title: Cyclopropanation of vinylidenecyclopropanes. Synthesis of 1-(dihalomethylene)spiropentanes.
abstract: Synthesis of 1-(dihalomethylene)spiropentanes via cyclopropylidenecyclopropanes generated by cyclopropanation of vinylidenecyclopropanes by dihalocarbenes is described. Reaction of diarylvinylidenecyclopropanes with dibromocarbene and dichlorocarbene exclusively gave 1-(dihalomethylene)spiropentanes in high yields. Reaction of monoarylvinylidenecyclopropanes with dihalocarbenes afforded cyclopropylidenecyclopropanes as the major product with the formation of a small amount of 1-(dihalomethylene)spiropentanes. The efficiency of the thermal rearrangement from the cyclopropylidenecyclopropanes to the 1-(dihalomethylene)spiropentane derivatives depended on the substituents and the reaction temperature. Reaction of diarylvinylidenecyclopropanes with diphenylcarbene and phenylthiocarbene gave the corresponding spiropentane derivatives. This type of thermal rearrangement was applicable to the cyclopropanation of 1,1-diarylallenes.