Recent Developments in Fulvene and Heterofulvene Chemistry

Cyclic conjugated systems involving an exo-methylene unit in the ring system are generally termed fulvenes. In order to elucidate their aromatic character, fulvenes have received much attention from a large number of chemists for a long time. Pentafulvenes, a structural isomer of benzene, triafulvenes, and heptafulvenes are representative examples of the class of compounds. Fulvene derivatives can serve as building blocks for the construction of the polycyclic conjugated systems. Triapentafulvalene, pentafulvalenes, sesquifulvalene, and heptafulvalene are representative examples of fulvalenes. This chapter explores heterofulvenes and heterofulvalenes from the viewpoint of their electronic properties; that is, how the replacement of the skeletal carbon atoms of fulvenes and fulvalenes byheteroatoms affects their properties. Among these derivatives the fully conjugated cyclic ketones cyclopropenone (CPN), cyclopentadienone (CPDN), and tropone are categorized as oxafulvenes. The reactions between benzotriafulvenes and transition metals afford metallacyclobutane derivatives as stable crystalline compounds in good yields.