Tetrabromobutatriene:  Completing the Perhalocumulene Series

Tetrabromobutatriene, C₄Br₄, can be prepared directly from dibromobutadiyne by reaction with Br₂ at −25 °C in concentrated hexanes solution. The cumulene precipitates out of the reaction mixture as a yellow powder. Under palladium-catalyzed coupling conditions, C₄Br₄ can undergo allylic rearrangement, giving a mixture of products, including some with butenyne backbones. However, in furan solution, C₄Br₄ reacts cleanly at its central double bond to give the furan Diels−Alder adduct. Under Suzuki conditions, this adduct reacts at the furan double bond rather than at bromide.