Dibromocyclopropanes elimination reactions

The reaction of dibromocarbene with haloalkenes (vinyl halides, alkenes with a halogen more distant from the double bond) leads to the formation of 1,1-dibromocyclopropanes in poor to good yields. However, reactions with allyl halides, particularly substituted allyl bromides, require comment. These alkenes furnish, apart from 1,1-dibromocyclopropanes 1, the alkylation products of tribromomethyl anion 2 and, occasionally, the products of their further transformations (dibromocarbene adducts 3, products of elimination of hydrogen bromide 4 etc.) if they react with bromoform under phase-transfer catalysis conditions (Houben-Weyl, Vol.E19b, pi620). 

The same elimination method works just as well with (bromomethyl)cyclopropanes, although far fewer examples are known (Table 3). This reaction is valuable in cases where preparation of the 1-bromo-l-methylcyclopropanes by dibromocyclopropanation followed by methylation is impractical. Most of the compounds prepared by this route were labeled with deuterium, as illustrated by the preparation of (methylene-t/jjcyclopropane (11) starting from methyl cyclo-propanecarboxylate (9). ... 

The reaction of a 1,1-dibromocyclopropane with an alkyllithium generally leads to 1,1-elimination producing a cyclopropylidene, which normally rearranges to an allene, but may be trapped by other carbenic reactions. When there is a silyl substituent on C2, the carbene is apparently trapped by a 1,2-silicon shift giving a silyl-substituted cyclopropene. ... 

Miscellaneous.—a-Elimination. The reaction of 1,1-dibromocyclopropanes with alkyl-lithiums continues to be used for routine generation of allenes. ... 

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