Bromine–magnesium exchange in gem-dibromocyclopropanes using Grignard reagents

Reaction of gem-dibromocyclopropanes with ethylmagnesium bromide at ambient temperature leads to very high yields of allenes; when cyclopropylidene–allene ring opening is suppressed, alternative carbenic products are observed, although other reactions compete. When the reactions are carried out at −60°C, a 1-bromo-1-(bromomagnesio)-cyclopropane is formed which may be trapped by a number of electrophiles.