1,5-Cyclooctadiene

Cycloocta-1,5-diene is a cyclic hydrocarbon with the chemical formula C8H12, specifically [\s(CH2)2\sCH\dCH\s]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 or on opposite sides  of the double bond's plane; the three possibilities are denoted, , and ; or , , and. (Because of overall symmetry, is the same configuration as .)

Generally abbreviated COD, the 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:
 * Synthesis of 9-BBN dimer.png

COD adds SCl2 (or similar reagents) to give 2,6-dichloro-9-thiabicyclo[3.3.1]nonane:
 * [[File:CODSCl2.png|450px|2,6-Dichloro-9-thiabicyclo[3.3.1]nonane, synthesis and reactions]]

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
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(C5H7O2)2]3 + 2COD + 2Al(C2H5)3 → Ni(COD)2 + 2Al(C2H5)2(C5H7O2) + C2H4 + C2H6

The related Pt(COD)2 is prepared by a more circuitous route involving the dilithium cyclooctatetraene:


 * Li2C8H8 + PtCl2(COD) + 3C7H10 → [Pt(C7H10)3] + 2LiCl + C8H8 + C8H12
 * Pt(C7H10)3 + 2COD → Pt(COD)2 + 3C7H10

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 + 3C2H4 → Pt(C2H4)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

 * EE-COD synthesis.svg

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.