Cyclononatetraene, anion

The reader may gain better appreciation of the many basic differences responsible for the division into different classes of heteronin by comparing certain representative members, directly or through appropriate models, in terms of the information presented in Table II. First, one notes that the classification of oxonin (24a) as atropic, jV-methylazonine (27a) as nondescript, and 1 //-azonine or its anion as diatropic, originally proposed on the basis of NMR chemical shifts (data shown in first three rows), was confirmed by the determination of solvent shift character (S values)38 39 that revealed 1//-azonine to possess significant diatropic influence (comparable to that of naphthalene +1.3538), the V-methyl counterpart to exhibit a far weaker effect in the same direction, and oxonin to be atropic or mildly paratropic under this criterion, its S value being closely similar to that of the family s 8 --electron polyenic model, all-cis-cyclononatetraene (24 X = CH2). Major differences between oxonin and parent azonine are also seen to exist in terms of thermal stability and 13C NMR and UV spectroscopy, all of which serve further to emphasize the close structural similarity of oxonin with n-... 

Compared to the cyclooctatetraenyl dianion 19, other cyclic anions (besides cyclopentadienyl anions discussed in Sect. 1.5) have received considerably less attention. Of those that have been studied, not all of them display electron photoejection as a reaction pathway. For example, the 8,8-dimethyl-2,4,6-cyclooctatrienyl anion 22 undergoes cyclization to give 8,8-dimethylbicy-clo[5.1.0]octa-3,4-dienyl anion 23 on photolysis as the exclusive photochemical pathway [42] (Eq. 6). Photolysis of the cyclononatetraenyl anion 24 results in protonation of the more basic excited state anion, to give transient cis, cis, cis, cis-1,3,5,7-cyclononatetraene 25 (Eq. 7), which subsequently undergoes intramole-... 

Use the inscribed polygon method to show the pattern of molecular orbitals in 1,3,5,7-cyclononatetraene and use it to label its cation, radical, and anion as aromatic, antiaromatic, or not aromatic. 

In 1945 Michael J. S. Dewar suggested that the tropylium ion (the cation derived from cycloheptatriene) should also be aromatic (Figure 9). This was confirmed in 1954 since then, the dianion of butadiene and the dication of cyclooctatetraene have also been shown to be aromatic. Like benzene, all four of these ions are planar rings with six tt electrons. According to Hiickel s rule the cyclopropene cation should also exhibit aromaticity, and it does. (In this case n = 0, and 4n + 2 = 2.) The planar anion of cyclononatetraene and the dianion of cyclooctatetraene should also be aromatic (n = 2, and 4n + 2 = 10), and both of them are. 

A very characteristic feature of cyclononatetraene derivatives is the collapse of the monocyclic nine-membered ring to a f)icyclo [4.3.0]triene, i.e. a dihydroindene, and this is a frequent result of reactions of the cyclononatetraenide anions with electrophiles. Thus reaction of potassium all-cfs cyclononatetraenide with carbon dioxide gives a dihydroindenecarboxylic acid and with methyl iodide gives a methyldihydroindene [52] ... 

The all-c is cyclononatetraenide anion extracts a proton from cyclopentadiene to give the cyclopentadicnide anion [61,66]. Ilquili-bration of the cyclononatetraenide ion with cyclopentadiene suggests that it is more thermodynamically stable than the cyclopentadicnide anion, and that the pK of the cyclononatetraene lies between 16 and 21, since no proton exchange takes place with indene [66]. 

As with cyclooctatetraene and its dianion, conversion of cyclononatetraene into its anion involves flattening a buckled molecule, at the same time Introducing angle and/or conformational strain. 

Cyclononatetraene.—Isomerization of the m,cw,cw,/ra 5-[9]annulene anion (151) to its all-cis-isomer has been studied. This isomerization, which occurs on warming a solution of (151) and has AG values in the range 29.6—34.8 kcal mol , depending upon the counterion and solvent, is catalysed by the addition of a trace of methanol or an alkali metal (Na/K alloy, K, or Cs) owing to reversible electron transfer from the strongly electropositive metal into the antibonding orbital of (151) to give a dianion radical, which rapidly isomerizes. Irradiation also causes isomerization of (151) to the all-cis-anion. The mechanism involved in isomerization of (151) into its all-cis-isomer is compared with the mechanism of its topomerization. ... 

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