Unusual regioselectivity of Pd(0)-catalyzed coupling reaction of electron-deficient alkenyl halides with allenic/propargylic zinc reagents. Highly selective synthesis of 6-phenylhex-5-yn-2(or 3)-enoates/enitrile and 4-phenyl-6-substituted-hexa-2,4,5-trienoates

Article abstract of DOI:10.1021/jo991243g

The Pd(0)-catalyzed coupling reaction of electron-deficient alkenyl halides with the organozinc formed by the subsequent treatment of 1- phenylalk-1-yne with n-BuLi and ZnBr₂ with or without a 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 of 1-phenylprop-1-yne afforded 6-phenylhex-5-yn-2(or 3)-enoates and -enitriles, while the reaction of 1-phenylhex-1-yne formed 4-phenyl-6- substituted-hexa-2,4,5-trienoates. A plausible explanation for the regioselectivity was discussed. The double bonds in 6-phenylhex-5-yn-2- enoates were prone to migrate 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 amount of allenic/propargylic zinc reagent or at low temperature.