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7-Methylenebicyclo[2.2.1] hept-2-ene and its derivatives are unusual in that the two double bonds are crossed. In contrast to the behaviour of most non-conjugated dienes toward palladium(ii), these dienes do not form stable chelated diolefin complexes. Instead, in solvents of low polarity, halogen-bridged dimeric 2-37-{ (bicyclo-[2.2.1]hept-2-en-7-yl)methyl) chelates are formed, in which C( 7) is bound to either CI. Br, or M e 0 (the latter if methanol is present in the reaction mixture). In the absence of methanol, the very fast overall reactions involve trans-addition of palladium and halogen to the exocyclic olefin, with co-ordination of the remaining double bond. The trans-addition must occur either by an ionic or a bimolecular path. In any event the facility for transhalogenopalladation observed here suggests that cisand trans-halogenopalladations of organic substrates may in general be competitive.CHLOROPALLADATION of unsaturated organic substrates is an important reaction in organopalladium chemistry.

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Die Reaktion des Pd(II)‐Komplexes (I) mit dem cis‐Bicycloocten (II) führt zu dem beständigen 1,4,5‐η‐Chlorcyclooctenyl‐Pd‐Komplex (III).