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

Abstract

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.

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

“…9 The regiochemistry observed for these cycloaddition reactions has been typically attributed to either steric requirements or to the electronic structure of endcapping groups. [10][11][12][13][14] For example, the thermal dimerization of a [5]cumulene with bulky alkyl substituents, such as [5]tBu (Scheme 1), results in dimerization at the central g-bond to form the symmetric [4]radialene 1a (Scheme 1). 7 [5]Cumulenes functionalized with smaller groups (e.g., [5]H or [5]Me) show dimerization at the a-bonds, to give cyclynes 2.…”

“…[10][11][12][13][14] For example, the thermal dimerization of a [5]cumulene with bulky alkyl substituents, such as [5]tBu (Scheme 1), results in dimerization at the central g-bond to form the symmetric [4]radialene 1a (Scheme 1). 7 [5]Cumulenes functionalized with smaller groups (e.g., [5]H or [5]Me) show dimerization at the a-bonds, to give cyclynes 2. 13,15,16 In the case of aryl substituents, the thermal reaction of [5]Ph forms 3 (Scheme 1), presumably via [4]radialene 1b and a subsequent [4+2] cycloaddition with [5]Ph followed by a [2+2] electrocyclization.…”

“…7 [5]Cumulenes functionalized with smaller groups (e.g., [5]H or [5]Me) show dimerization at the a-bonds, to give cyclynes 2. 13,15,16 In the case of aryl substituents, the thermal reaction of [5]Ph forms 3 (Scheme 1), presumably via [4]radialene 1b and a subsequent [4+2] cycloaddition with [5]Ph followed by a [2+2] electrocyclization. 17 Finally, the dimerization of [5]cumulenes under Ni-catalysis has been reported by Iyoda and coworkers, 12 and the regiochemical outcome of these reactions centers on the reaction at either the b-or g-bond, and is dependent on the size and electronic makeup of the end groups.…”

“…13,15,16 In the case of aryl substituents, the thermal reaction of [5]Ph forms 3 (Scheme 1), presumably via [4]radialene 1b and a subsequent [4+2] cycloaddition with [5]Ph followed by a [2+2] electrocyclization. 17 Finally, the dimerization of [5]cumulenes under Ni-catalysis has been reported by Iyoda and coworkers, 12 and the regiochemical outcome of these reactions centers on the reaction at either the b-or g-bond, and is dependent on the size and electronic makeup of the end groups. The reactivity of longer ''odd'' [n]cumulenes (i.e., n = 7 or 9) in cycloaddition reactions has not, to our knowledge, been reported to date.…”

“…Intrigued by the formation of 3 from [5]Ph as reported by Kawamura and coworkers, the thermal reaction of [5]tBuPh, recently synthesized in our laboratory, 18 was examined. After heating in toluene at reflux for 4 d (Scheme 2), the crude reaction mixture showed many products (by TLC analysis).…”