Alkyl halides halogens + dienes

The photochemistry of ir-allylpalladium complexes has been studied to a limited extent. Two basic reactions have been observed. Irradiation at 366 nm of ir-allylpalladium complexes produced 1,5-diene dimers, reportedly via a radical coupling mechanism.334 333 Similar irradiations in the presence of species capable of trapping the presumed allyl radical intermediate, such as BrCCb, BrCH2Ph or allyl bromide, now yield alkylated and halogenated allyls, in addition to 1,5-diene dimer. This reaction fails for simple alkyl or aryl halides due to the instability of the associated radical (equations 130 and 131 ).336... 

Alkyl- and aryl-acetylenes, e.g. (70), but not acetylene itself, co-dimerize with allylic halides (71) in the presence of catalytic amounts of palladium complexes to give halogenated 1,4-dienes (72) in excellent yields. " The most active catalyst appears to be the [PdX2(PhCN)2] complex. The procedure involves very careful addition of the acetylenic compound to the allylic halide solution at 20 °C to prevent polymerization in this exothermic reaction. The co-dimerization of isobutene with trichloroethylene can produce useful quantities of l,l-dichloro-4-methylpenta-1,4-diene in the presence of t-butyl peroxide at 500 °C in a gas-phase reactor. The reaction probably occurs by a radical transfer mechanism. 

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