Cumulene

Not to be confused with cumene.

A cumulene is a hydrocarbon with three or more cumulative (consecutive) double bonds. A member of this compound class is butatriene (which is also called simply cumulene), H2C=C=C=CH2.[1] Unlike most alkanes and alkenes, cumulenes tend to be rigid, which makes them appealing for molecular nanotechnology. Polyynes are another kind of rigid carbon chains. Cumulenes are found in regions of space where hydrogen is rare (see astrochemistry). Cumulenes containing heteroatoms are called heterocumulenes. An example is carbon suboxide.

A collection of simple allenes and cumulenes. From left to right: propadiene (allene), butatriene (cumulene), buta-1,2-diene, penta-2,3-diene.

Synthesis

The first reported synthesis of a butatriene is that of tetraphenylbutatriene in 1921.[2] The most common synthetic method for butatriene synthesis is based on reductive coupling of a geminal dihalovinylidene.[3] Tetraphenylbutatriene was reported synthesized in 1977 by homocoupling of 2,2-diphenyl-1,1,1-tribromoethane with elemental copper in dimethylformamide.[4]

Structure

The rigidity of cumulenes arises from the fact that the internal carbon atoms carry two double bonds. Their sp hybridisation results in two bonds separated by 90°. Cumulenes with (1) an even number of consecutive double bonds and (2) dissimilar substituents on either end can be chiral even though they lack a classical stereocenter. On the other hand odd numbers of double bonds in a cumulated system with proper substituents show cis–trans isomerism.

Reactions

The reactions of cumulene are those of the isolated double bond. Although this molecule possesses two π-bonds in very close vicinity, in essence these act as isolated double bonds. This is because the two π-bonds on the central carbon atom are formed by the non-hybridised p-orbitals. As these orbitals are perpendicular towards each other and occupy each other's nodal planes, the two bonds are in essence isolated.

Transition metal cumulenes

The first reported complex containing a vinylidene ligand was (Ph2C2Fe2(CO)8, derived from the reaction of diphenylketene and Fe(CO)5 Structurally, this molecule resembles Fe2(CO)9, wherein one μ-CO ligand is replaced by 1,1-diphenylvinylidene, Ph2C2. The first monometallic vinylidene complex was (C5H5)Mo(P(C6H5)3)(CO)2[C=C(CN)2]Cl. [5]

See also

References

  1. ↑ IUPAC Gold Book IUPACs definition of cumulenes
  2. ↑ Brand, K. (1921), Über Untersuchungen in der Tetraarylbutan-Reihe und über das 1.1 4.4-Tetraphenyl-butatrien. (4. Mitteilung über die Reduktion organischer Halogen-verbindungen.). Berichte der deutschen chemischen Gesellschaft (A and B Series), 54: 1987–2006. doi:10.1002/cber.19210540828
  3. ↑ Synthesis of the Butatriene C4 Function: Methodology and Applications Léo Leroyer, Valérie Maraval, and Remi Chauvin Chemical Reviews Article ASAP doi:10.1021/cr200239h
  4. ↑ Convenient Preparation of Tetraarylbutatrienes TAKEHISA KUNIEDA and TAKEO TAKIZAWA CHEMICAL & PHARMACEUTICAL BULLETIN Vol.25 , No.7(1977)pp.1809-1810 doi:10.1248/cpb.25.1809
  5. ↑ R. B. King (2004). "The Beginnings of Terminal Vinylidene Metal Complex Chemistry Through the Dicyanomethylene/Oxygen Analogy: Dicyanovinylidene Transition Metal Complexes". Coordination Chemistry Reviews 248 (15–16): 1533–1541. doi:10.1016/j.ccr.2004.05.003.
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