1,5-Cyclooctadiene

1,5-Cyclooctadiene is the organic compound with the chemical formula C₈H₁₂. Generally abbreviated COD, this diene is a useful precursor to other organic compounds and serves as a ligand in organometallic chemistry.

Synthesis

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.

Reactions and applications

Organic reactions

COD reacts with borane to give [[9-BBN|9-borabicyclo[3.3.1]nonane]], commonly known as 9-BBN, a reagent in organic chemistry used in hydroborations. COD adds SCl₂ (or similar reagents) to give 2,6-dichloro-9-thiabicyclo[3.3.1]nonane:

The resulting dichloride can be further modified as the di-azide or di-cyano derivative in a nucleophilic substitution aided by anchimeric assistance.

Metal complexes

1,5-COD typically binds to low-valence metals via both alkene groups. The complex Ni(cod)₂ is a precursor to several nickel(0) and Ni(II) complexes. 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 + 2 COD + 2 Al(C₂H₅)₃ → Ni(COD)₂ + 2 Al(C₂H₅)₂(C₅H₇O₂) + C₂H₄ + C₂H₆

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

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

Pt(C₇H₁₀)₃ + 2 COD → Pt(COD)₂ + 3 C₇H₁₀

Extensive work has been reported on complexes of COD, much of which can has been described in volumes 25, 26, and 28 of Inorganic Syntheses. The platinum complex has been used in many syntheses:

Pt(COD)₂ + 3 C₂H₄ → Pt(C₂H₄)₃ + 2 COD

COD complexes are useful as starting materials, one noteworthy example is the reaction:

Ni(cod)₂ + 4 CO_(g) $rightleftharpoons$ Ni(CO)₄ + 2 COD

The product Ni(CO)₄ is highly toxic, thus it is advantageous to generate it in the reaction vessel as opposed to being dispensed directly. Other low-valent metal complexes of COD include Mo(COD)(CO)₄, [RuCl₂(COD)]_n, and Fe(COD)(CO)₃. COD is an especially important in the coordination chemistry of rhodium(I) and iridium(I), examples being Crabtree's catalyst and cyclooctadiene rhodium chloride dimer. The square planar complexes [M(COD)₂]⁺ are known (M = Rh, Ir).

References