Strictly pair-selective and economical synthesis of conjugated diynes via Pd-catalyzed reaction of terminal alkynes with 1,1-dichloroethylene, elimination with LDA, and subsequent transformations

Various unsymmetrically substituted conjugated diynes can be synthesized in a completely pair-selective manner via Pd-catalyzed reaction of terminal alkynes with 1,1-dichloroethylene in the presence of Pd(PPh34) (followed by elimination with 2 equiv of lithium diisopropylamide (LDA), zincation with ZnBr)(2) (or ZnCl)(2), and Pd-catalyzed cross-coupling with aryl and alkenyl iodides and bromides. The desired unsymmetrically substituted conjugated diynes have been obtained in > 80% yields except in two cases where an alkenylzinc reagent generated in situ from (E)-3-iodo-2-propenol was used. The use of 1,1-dichloroethylene renders this method more economical than those involving 1,2-dihaloethylenes previously reported.