Stereochemistry of the Palladium(0)-Catalyzed Phenylation of 1-Halogenoallenes

Article abstract of DOI:10.1021/jo00217a004

The stereochemistry of the Pd4-catalyzed phenylation of the 1-halogenoallenes RCH=C=CHX (R=Ph, t-Bu; X=Cl, Br, or I) with phenylzinc reagents has been elucidated by using optically active 1-halogenoallenes.The products RCH=C=CHPh were obtained with a moderate to high degree of inversion of configuration around the allenyl moiety when X was Cl and Br.When X was I, retention of configuration was observed.A rationalization of the results is given on the basis of two competing processes, viz., insertion of palladium in the C-X bond (oxidative addition) to give RCH=C=CH(Pd2X) with retention of configuration and a Pd2-induced anti-SN2'-like reaction followed by a suprafacial <1,3>-shift of the Pd2 group leading to RCH=C=CH(Pd2X) with inversion of the configuration.For the latter reaction mode, such an allenyl palladium(0) intermediate was isolated and its absolute configuration determined.