Cycloheptatrienyl anion

Even though resonance tells us that the negative charge in cycloheptatrienyl anion can be shared by all seven of its carbons, this delocalization offers little in the way of stabilization. Indeed with eight tt electrons, cycloheptatrienyl anion is antiaromatic and relatively unstable. 

Staley, S.W. Orvedal, A.W. J. Am. Chem. Soc., 1974, 96, 1618. In this case, the reagent converted one molecule of cycloheptatriene to the cycloheptatrienyl anion (p. 52). which then added stepwise to the other molecule. 

Problem 10.30 Design a table showing the structure, number of tt electrons, energy levels of tt MO s and electron distribution, and state of aromaticity of (a) cyclopropienyl cation, b) cyclopropenyl anion, (c) cyclobutadiene, (d) cyclobutadienyl dication, (c) cyclopentadienyl anion, (/) cyclopentadienyl cation, (g) benzene, (h) cycloheptatrienyl anion, (/) cyclooctatetraene, (/ ) cyclooctatetraenyl dianion. ... 

Although the tropylium ion forms easily, the corresponding anion is difficult to form because it is antiaromatic. Cycloheptatriene (pKa = 39) is barely more acidic than propene (piTa = 43), and the anion is very reactive. This result agrees with the prediction of Htickel s rule that the cycloheptatrienyl anion is antiaromatic if it is planar. 

Compare the piQ of cyclopentadiene with that of cycloheptatriene. Whilst the anion of the former has 6 7t electrons (which makes it isoelectronic with benzene), the anion of the latter has 8 ti electrons. Remember that on p. 176 we saw how 4n n electrons made a compound anti-aromatic The cycloheptatrienyl anion does have 4 7t electrons but it is not anti-aromatic because it isn t planar. However, it certainly isn t aromatic either and its pKa of around 36 is about the same as that of propene. This contrasts with the cyclopropenyl anion, which must be planar since any three points define a plane. Now the compound is anli-aromatic and this is reflected in the very high pKit (about 62). Other compounds may become aromatic on losing a proton. We looked at fluorene a few pages back now you will see that fluorene is acidic because its anion is aromatic (14 n electrons). 

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... 

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