Azulene dianion

Many aromatic compounds have considerable resonance stabilization but do not possess a benzene nucleus, or in the case of a fused polycyclic system, the molecular skeleton contains at least one ring that is not a benzene ring. The cyclopentadienyl anion C5HJ, the cycloheptatrienyl cation C7H+, the aromatic annulenes (except for [6]annulene, which is benzene), azulene, biphenylene and acenaphthylene (see Fig. 14.2.2(b)) are common examples of non-benzenoid aromatic hydrocarbons. The cyclic oxocarbon dianions C Of (n = 3,4,5,6) constitute a class of non-benzenoid aromatic compounds stabilized by two delocalized n electrons. Further details are given in Section 20.4.4. 

Figure 3. Some 4n 7r-electron and related compounds reduced to dilithium dianions (A) cyeloootatetraene , (B) Ih-annulene" , (C) l,9-dimethyldibenzo[b,f]pentalene , (D) azulene , (E) heptalene °, (F) octalene, (G) pyrene , (H) bis(cyclohepta)[cd,gh]pentalene, (I) bis(cyclopenta)[ef,kl]heptalene . (J) 15,16-dimethyl-15,16-dihydropyrene , (K) acepleiady-lene .

The azulene anion 752- was reinvestigated by Edlund 92) by the lithium reduction of the neutral azulene (75) in THF-d8. This research followed an earlier study in which a dimeric dianion 192 was mistaken for the dianion 152 93). This anion is a 4nic-... 

Fig. 5. 250.13 MHz H NMR spectrum of the lithium dianion of azulene (152 ) in [2H8]tetrahydro-furan. Residual signals are due to the doubly charged dimer I92...

Hence, the chemical evidence presented for anthracene was confirmed spectroscopically. Furthermore, Mullen et al. (12) categorized a number of hydrocarbons in Li-NH3 solutions into two classes group 1 hydrocarbons that exist as monoanions in ammonia and group 2 hydrocarbons that exist as dianions in ammonia. Group 1 includes anthracene and its 9-methyl, 9-phenyl, and 9,10-diphenyl derivatives acenaphthylene azulene fluoranthene phenanthrene and pyrene. Group 2 includes aceheptylene, cyclooc-tatetraene, and perylene. 

Reaction of azulene with lithium gives an azulenide dianion, probably formed by disproportionation of an intermediately formed radical [142,143], This product is remarkably stable for a charged paratropic species and can be kept in solution in tetrahydrofuran in a sealed n.m.r. tube for weeks. Exposure to dry air leads immediately to the reformation of azulene [143]. N.m.r. spectra show the para-tropicity of this anion [142,143],... 

Lithium reacts with azulene to give an azulenyl radical which is in equilibrium both with an azulenide dianion and a dimeric dianion [143], Aqueous work-up provides a 6,6 -bis(dihydroazuIenyl). 

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