1,2-Dibromoalk-2-enylidenes by ring-opening of 1,2-dibromo-3-alkylcyclopropenes at ambient temperature

1,2-Dibromo-3-alkylcyclopropenes have been obtained by 1,2-debromination of 1,1,2,2-tetrabromocyclopropanes by reaction with 1 mol equiv. of methyllithium. They ring-open at ambient temperature and, in the presence of an electron-rich or electron-poor alkene, a cyclopropane is formed by apparent addition of an intermediate vinylcarbene; addition to (E)- and (Z)-but-2-enes occurs with retention of alkene relative stereochemistry, indicating that, in this case at least, the carbene is trapped as a singlet.