1,3,5,7-tetramethylcyclooctatetraene

A dication derived from 1,3,5,7-tetramethylcyclooctatetraene is formed at —78 C in SO2CI by reaction with SbFj. Both the proton and carbon NMR spectra indicate that the ion is a symmetrical, diamagnetic species, and the chemical shifts are consistent with an... 

The tropylium ion has an aromatic sextet spread over seven carbon atoms. An analogous ion, with the sextet spread over eight carbon atoms, is 1,3,5,7-tetramethylcyclooctatetraene... 

Since improper rotation axes include 5, = <7, and S2 — /, the more familiar (but incomplete ) statement about optical isomerism existing in molecules that lack a plane or center of symmetry is subsumed in this more general one. In this connection, the tetramethylcyclooctatetraene molecule (page 37) should be examined more closely. This molecule possesses neither a center of symmetry nor any plane of symmetry. It does have an SA axis, and inspection will show that it is superimposable on its mirror image. 

Cyclooctatetraene is not aromatic. 1,2,3,4-Tetramethylcyclooctatetraene and 1,2,3,8-tetramethyl-cyclooctatetraene are constitutional isomers. 

A very similar molecular structure has been observed (56) for the chromium complex of 1,3,5,7-tetramethylcyclooctatetraene (TMCOT), (TMCOT)Cr(CO)3 (30). The Cr(CO)3 group is linked to six of the ring... 

If 1,3,5,7-tetramethylcyclooctatetraene is treated at low temperature (-78 °C) with SbF5/S02ClF (strongly oxidizing conditions) a dication is formed. This cation, unlike the neutral compound, is planar and all the C-C bond lengths are the same. 

Studies of the electrochemical reduction of tetraphenylcycloocta-tetraenes showed that the 1,3,5,7-tetraphenyl derivative was reduced more easily than cyclooctatetraene itself whereas the 1,2,4,7-tetra-phenyl isomer was more difficult to reduce [34]. This accords well with the concept that the presence of phenyl substituents should, on the one hand, make reduction easier because of increased delocalisation of charge in the resultant anion but, on the other hand, make reduction more difficult due to increased steric hindrance when the molecule is flattened [34]. Steric factors similarly result in 1,2,3,8-tetramethylcyclooctatetraene being more easily reduced than its 1,2,3,4-tetramethyl isomer [13],... 

Adjacent substituents should inhibit formation of planar, or even partially planar conformations. Thus 1,2,3,4- and 1,2,3,8-tetra-methylcyclooctatetraenes have been isolated as separate compounds which do not interconvert at room temperature [12,13]. The 1,2,3,8--isomer can be converted slowly into the 1,2,3,4-isomer at temperatures above 350 but the reverse isomerisation was not observed even at 420 [12]. In the case of 1,2,5,6- and 1,4,5,8-tetramethylcyclo-octatetraenes bond shift is somewhat inhibited but not sufficiently so to give the separate isomers shelf life [13]. It has been pointed out that steric interactions between the methyl groups will in fact be less in the delocalised planar form than in a localised planar form [14] and for 1,2,3,4-tetramethylcyclooctatetraene AG j for bond shift and ring inversion are equal [15]. 

The crystal structure of the potassium salt of 1,3,5,7-tetramethylcyclooctatetraene dianion has been determined by X-ray diffraction. The eight-membered ring is planar, with aromatic C—C bond lengths of about 1.41 A without significant alternation. The spectroscopic and structural studies lead to the conclusion that the cyclooctatetraene dianion is a stabilized delocalized structure. 

It has been possible to isolate two different tetramethylcyclooctatetraenes with four adjacent methyl groups. These correspond to the isomers differing in doublebond positions ... 

Figure 3.11 Chemical compounds with S4 symmetry (a) chemical structure and (b) three-dimensional model of 1,3,5,7-tetramethylcyclooctatetraene in both cases H atoms are omitted for clarity, (c) A cyclic pbospbazene with the formula N4P4CIS also belongs to the S4 point group.

---