Cyclooctatetraene

Cyclooctatetraene (COT) is a polyunsaturated cyclic hydrocarbon. Its history goes back to 1905, when Richard Willstätter¹* and Ernst Waser at Eidgenössische Technische Hochschule² (ETH) Zürich reported its preparation from pseudopelletierine³, a natural eight-carbon bicyclic alkaloid. The same chemists described an improved synthesis in 1912; the final step was the reaction of “tetramethyldiaminocyclo-octadiene methiodide”⁴ with silver oxide.

When Willstätter and Waser determined the chemical properties of COT, they realized that they were quite different from the properties of benzene:

• COT quickly adds four hydrogen atoms in the presence of a platinum catalyst; benzene cannot be hydrogenated that easily.
• COT reduces permanganate very readily and instantly adds bromine; benzene does not.
• Treating COT with sulfuric acid–nitric acid produces only a resin and not a nitro compound, as does benzene.
• COT isomerizes rather quickly into compounds with bridged rings; benzene does not isomerize.

These findings launched a controversy in the world of organic chemistry. COT and benzene can both be written as cyclic structures with alternating double bonds, so why are they so different? (Chemists eventually found that benzene has a planar structure, whereas COT has a boatlike structure; see images.) At the time, German chemist August Kekulé’s theory of the structure of benzene, including the idea of resonance structures, was not completely understood. It eventually took the work of German physicist/physical chemist Erich Hückel, who in 1931 used quantum mechanics to establish what became known as Hückel’s rule: If a planar ring with only sp²-hybridized atoms contains 4n + 2 π-electrons, where n is a positive integer, it is aromatic. Conjugated cyclic molecules such as COT do not satisfy this rule and therefore are nonaromatic (and may not be planar).

A much-improved synthesis of COT appeared in 1948, when Walter Reppe and co-workers at BASF (Ludwigshafen, West Germany) heated acetylene over a nickel(II) catalyst to produce COT in 95% yield. The predominant use of COT since the 1960s is its use as a ligand for metal atoms; in an early example from 1960, J. A. Manuel and F. G. A. Stone* at Harvard University (Cambridge, MA) prepared three complexes of COT with iron tricarbonyl⁵ [Fe(CO)₃]: COT•Fe(CO)₃, COT•[Fe(CO)₃], and COT•Fe₂(CO)₇. The authors reported the properties of these complexes and others in detail.

1. Willstätter was awarded the 1915 Nobel Prize in Chemistry.
2. Federal Institute of Technology.
3. CAS Reg. No. 552-70-5.
4. Most likely 3,7-bis(dimethylamino)-1,5-cyclooctadiene dimethiodide.
5. CAS Reg. No. 52491-41-5.