Pentadienals, halo

The reaction of allyl halides with terminal alkynes by use of PdClifFhCNji as a catalyst affords the l-halo-l,4-pentadienes 297. 7r-AlIylpalladium is not an intermediate in this reaction. The reaction proceeds by chloropalladation of the triple bond by PdCh, followed by the insertion of the double bond of the allyl halide to generate 296. The last step is the regeneration by elimination of PdCh, which recycles[148]. The cis addition of allyl chloride to alkynes is supported by formation of the cyclopentenone 299 from the addition product 298 by Ni(CO)4-catalyzed carbonylation[149]. 

The synthesis of cyclopropanes by simple cyclization of 2-halo-1,4-pentadienes has not been achieved, presumably because the product 1,2-dimethylenecyclopropane would not be stable under the reaction conditions necessary for ring closure. However, the formation of bicyclo[n.l.O]alkanes from certain 2-halo-l,3-pentadienes (Sect. IV.2.2.1.C.iii.a) or by a cascade reaction (Sect. IV.3.1) has been achieved. 

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