Acumulene is a compound having three or morecumulative (consecutive)double bonds.[1] They are analogous toallenes, such aspropadiene, but with at least one additional alkene unit. The simplest molecule in this class is thusbutatriene (H2C=C=C=CH2), which is also called simplycumulene. Unlike mostalkanes andalkenes, cumulenes tend to be rigid, comparable topolyynes. Cumulene carbenesH2Cn forn from 3 to 6 have been observed in interstellarmolecular clouds[2][3] and in laboratory experiments[4] by using microwave and infrared spectroscopy. (The more stable cumulenesH2CnH2 are difficult to detect optically because they lack an electric dipole moment.) Cumulenes containingheteroatoms are calledheterocumulenes;[5] an example iscarbon suboxide.
The first reported synthesis of a butatriene is that of tetraphenylbutatriene in 1921.[6] The most common synthetic method for butatriene synthesis is based onreductive coupling of ageminaldihalovinylidene.[7] Tetraphenylbutatriene was reported synthesized in 1977 by homocoupling of 2,2-diphenyl-1,1,1-tribromoethane with elementalcopper indimethylformamide.[8]
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 twoπ bonds, one to each neighbor, which are perpendicular to each other. This bonding reinforces alinear geometry of the carbon chain.
Cumulenes with non-equivalent substituents on each end exhibitisomerism. If the number of consecutive double bonds is odd, there iscis–trans isomerism as for alkenes. If the number of consecutive double bonds is even, there isaxial chirality as for allenes.
The first reportedcomplex containing a vinylidene ligand was (Ph2C2Fe2(CO)8, derived from the reaction ofdiphenylketene andFe(CO)5. Structurally, this molecule resemblesFe2(CO)9, wherein oneμ-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]
^King, R. Bruce (August 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.
