Cyclooctatetraene conformational isomers

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. 

Because of the inherent complexity of the cyclooctatetraene system,these results can be explained without invoking adsorbed intermediates 102 but the possibility remains that adsorption plays an important role. By analogy with electrophilic additions to cyclooctatetraene 1 °3, an adsorbed cyclooctatetraene molecule would be attacked preferentially from the fold of the tub conformation with formation of encfo-8-acetoxyhomotropylium ion (11). Attack by acetate ion on 11 would give the c/s-isomer, whereas the trans-isomer might be formed from exo-8-acetoxyhomotropylium ion 12 (Eq. (33) ). 

Methylated and unmethylated methoxyazocine appear, on the basis of their chemistry and spectroscopy, to exist in the tub conformation (39), similar to cyclooctatetraene. These compounds react with dienophiles and potassium f-butoxide to give Diels-Alder adducts and benzonitriles, respectively, presumably via an azabicyclo[4.2.0]octane isomer (40). The properties of 39 indicate that the methoxy group is attached to the imino and not an olefinic carbon. This has been confirmed by recent ab initio calculations.69... 

Pettit and co-workers (89) have recently isolated three isomeric complexes of formula 7r-CgHgFe2(CO)6 from the reaction of Fe2(CO)9 with cyclooctatetraene. One is identical with the known compound obtained from Fe(CO)s or Fe3(CO)i2 and the olefin, with a trans configuration of Fe(CO)3 groups (40). The second is the cis isomer (XXIV) with the Fe(CO)3 groups on the same side of the ring in its chair conformation, as shown by the NMR... 

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. 

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