatment removes any HC1 which may have been produced during the reaction. The mixture is then filtered directly into a 1-1. round-bottomed flask containing I35g. (0.05 mole) of dimethyl aeefylenedi-earboxylate. A condenser is fitted to the Hash and the solution is... 

CUoro-, bromo-, and iodocyclooctatetraeneJ The c/,r-7,8-dihalucycIoocta-I,3,S-trienes which are formed by chlorination or bromination of cyclooctatetiaene at -60° in dichloromethane are dehydrohalogenated in silu by potassium /err-buloxide at 45° yields of 85% of bromo- and 74-83% of chlorocyclooctatetraene are obtained. lodo-cyclooctatetraene is accessible from cyclooctatetraenyllithium and iodine. Rational... 

Preparation. Dry chlorine gas is admitted to a solution of cyclooctatetraene (1) in carbon tetrachloride maintained at —28° to —30° over a period of about 1 hr., until 1 mole of chlorine has been absorbed. The mixture is allowed to warm to 0°,... 

If a stream of chlorine is led into a solution of cyclooctatetraene in ten times the quantity of chloroform at —30 to —25° until no more is absorbed and the mixture is left to react for some hours at the stated temperature and then for 8 hours at room temperature, then a hexachlorobicyclooctane, m.p. 126-127°, is obtained together with isomeric liquid hexachloro compounds. 

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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