Date: 07 Dec 1999

Bicyclopropylidene — A Unique Tetrasubstituted Alkene and a Versatile C6-Building Block

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Strain in an organic molecule often correlates with increased reactivity, at least for certain types of reactions. Thus, the elucidation of the chemistry of highly strained alkenes like the unusual tetrasubstituted alkene, bicyclopropylidene (1), which is a particularly strained derivative of methylenecyclopropane (2), has proved to be fruitful both with respect to synthetic applications as a C6 building block, as well as understanding certain reaction principles. The different, steadily improved methods which have been developed for the preparation of this unusual alkene in the last thirty years as well as rather recent methods for the synthesis of functionally substituted, and spirocyclopropanated derivatives as well as bis(bicyclopropylidenyls) are presented. The special aspects of molecular geometry in bicyclopropylidenes 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 cyclopropanated analogs of natural products, are presented.