Conjugated dienes hydrozirconation

The alkenylzirconium 685, prepared by hydrozirconation of a terminal alkyne with hydrozirconocene chloride, reacts with alkenyl halide to afford the conjugated diene 686(545]. The Zr reagent can be used even in the presence of the carbonyl group in 687, which is sensitive to Al and Mg reagents. 

Hydrozirconation of the sterically less hindered double bond in conjugated dienes is generally observed [103]. However, the presence, as the minor regioisomer, of the allyhc product has been reported [5, 86]. Depending on the reaction conditions and the reagents used in the subsequent transformation, the allylic isomer did not form (Scheme 8-12) [2]. 

Dienes. The (E)-l-alkcnylzirconium compounds, obtained by hydrozirconation of 1-alkynes (6, 177-178), undergo cross coupling with alkenyl halides to form conjugated dienes (cf. the similar reaction of alkcnylalanes with alkenyl halides, 7, 95-96). In the present synthesis tetrakis(triphenylphosphine)-palladium (6, 571-573) can usually serve as catalyst. Nickel catalysts are less effective than Pd catalysts in this case. ... 

Highly efficient (up to 80%) and selective syntheses of several conjugated dienes (335-336) and trienes (337-338) via alkyne hydrozirconation and Pd-catalyzed alkenyl-alkenyl cross-coupling using (339) (PEPSI-iPr) as a... 

Although further comparisons of alkenylzirconiums to other alkenylmetals in Pd-catalyzed cross-coupling are desirable, the results discussed above clearly indicate that alkenylzirconiums are among the best alkenylmetals in alkenyl-alkenyl coupling, and the hydrozirconation-cross-coupling tandem process provides a convenient and satisfactory route to conjugated dienes and polyenes. 

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