Don't Stop with Benzene! The Educational Value of the Cyclooctatetraene (C8H8) Molecule Cindy Samet Dickinson College, Carlisle, PA, 17013
As educators, we ofien search for model compounds that are both im~ortantto the rescarch community and valuable as pedagogical tools in the classroom. of-course, one such model com~oundis benzene. The chemistrv of benzeneisdiscussedat great length in standard textbooks and in the literature. It is an excellent wol for teaching aromnticity and resonance, and it is a very diverse mol&le, as previously discussed in this Journal (1). However, we often stop here and ignore larger ring systems, thus suggesting to our students that benzene covers all the important aspects of the chemistry of annulenes. There is another fascinating model hydrocarbon that is interesting both in itself and in its comparison with benzene. This molecule is cyclcmtatetraene (COT), which has the formula C8H8 (1).If you have not yet come across it, chances are you have been barking up the wrong tree, that is, not a pomegranate tree--the source of COT! As shown in this article, the undergraduate curriculum would be enriched by including COT in its discussions of concepts, including aromaticity and resonance stabilization Hiidre1 molecular orbital theory symmetry spectmscopy Other topics, such as the Jahn-Teller effect, would also he excellent candidates for upper-level or special-topics courses. The Discovery of Cyclooctatetraene In 1911, Willstater and Waser (2) discovered COT while attempting to create a new nonbenzenoid aromatic system. They obtained i t from pseudo-pelletierine, a n alkaloid from the bark of the pomegranate tree. In 1940, COT was
first successfully synthesized in the laboratories of Hadischrhlin- and Soda-Fabrik by thrcyclotetramerization of acetylene. Thus, large quantities of the pure compound could be easily prepared. COT has been identified as one of the ~roductsof the thermal polymerization of acetylene, being obtained at low rields usine W irradiatiun. COT is also formed bv various isomerization processes occurring in other CsHs compounds. In addition, COT has been identified as one of the compounds responsible for the odor of tomatoes, and thus may be a natural product (3)! Since its discovery by Willstater and Waser over eight decades ago, the COT molecule has been important from both a theoretical and synthetic standpoint, and there exist several excellent and comprehensive treatises on the chemistry of COT and its derivatives. The purpose of this discussion is to bring this versatile molecule to the attention of undergraduate educators because, unlike benzene, COT is rarely represented in standard chemistry texts.
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Synthetic Uses of COT Skeletal Transformations
COT, the vinylog of benzene, undergoes a wide array of reactions in which the COT nucleus (2), that is, the eightmembered ring, does not retain its original structure. These structural changes, which do not occur in benzene and benzenoid compounds, lead to many fascinating species (3)including semibullvalene(3) the homotmpylium cation (4) the dianion (5) basketme (6) [16lannulene (7) The abilitv of the COT molecule to &ergo these diverse skeletal transformations makes i t a n important synthetic tool. A Planar Transition State in a Nonplanar Molecule
Neutral COT exists i n a strain-free, nonplanar D2d tub conformation (2). This species is a 4n-electron system, and a significant loss in resonance energy is predicted for a nonplanar structure of alternating single and double bonds. Nevertheless the molecule is quite stable. It is generally assumed that r i n g inversion (eq 1) a n d hond-shift processes or valence tautomerism (eq 2) occur via planar transition states. Either alternating Volume 70 Number 4 April 1993
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Preparation of a Constrained Product The second way to obtain planar COT species is to synthesize cyclooctatetraenes that are constrained to planar conformations by small-ring fusions. One such molecule is perfluorotetracyclobutacyclo octatetraene (101, which was synthesized by coupling tetrailuoro-1,2-diiodocyelobu tene with Cu powder in the presence of dimethylformamide (12). Very recently, Pirrung et al. (13) have synthesized the first example (11) of a class of cyclobutenocyclooctatetraenes that have both stability at room temperature and a carbon skeleton closer to being planar than any bicyclic COT yet prepared. bond lengths (D4hsymmetry)(8)or equal bond lengths (D8h symmetry)(9)may be present (4). Preparation of Planar Species
Therefore, considerable attention has been focused on the existence and synthesis of species containing planar, conjugated eight-membered ring systems. In general, there are two ways to prepare planar COT species: by producing an anionic planar intermediate or by synthesizing a product that is constrained to a planar conformation. Prwlucing an Anionic Planar Intermediate One is to reduce neutral COT to either its radical anion (COT) or Huckel dianion (COT") (5).This happens quite readily because COT has a higher electron affinity than do most olefins (5).Complete ring flattening occurs when COT is reduced to its ions. Early studies (6, 7)show that the dianion COTZis planar symmetry)with aromatic features such as a ring current. The structure of the monoanion has been in question for years, but several studies (8,9)show that it is probably planar. Theoretical Support These findings support Huckel's theory that aromatic hydrocarbons with a 4n electronic configuration will readily accept electrons to form a closed shell of 4n + 2 electrons. Planar aromatic hydrocarbons with this electronic configuration (e.p., benzene) are extremely stable. COT reacts k t h alkali metals in ethers or liquid ammonia to form salts as follows (6). COT + 2M' CO?
where M is Li, Na, K, or Rb. For the planar COTmonoanion, molecular orbital theory predicts an orbitally degenerate ground state subject to Jahn-Teller distortions. The operation of the Jahn-Teller effect in organic molecules and especially in negative aromatic ions has been the subject of extensive theoretical and experimental work (10). A recent study done by Samet et al. (11)addresses the Jahn-Teller effect in COT. 292
Journal of Chemical Education
COT-Metal Complexes
The synthesis of these COT analogs has contributed much to the understanding of antiaromaticity in [8lannulenes as well as sandwich complexes involving metals and COT. COT bonds with metals in many ways. In addition to acting as an electron-acceptor, as shown above, COT also donates electrons. In some transition metal complexes, the CsHs ligand flattens completely to resemble the dianion. In others it
ranged (12) in a manner analogous to other metallocenes. However, for (COT)ZF~ (13)one of the COT rings uses a 1.3.5-triene svstem for coordination. while the other rine is bbnded to a i,3-diene system. In addition, this mole&le exhibits fluxional behavior (ea . * 3) in solution. as do several other (COThM complexes. Finallv. extraordinarv com~lexesinvolvine COT and carborane cages have been synchesized (3)by reacting the sodium salts of carborane dianions with com~lexessuch as [(COT)TiC1I2to yield a mixed-ligand carborane (14). Summary As can be seen. the chemistw of cvclooctatetraene and its ions is quite spectacular. More exciting than this, how-
ever. is the realization that this versatile model molecule maybe used in many ways to narrow the gap between the real chemistry pursued in laboratories worldwide and the chemistry taught in the undergraduate classroom and laboratory. Making important connections between doing chemistry and learning chemistry brings us a giant step closer to fulfilling our primary goal as educators: to help and encourage our students to become chemists! Literature Cited 1. Pdheter. J. H.J. Chem Educ. 19Sl. 68(4). 280.
retains its tub conformation and may even act as an olefin. There is a variety of COT-metal complexes that fall somewhere between these two extremes oi'ligand structure. In some cases, COT forms multinuclear complexes containing metal-metal bonds, and the monocyclic tetraene ligand is transformed to a different species (e.g., two rings fused) (3).Such a vast array of ligand forms is not seen with other C,H, ligands, where n = 5 , 6 , and 7. COT also forms sandwich complexes with metals such as Fe and U (31.The hgands in (COTj2U(uranocene) are ar-
5. Katz, T. J. J Am. Chem. Soc 1960,8Z, 3784.~ 6. Sheuss, H.L.;Kate,T. J.; Fraenb1,G. K J A m . Chem. Sa.lsBS,86,'23BO-2SM. 7. Noardb, J H.; van den Hark,Th. E. M. A e h Crystallagr, Sect, B. 1914.30.853. 8. Kimmel,P I.;Strauss, H. L. J Phys. Chem. 196%72,2813-2817. 9 . Shida, T.; lwata, S. J. Am. Chem Sae 1912,95,3413. 10. Hobey, W. D.; McLachlan, A. D.J Chem. Phys. 1960,33,169Fr1698. 11. Samet, C.; Rase, J. L.;Piepha, S. B.; Sehatz, P. N. manuscript in preparation. 12. B"iton, W E.; Ferrarirrari,J.P.; Soulen, R.L. J A m Chrm. Sa.1982,104,53225325. 13. PYrung,M. C.; Knshnamurthy, N.;Nunn, D. S.; McPhaiL A. T. J. A m Chem. Sa. 1991,113.491C-4917.
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