Cumulene


A cumulene is a hydrocarbon with three or more cumulative double bonds. They are analogous to allenes, only having a more extensive chain. The simplest molecule in this class is butatriene, which is also called simply cumulene. Unlike most alkanes and alkenes, cumulenes tend to be rigid, comparable to alkynes, which makes them appealing for molecular nanotechnology. Polyynes are another kind of rigid carbon chains. Cumulenes are found in regions of outer space where hydrogen is rare. Cumulenes containing heteroatoms are called heterocumulenes; an example is carbon suboxide.

Synthesis

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

Structure

The rigidity of cumulenes arises from the fact that the internal carbon atoms carry two double bonds. Their sp hybridisation results in two π bonds, one to each neighbor, which are perpendicular to each other. This bonding reinforces a linear geometry of the carbon chain.
Cumulenes with non-equivalent substituents on each end exhibit isomerism. If the number of consecutive double bonds is odd, there is cis–trans isomerism as for alkenes. If the number of consecutive double bonds is even, there is axial chirality as for allenes.

Reactivity

The reactions of cumulene are those of simple isolated double bonds because the consecutive double bonds are not conjugated to each other.

Transition metal cumulenes

The first reported complex containing a vinylidene ligand was 8, derived from the reaction of diphenylketene and Fe5 Structurally, this molecule resembles Fe29, wherein one μ-CO ligand is replaced by 1,1-diphenylvinylidene, Ph2C2. The first monometallic vinylidene complex was Mo2Cl.