(E)/(Z)-1,2-Dibromoethylene (I) underwent palladium-catalyzed diastereoselective alkynylation reaction with alkynylzinc chlorides RC≡CZnCl (R = Bu, BuCH2, Me3Si, Ph, 2-thienyl) to afford bromoenynes (E)-RC≡CCH:CHBr (II) and endiynes RC≡CCH:CHC≡CR (III) with very high stereoisomeric purity. Compds. II and III can be obtained selectively according to the molar ratio between the reagents. Evidence has been obtained that the diastereoselectivity of the alkynylation reactions can be attributed to a diastereoselective oxidative addn. of (E)/(Z)-I to the palladium(0) complex used as catalyst in these reactions. It has also been shown for the 1st time that the complexes which derive from the oxidative addn. of (E)-I or (E)-1,2-dibromo-1,2-diphenylethylene (IV) to Pd(PPh3)4 easily undergo decompn. affording both PdBr2(PPh3)2 and acetylene or diphenylacetylene, resp. Finally, it has been shown that the reaction of IV with organozinc derivs., in the presence of catalytic amts. of Pd(PPh3)4, affords diphenylacetylene, selectively, instead of cross-coupling products.
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