Bicyclopropylidene - A unique tetrasubstituted alkene and a versatile C-6-building block

Strain in an organic molecule often correlates with increased reactivity, a t least for certain types of reactions. Thus, the elucidation of the chemis try of highly strained alkenes like the unusual tetrasubstituted alkene, bi cyclopropylidene (1),which is a particularly strained derivative of methyle necyclopropane (2), has proved to be fruitful both with respect to syntheti c applications as a Cg building block, as well as understanding certain rea ction principles. The different, steadily improved methods which have been developed for the preparation of this unusual alkene in the last thirty yea rs as well as rather recent methods for the synthesis of functionally subst ituted, and spirocyclopropanated derivatives as well as bis(bicyclopropylid enyls) are presented. The special aspects of molecular geometry in bicyclop ropylidenes as well as strain and its influence on chemical properties are discussed. The rich chemistry of bicyclopropylidene beginning with its well -known thermal rearrangement and dimerization, its dihalocarbene additions all the way to its recently developed organometallic chemistry, especially its reactions under the catalysis of palladium and other transition metals, is covered. Finally some synthetically useful chemical transformations of bicyclopropylidene derivatives, e.g., synthetic approaches to the cycloprop anated analogs of natural products, are presented.