Reactions of palladium(II) halides with 7-methylenebicyclo[2.2.1]hept-2-ene derivatives; unique trans-halogenopalladation of the exocyclic double bond

Abstract

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 2–3η-{(bicyclo-[2.2.1]hept-2-en-7-yl)methyl} chelates are formed, in which C(7) is bound to either Cl, Br, or MeO (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 trans-halogenopalladation observed here suggests that cis- and trans-halogenopalladations of organic substrates may in general be competitive.