Cyclononatetraenyl anion

Nine-tnetnbered ir-excessive heterocycles with the general formula 1 are commonly referred to as heteronines. They can be regarded as direct analogs of the cyclononatetraenyl anion and have been extensively studied with regard to the question of their potential 1 On-aroma tic character. Heteronines have been the subject of several reviews.1 4... 

Das Singulettspektrum des Cyclononatetraenyl-Anions ist auBer fiir Lithium und Kalium auch fiir Tetraathylammonium als Gegenion ge-messen worden. Die chemischen Verschiebungen fiir Li und (HsCa N in Tetrahydrofuran bzw. Dimethylsulfoxid sind praktisch gleich (3,15 bzw. 3,18 t), wahrend das Signal des Kaliumsalzes bei etwas niedrigerem T-Wert (2,96) erscheint 148,149). 

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-... 

Four compounds, 89 240>, 732il 742i2 > and 75 242) in the nine membered IOtt heteroannulene series have now been reported these compounds are isoelectronic with the cyclononatetraenyl anion, 60, a planar aromatic system. Of the three heteronin systems (N, O, S) reported however, only 1-H azonin, 89, appears to possess aromatic character, as judged by the n.m.r. spectra 240>. 

Explain the following (a) Cyclononatetraenyl anion is planar (in spite of the angle strain involved) and appears to be aromatic. 

Explain the following (a) Cyclononatetraenyl anion is planar (in spite of the angle strain involved) and appears to be aromatic, (b) Although [16]annulene is not aromatic, it adds two electrons readily to form an aromatic dianion. 

Cyclononatetraenyl anion has Dzh symmetry, which is reduced to Cz in the triplet state. However, this triplet is calculated to have a Mobius r-system which leads to some stabilization. Here, the calculated singlet-triplet gap is 41 kcal/mol, significantly smaller than for the other 4n -I- 2 systems. As noted above, the cation with Cz symmetry has a Mobius r-system and the triplet has Cz symmetry. The singlet—triplet gap is 16 kcal/mol. 

Which would you expect to be most stable, cyclononatetraenyl radical, cation, or anion ... 

The An+ 2 cyclononatetraenyl system is the anion which is prepared by the reaction of chlorobicyclo-[6.1. OJnonatetraene with potassium. The planar Dgh structure has been confirmed by low-temperature H NMR spectroscopy. This is remarkable in view of the 140 °C—C—C bond angles, which lead to consid-... 

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