Johanna A. Januszewski

Unexpected Formation of a [4]Radialene and Dendralenes by Addition of Tetracyanoethylene to a Tetraaryl[5]cumulene

The use of cumulenes in synthetic transformations offers the possibility to form structurally interesting and potentially useful conjugated molecules. The cycloaddition reaction of a tetraaryl[5]cumulene with the electron-deficient olefin tetracyanoethylene affords unusual products, including functionalized dendralenes and alkylidene cyclobutanes, as well as a symmetric [4]radialene that shows unique solvatochromism, with λ(max) values approaching the near-IR region. These carbon-rich products have been investigated spectroscopically and by X-ray crystallographic analysis (five structures). The cycloaddition reaction sequence has also been explored by mechanistic and theoretical studies. The obtained results clearly demonstrate the potential of [5]cumulenes to serve as precursors for unprecedented conjugated structures.

Oligomers from sp-Hybridized Carbon: Cumulenes and Polyynes

Carbon-rich organic compounds are candidates for use in a wide range of new technologies. Particularly interesting are materials with a framework of pure carbon, namely the allotropes. Much is known about the most common allotropes, diamond (sp3-carbon) and graphite (sp2-carbon), as well as the newer allotropes, fullerenes, nanotubes, and graphene. On the other hand, little is known about the allotrope constructed from sp-hybridized carbon atoms, a material commonly called carbyne. Without a defined sample of carbyne to study, synthetic chemists have attempted to model the properties of carbyne through the formation of defined length oligomers based on a cumulene or polyyne skeleton. Spectroscopic analysis of these series of oligomers by vibrational (IR and Raman), NMR, and UV/Vis spectroscopy has outlined some of the potential properties of carbyne. Likewise, X-ray crystallographic analyses give further clues as to the structure of carbyne. This review will highlight selected synthetic methods for cumulenes and polyynes, and it then summarizes characterization data related to structure and bonding in these molecules.