A mechanistic rationale for the outcome of Sonogashira cross-coupling of 9-bromoanthracene and ethynyltrimethylsilane: An unexpected product 4-(9-anthracenyl)-1,3-bis(trimethylsilyl)-3-en-1-yne

The Sonogashira reaction of 9-bromoanthracene and ethynyltrimethylsilane furnishes not only the anticipated 9-(trimethylsilylethynyl)anthracene and 2-(trimethylsilyl)aceanthrylene but also (E)-4-(9-anthracenyl)-1,3-bis(trimethylsilyl)-but-3-en-1-yne, all in moderate yields. A mechanistic rationale is proposed that invokes the intermediacy of bromo(anthracenyl)bis(triphenylphosphine)palladium(II) that can undergo coupling either directly, or after coordination and migratory insertion of the free alkyne.