Dicarbanion Compounds: The Bridge between Organometallic Reagents and Mononuclear Heterocycles

Dicarbanion compounds, namely organo-bi-metallics in which two metal atoms are separated by all-carbon backbones, are of great importance in organometallic chemistry for not only the interest in the structure but also the wide application in synthesis, especially of mononuclear heterocycles. The synthetic methods and reactivities of dicarbanion compounds highly depend on the type of metals as well as the structure of backbones. For metal types, while there are examples of other metals, lithium and magnesium play important roles in the chemistry of dicarbanion compounds. For the backbone structure, different unsaturated bonds in the backbones result in different rigidities and orientations of carbon–metal bonds and thus have a great impact on the reactivity of dicarbanion compounds. Herein, a general review of the preparation, structure, and reactivity of dicarbanion compounds with different metals and backbones are presented, mainly focusing on the dicarbanion compounds that have been or may potentially be employed in mononuclear heterocycle synthesis. Heterocycles mentioned in this review are mostly metallacycles because of the limitation of synthetic methods.