Nickel-Catalyzed Dimerization of [5]Cumulenes (Hexapentaenes)

Abstract

Tetraaryl[5]cumulenes react with low-valent nickel complexes at the second double bond to produce novel [4]radialene derivatives that are head-to-head dimers of [5]cumulenes. The head-to-head dimers are also synthesized by a stepwise route. On the other hand, the nickel-catalyzed dimerization of [5]cumulenes with bulky substituents produces other types of extended [4]radialenes and [5]radialenones depending on the bulkiness of the terminal alkyl substituents. Thus, tetrakis-t-butyl[5]cumulene and 1,4-bis(2,2,6,6-tetramethylcyclohexylidene)[3]cumulene react at the central double bond to give the corresponding [4]radialene and [5]radialenone, whereas 1,4-bis(2,2,5,5-tetramethylcyclopentylidene)[3]cumulene and its benzo-annelated derivative react at the second double bond in a head-to-tail manner to afford the corresponding extended [4]radialenes. Tetrakis-t-butyl[5]radialenone was converted into [5]radialene by using a methylation–dehydration procedure. The extended [4]- and [5]radialenes and [5]radialenones have been fully characterized by spectroscopic analyses, X-ray crystallography, and/or independent chemical synthesis. The properties of these novel exocyclic π-electron systems have been investigated in detail. The aryl-substituted [4]radialenes exhibit facile bond rotation with low energy barriers. The [5]radialene and [5]radialenone form the corresponding cations easily by the addition of an acidic proton. The selectivity of the nickel-catalyzed dimerization of [5]cumulenes is discussed on the basis of theoretical calculations.

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

“…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.…”

“…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] 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.…”

“…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.…”

“…In the best case, the yield of 5 was 24%. Drawing from the seminal work by Iyoda and coworkers, 12 [Ni(CO) 2 Cl 2 ] was explored as a catalyst toward improving the yield of 5, but these attempts were not successful. Finally, it is worth noting that the dimerization reaction could be prevented if solutions of [7]Tol were maintained at a low temperature (À20 1C).…”

Carbyne: The Molecular Approach

“…[93,94] We quickly learned that the study of cumulenes had lain essentially dormant since the 1950s and 1960s, beyond the work of Kuhn, [95][96][97] Bohlmann, [98,99] and a few others, [100,101] aside from studies such as those of Diederich, [102][103][104] Bildstein [105,106] and Iyoda. [107] Thus, many unanswered questions remained about the synthesis and physical properties of this class of molecules as an alternative model for carbyne. One of the first graduate students to join my group in Erlangen, Johanna Januszewski, chose to develop the cumulene chemistry as her Diplomarbeit.…”