1,3,5,7-Cyclooctatetraene (COT) is an unsaturated derivative of cyclooctane, with the formula C8H8. It is also known as annulene. This polyunsaturated hydrocarbon is a colorless to light yellow flammable liquid at room temperature. Because of its stoichiometric relationship to benzene, COT has been the subject of much research and some controversy.
Unlike benzene, C6H6, however, cyclooctatetraene, C8H8, is not aromatic. Its reactivity is characteristic of an ordinary polyene, i.e. it undergoes addition reactions. Benzene, by contrast, characteristically undergoes substitution reactions, not additions.
Willstätter noted that the compound did not exhibit the expected aromaticity. Between 1939 and 1943, chemists throughout the US unsuccessfully attempted to synthesize COT. They rationalized their lack of success with the conclusion that Willstätter had not actually synthesized the compound but instead its isomer, styrene. Willstätter responded to these reviews in his autobiography, where he noted that the American chemists were 'untroubled' by the reduction of his cyclooctatetraene to cyclooctane (a reaction impossible for styrene). In 1947, Walter Reppe at Ludwigshafen at last repeated Willstätter's synthesis.
Richard Willstätter's original synthesis (4 consecutive elimination reactions on a cycloctane framework) gives relatively low yields. Reppe's synthesis of cyclooctatetraene, which involves treating acetylene at high pressure with a warm mixture of nickel cyanide and calcium carbide, was much better, with chemical yields near 90%
Because COT is unstable and easily forms explosive organic peroxides, a small amount of hydroquinone is usually added to commercially available material. Testing for peroxides is advised when using a previously opened bottle; white crystals around the neck of the bottle may be composed of the peroxide, which may explode when mechanically disturbed.
The π bonds in COT react as usual for olefins, rather than as aromatic ring systems. Mono- and polyepoxides can be generated by reaction of COT with peroxy acids or with dimethyldioxirane. Various other addition reactions are also known. Furthermore, a stable polyacetylene has been synthesized via the ring-opening polymerization of an alkyl-substituted cyclooctatetraene. COT itself—and also analogs with side-chains—have been used as metal ligands and in sandwich compounds.
COT readily reacts with potassium metal to form the salt K2COT, which contains the dianion C8H82− . The dianion is both planar in shape and aromatic with a Huckel electron count of 10. Cyclooctatetraene forms complexes with some metals, including yttrium and lanthanides. One-dimensional Eu-COT sandwiches have been described as nanowires. The sandwich compounds U(COT)2, or uranocene and Fe(COT)2, are known.
Because COT changes conformation between tub-shaped and planar with addition or subtraction of electrons, it could, in principle, be used to construct artificial muscles. Such devices have been contemplated to be makeable by grafting COT derivatives to a backbone of a suitable conducting polymer, which would supply or remove the reducing equivalents.
10. Experimental/Computational Study of the Electrochemical Oxidation of Cyclooctatetraene in Protic Media. Solvent Effects, Grayson Connors, Xin Wu, Albert J. Fry. J. Am. Chem. Soc.; 2007; http://pubs.acs.org/cgi-bin/asap.cgi/orlef7/asap/html/ol070349s.html