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| - Unusual Regioselectivity of Pd(0)-Catalyzed Coupling Reaction ofElectron-Deficient Alkenyl Halides with Allenic/Propargylic ZincReagents. Highly Selective Synthesis of 6-Phenylhex-5-yn-2(or3)-enoates/Enitrile and4-Phenyl-6-substituted-hexa-2,4,5-trienoates
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| - The Pd(0)-catalyzed coupling reaction of electron-deficient alkenyl halides with the organozincformed by the subsequent treatment of 1-phenylalk-1-yne with n-BuLi and ZnBr2 with or withouta catalytic amount of HgCl2 was studied. Both the allene-formation- and the alkyne-formation-type coupling reactions were observed with high regio- and stereoselectivity: the reaction of1-phenylprop-1-yne afforded 6-phenylhex-5-yn-2(or 3)-enoates and -enitriles, while the reaction of1-phenylhex-1-yne formed 4-phenyl-6-substituted-hexa-2,4,5-trienoates. A plausible explanation forthe regioselectivity was discussed. The double bonds in 6-phenylhex-5-yn-2-enoates were prone tomigrate to the position conjugated with the carbon−carbon triple bonds to form 6-phenylhex-5-yn-3-enoates at higher temperature. The migration did not occur in absence of an excess amountof allenic/propargylic zinc reagent or at low temperature.
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