Synthesis of extended polyynes: Toward carbyne

This account will focus on the synthesis and characterization of polyynes. After a brief discussion of synthetic tools commonly employed in the assembly of sp-hybridized carbon frameworks, a more detailed discussion of polyynes composed of at least five contiguous acetylenic units will follow. The construction of polyynes up to hexaynes is a rather well-developed area where traditional synthetic strategies are typically still useful, i.e., metal-catalyzed homo- or heterocoupling and exhaustive elimination reactions. The synthesis of longer polyynes (heptaynes, octaynes, etc.), on the other hand, can require more subtle synthetic approaches due to inherent problems of reagent compatibility and the reactive nature of many polyyne intermediates and products. A description of the innovative synthetic methods that have been used over the past decade to overcome some of these challenges will be presented. Furthermore, a review of all polyynes reported to date with a length of at least 16 sp-carbons is included, combined with an account of methods used for their characterization.