# 1,5-Cyclooctadiene

1,5-Cyclooctadiene (also known as cycloocta-1,5-diene) is a cyclic hydrocarbon with the chemical formula C$_8$H$_{12}$, specifically [−(CH$_2$)$_2$−CH=CH−]$_2$.
There are three configurational isomers with this structure, that differ by the arrangement of the four C–C single bonds adjacent to the double bonds. Each pair of single bonds can be on the same side (cis,Z) or on opposite sides (trans,E) of the double bond's plane; the three possibilities are denoted cis,cis, trans,trans, and cis,trans; or (Z,Z), (E,E), and (Z,E). (Because of overall symmetry, trans,cis is the same configuration as cis,trans.)
Generally abbreviated COD, the cis,cis isomer of this diene is a useful precursor to other organic compounds and serves as a ligand in organometallic chemistry. It is a colorless liquid with a strong odor. 1,5-Cyclooctadiene can be prepared by dimerization of butadiene in the presence of a nickel catalyst, a coproduct being vinylcyclohexene. Approximately 10,000 tons were produced in 2005.

## Organic reactions

COD reacts with borane to give 9-borabicyclo[3.3.1]nonane, commonly known as 9-BBN, a reagent in organic chemistry used in hydroborations:
COD adds SCl$_2$ (or similar reagents) to give 2,6-dichloro-9-thiabicyclo[3.3.1]nonane:
The resulting dichloride can be further modified as the diazide or dicyano derivative in a nucleophilic substitution aided by anchimeric assistance.
COD is used as an intermediate in one of the syntheses of disparlure, a gypsy moth pheromone.

## Metal complexes

* Selected metal 1,5-COD complexes.
* Bis(cyclooctadiene)nickel(0).
* Crabtree's catalyst.
* The complex Rh$_2$(COD)$_2$Cl$_2$.
* Co(1,5-cyclooctadiene)(cyclooctenyl).
* Dichloro(1,5-cyclooctadiene)palladium
* cyclooctadiene iridium chloride dimer
* Dichloro(1,5-cyclooctadiene)platinum
1,5-COD binds to low-valent metals via both alkene groups. Metal-COD complexes are attractive because they are sufficiently stable to be isolated, often being more robust than related ethylene complexes. The stability of COD complexes is attributable to the chelate effect. The COD ligands are easily displaced by other ligands, such as phosphines.
Ni(COD)$_2$ is prepared by reduction of anhydrous nickel acetylacetonate in the presence of the ligand, using triethylaluminium
    1⁄3[Ni(C$_5$H$_7$O$_2$)$_2$]$_3$ + 2COD + 2Al(C$_2$H$_5$)$_3$ → Ni(COD)$_2$ + 2Al(C$_2$H$_5$)$_2$(C$_5$H$_7$O$_2$) + C$_2$H$_4$ + C$_2$H$_6$
The related Pt(COD)$_2$ is prepared by a more circuitous route involving the dilithium cyclooctatetraene:
    Li$_2$C$_8$H$_8$ + PtCl$_2$(COD) + 3C$_7$H$_{10}$ → [Pt(C$_7$H$_{10}$)$_3$] + 2LiCl + C$_8$H$_8$ + C$_8$H$_{12}$
    Pt(C$_7$H$_{10}$)$_3$ + 2COD → Pt(COD)$_2$ + 3C$_7$H$_{10}$
Extensive work has been reported on complexes of COD, much of which has been described in volumes 25, 26, and 28 of Inorganic Syntheses. The platinum complex is a precursor to a 16-electron complex of ethylene:
    Pt(COD)$_2$ + 3C$_2$H$_4$ → Pt(C$_2$H$_4$)$_3$ + 2COD
COD complexes are useful as starting materials; one noteworthy example is the reaction:
    Ni(COD)$_2$ + 4CO → Ni(CO)$_4$ + 2COD
The product Ni(CO)$_4$ is highly toxic, thus it is advantageous to generate it in the reaction vessel upon demand. Other low-valent metal complexes of COD include cyclooctadiene rhodium chloride dimer, cyclooctadiene iridium chloride dimer, and Fe(COD)(CO)$_3$, and Crabtree's catalyst.
The M(COD)$_2$ complexes with nickel, palladium, and platinum have tetrahedral geometry, whereas [M(COD)$_2$]$^+$ complexes of rhodium and iridium are square planar.

## (E,E)-COD

    E,E-COD synthesis (Stöckmann et al. 2011)
The highly strained trans,trans isomer of 1,5-cyclooctadiene is a known compound. (E,E)-COD was first synthesized by George M. Whitesides and Arthur C. Cope in 1969 by photoisomerization of the cis,cis compound. Another synthesis (double elimination reaction from a cyclooctane ring) was reported by Rolf Huisgen in 1987. The molecular conformation of (E,E)-COD is twisted rather than chair-like. The compound has been investigated as a click chemistry mediator.

## Related compounds

* Dibenzocyclooctatetraene