An evolving multifunctional molecular building block: Bicyclopropylidene

The elucidation of the chemistry of the highly strained and unusually tetrasubstituted alkene bicyclopropylidene (2), has proved to be fruitful, both with respect to synthetic applications as a multifunctional Cg building block and for the understanding of certain reaction principles. The different, and steadily improved, methods developed over the last thirty years for the preparation of this unusual alkene, and the more recent methods for the synthesis of functionally substituted as well as spirocyclopropanated derivatives, are presented. The rich chemistry of bicyclopropylidene, beginning with its well-known thermal rearrangement and demerization, its [2+n] cycloadditions with various carbenes, alkenes, 1,3-dipoles, and dienes all the way to its recently developed organometallic chemistry, especially its reactions under the catalysis of palladium and other transition metals, is covered. Some of the peculiar physical properties of bicyclopropylidene (2) which explain its unique reactivity, are also discussed. Finally, some synthetically useful chemical transformations of bicyclopropylidene derivatives, for example, synthetic approaches to certain cyclopropanated analogs of natural products, are presented.