Thermal dimerization of [n]cumulenes (n = 5, 7, 9).

The thermal dimerization of three [n]cumulenes (n = 5, 7, and 9) has been investigated, and a common reactivity pattern is observed that gives radialenes and expanded radialenes through regioselective cycloaddition reactions; all three products are characterized using X-ray crystallography.

Cumulene rotaxanes: stabilization and study of [9]cumulenes.

The stabilization of long [n]cumulenes has traditionally been achieved by placing sterically bulky "protecting groups" at the termini, which shield the reactive carbon chain from unwanted reactions. Herein, we present an alternative strategy: stabilization through threading the sp-hybridized carbon chain through a phenanthroline-based macrocycle. The result is stable [9]cumulene rotaxanes that enable the study of properties as a function of length for [n]cumulenes in unprecedented detail, including by quantitative UV/Vis spectroscopy, cyclic voltammetry, and differential scanning calorimetry.

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.

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).