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