Conformation of cyclooctatetraene

Karle, I. L. /. Chem. Phys. 1952,20,65. Dewar reported calculations showing that the chair-like conformation of cyclooctatetraene is unstable with respect to the tub conformation shown here Dewar, M. J. S. Merz, K. M., Jr. /. Chem. Soc. Chem. Commun. 1985, 343. 

Molecular orbital energy diagram for a planar conformation of cyclooctatetraene. Three pairs of electrons fill the three low-lying 77-bonding molecular orbitals. Two electrons are unpaired in degenerate 77-nonbonding molecular orbitals. 

There is, however, nmr evidence that indicates that the tub form is in rapid equilibrium with a very small amount of the planar form at room temperature. There is about a 15-kcal mole-1 energy difference between the two forms. The dication, C8HS2 , and the dianion of cyclooctatetraene, C8HR2e, which have (4 n + 2)tt electrons, appear to exist in planar conformations (see Exercise 21-16, p. 996). 

Make a model of cyclooctatetraene in the tub conformation. Draw this conformation, and estimate the angle between thep orbitals of adjacent pi bonds. 

Cyclooctatetraene reacts with Co(CO)2(C5H5) to produce (CgHg)-Co(C5H5) 158, 268, 431) and (CsH, )[60(05115)]2 227). Preparation of the complexes by reaction of cyclooctatetraene with cobaltous acetyl-acetonate and ethoxydiethylaluminum has also been reported 611). Spectral data indicate that in the mononuclear complex the olefin is coordinated as a 1,5-diene with the tub conformation 158, 226, 227, 268). X-Ray data 220) show structure (169) for the binuclear complex. 

Gyclooctatrienyne zirconocene complexes of type 704 can be prepared by /3-hydrogen elimination from zirconocene biscyclooctatetraenyl complexes (Scheme 171).528 These complexes are fluxional by a ring inversion process, with activation barriers in the range typical of cyclooctatetraenes. An X-ray crystal structure (R = Ph) reveals a boat conformation of the cyclooctatrienyne ring with no significant flattening when compared with cyclooctatetraene, thus the structure more closely resembles substituted cyclooctatetraenes than that expected of a cyclooctatrienyne. 

Optically active polyacetylene derivatives 97 were synthesized through ring-opening polymerization of the corresponding cyclooctatetraene derivatives.25 A twisted conformation of the main chain was proposed on the basis of CD and UV absorptions. Various optically active polyacetylenes have also been prepared from chiral monomers.24,25,263,177-183 The examples include a phenylacetylene derivative (98),26a alkylacetylenes 99,24 propionic esters such as 100,177,178 a Si-containing monomer (101),179 and disubstituted monomers such as 102.180 Poly-(A)-98 synthesized using a [RhCl(norbornadiene)]2 catalyst shows intense CD bands in the UV—vis region, probably based on a predominant helical sense of the main chain.263 This polymer effectively resolves several racemic... 

Annulene is a 4n -electron system, and thus, the (neutral) planar form would be expected to exhibit antiaromatic behavior. Indeed, Frost s circle analysis predicts an open-shell species (as with [4]annulene see Figs. 2 and 6). Extensive studies of cyclooctatetraene were performed, and it was established that the neutral state avoids the problem of antiaromaticity by adopting a tub conformation. Both the anion and dianion are known to exist in a planar conformation, the latter being aromatic. This fact is also predicted by simple analysis of the [8]annulene dianion by Frost s circle (see Fig. 6). It is predicted that the planar [8]annulene dianion is a closed-shell system. 

It has long been known that the interconversion of cyclooctatetraene (COT) and bycyclo[4.2.0]octa-2,4,7-triene (BCO) is rapid in both directions at relatively low temperatures (ca. 100°C) [19, 20] the product has a cis junction, so the disro-tatory pathway is evidently preferred. The conventional procedure would be to freeze one tt bond and regard the reaction as being an allowed disrotatory six-electron cyclization of the hexatriene moiety. One could, of course, freeze one pair of adjacent tt bonds instead, and produce the imns-joined product by a no less allowed conrotatory four-electron cyclization of the residual butadiene system. In view of the fact that adjacent double bonds of cyclooctatetraene (COT) are twisted away from each other in its stable tub conformation, the latter mode would seem to be the more facile, so the failure to observe the tran - o ned isomer has to be rationalized in terms of its lesser thermodynamic stability, rather than a symmetry-imposed potential barrier along the conrotatory pathway. 

In a planar conformation, the p orbitals of cyclooctatetraene overlap continuously around the ring. 

Figure 3.46. Structural and conformational isomers of cyclooctatetraene derivatives.

Other equilibria which have been studied by NMR spectroscopy include those of oxepine-benzene oxide isomerization (Figure 3.45), structural and conformational isomers of cyclooctatetraene derivatives (Figure 3.46), car-bocations, and organometallic compounds. 

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