Addition of dibromocarbene to cyclobutene

Cyclobutene reacted with dibromocarbene in solution to give 1,5-dibromocyclopent-1-ene (9), 1,2,6, 6-tetrabromobicyclo[3.1.0]hexane (10), and 1,2,3,6-tetrabromocyclohex-1-ene (11), in a ratio of 1:4:8, respectively. Compounds 10 and 11 were found to be formed from a second carbene addition and rearrangement under the given reaction conditions.