A direct and a formal trapping of propa-1,2-dienylidene

1,2-Dibromocyclopropene ring-opens in solution at 0 – 20 °C to give 1,2-dibromoprop-2-en-1-ylidene, which is trapped by alkenes; the derived cyclopropanes react efficiently with methyl lithium to produce eth-1-en-1-ylidenecyclopropanes. Reaction of the cyclopropene with methyl lithium in ether in the presence of an alkene leads to the same allenes by trapping of the parent allenic carbene, propa-1,2-dienylidene, albeit in low yield