Synthesis and properties of long [n] cumulenes (n >= 5)

Molecules composed of a contiguous sequence of double bonds, the [n] cumulenes, share structural similarities to both of their conjugated relatives, the polyenes and polyynes. The synthesis and properties of [n] cumulenes are, however, quite different from those of either polyenes or polyynes. At an infinite length, [n] cumulenes would provide one structural form of the hypothetical sp-hybridized carbon allotrope carbyne, while shorter derivatives offer model compounds to help to predict the properties of carbyne. Finally, derivatization of the p-electron framework of [n] cumulenes provides a number of different synthetic transformations, with cycloaddition reactions being the most common. In this review, both historical and recent synthetic achievements toward long [n] cumulenes (n >= 5) are discussed. This is followed by a description of our current understanding of the physical and electronic structure of [n] cumulenes based on UV/vis spectroscopy and X-ray crystallography. Finally, the reactivity of long [n] cumulenes is described.