Palladium Migration along Linear Carbon Chains:  The Detection of η¹-η²-Enyl Intermediates and the Study of Their Rearrangement

The reactions of [PdPfBr(NCMe) 2 ] (Pf = C 6 F 5 ) with stoichiometric amounts of 1,5-hexadiene, 1,6-heptadiene, or 1,7-octadiene at low temperature result in the formation of several (η 1 -η 2 -enyl)palladium complexes that isomerize sequentially at different temperatures depending on the ring size of the palladacycles ( T isom: 7.5- < 6.5- < 5.5-membered). These (η 1 -η 2 -enyl)palladium derivatives are intermediates in the Pd-migration process, arrested by coordination of the unattacked double bond. The final products of their isomerization are several isomeric Pf−(η 3 -allyl)palladium complexes (Pf = C 6 F 5 ). The major allylic derivative in each case arises from Pd migration to the terminal double bond. Minor amounts of (η 3 -allyl)palladium complexes formed by double bond switches in the process of Pd migration are also seen, but this occurs only on putative 1,5- or 1,6-diene−hydrido−palladium intermediates. A small amount of cyclic organic derivatives coming from the cyclization of (η 1 -η 2 -enyl)palladium intermediates is detected in each case. The use of excess diolefin gives rise to additional (η 3 -allyl)palladium complexes without the Pf group and to the corresponding Pf-substituted linear dienes. These arise via displacement of the Pf dienes by the starting diolefin in a hydrido−palladium intermediate during the Pd-migration process.