Cycloheptatrienide anion

The cyclooctadiendiynes 93 and 94 are planar conjugated eight-electron systems (the four extra triple-bond electrons do not participate), which nmr evidence show to be antiaromatic.There is evidence that part of the reason for the lack of planarity in 78 itself is that a planar molecular would have to be antiaromatic. The cycloheptatrienyl anion (61) also has eight electrons, but does not behave like an aromatic system. The bond lengths for a series of molecules containing the cycloheptatrienide anion have recently been published. " The NMR spectrum... 

Resonance stractures can be written that show delocahzation of the negative charge over all of its seven carbons nevertheless, because cycloheptatrienide anion contains eight tt electrons, it is not aromatic. The equilibrium constant for formation from the parent hydrocarbon is more favorable by 10 ° (20 pA a units) for the aromatic cyclopentadienide anion than for the nonaromatic cycloheptatrienide anion. 

A thiepine is formally isoelectronic with the 871-electron cyclooctatetraene and cycloheptatrienide anion as well as other heteropines, and if planar, should be antiaromatic as the HOckel rule predicts. However, as discussed under x-ray diffraction (Section 9.03.2.1), the thiepine ring is boat shaped. H NMR spectral analysis indicates that thiepines are not paramagnetic but atropic (Section 9.03.2.2) (see also Section 9.01.1). 

Electrochemical studies suggest that the pX of cycloheptatriene is about 36 [385-387]. Two waves are observable, reflecting successive reduction to the radical and anion. Both before and after the second wave the major product is bis(cycloheptatrienyl) and it is suggested that after the second wave this arises from rapid electron transfer from a cycloheptatrienide anion to a tropylium cation, followed by combination of the resultant radicals [386,388]. 

In the remainder of this chapter tropylium salts, tropones and tropolones will be described in turn. Heptafulvenes (XIX) will also be mentioned and, finally, the cycloheptatrienide anion (XX), which has eight ir-electrons and hence might be destabilised, will be discussed briefly. 

The 4/7 cycloheptatrienide anion was obtained by the treatment of 7-methoxycycloheptatriene with K-Na alloy in THE at —20 °C and has high reactivity. NMR spectroscopy of some monosubstituted derivatives showed that they have nonplanar structures, in contrast to the cation. The AMcid of cycloheptatriene is 375 3 kcal/mol, making it 34 3 kcal/mol more acidic than ethylene. The calculated values in Table 3 are in good agreement with this difference. This is in sharp contrast with cyclopropene, which is 11 kcal/mol less acidic than ethylene. A number of calculations have been reported for cycloheptatrienyl anion, as well as studies of its formation and reactions in the gas phase. ... 

The cycloheptatrienide, or tropenide, anion (LII) has eight electrons, and, if these were delocalised over the ring as in (LIll) would provide a system which might be destabilised because of its being a 4n-Tt-electron system,... 

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