A cumulene is a compound having three or more cumulative (consecutive)
allenes, only having a more extensive chain. The simplest molecule in this class is butatriene (H2C=C=C=CH2), which is also called simply cumulene. Unlike most alkanes and alkenes, cumulenes tend to be rigid, comparable to polyynes. Cumulene carbenes H2Cn for n from 3 to 6 have been observed in interstellar molecular clouds[2][3] and in laboratory experiments[4] by using microwave and infrared spectroscopy. (The more stable cumulenes H2CnH2 are difficult to detect optically because they lack an electric dipole moment.) Cumulenes containing heteroatoms are called heterocumulenes;[5] an example is carbon suboxide
.
Synthesis
The first reported synthesis of a butatriene is that of tetraphenylbutatriene in 1921.
vinylidene.[7] Tetraphenylbutatriene was reported synthesized in 1977 by homocoupling of 2,2-diphenyl-1,1,1-tribromoethane with elemental copper in dimethylformamide.[8]
Structure
Cis–trans isomerism of a triene vs axial chirality of an allene
The rigidity of cumulenes arises from the fact that the internal carbon atoms carry 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.
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.[9]
