Azaazulene

Interestingly in these cases protonation takes place at the ring carbon, whereas for 5-azaazulene protonation occurs at the -equivalent nitrogen, thus retaining the aromatic rr-electron structure.239... 

Following on from a preliminary report <9lT(.6727>, Nitta and co-workers have published details of the formation of a number of 5-azaazulenes by the reaction of cyclopentadienyl aldehydes with vinylphosphoranes, the simplest product being the phenyl derivative, 3-phenylcyclohepta[c]azepine <95JCS(Pl )iooi>. 

For nonalternant hydrocarbons, however, a bathochromic or a hypso-chromic shift may result, depending on the absolute magnitude of the LCAO coefficients of the HOMO and LUMO. This is also shown in Figure 2.26 and is clearly illustrated by the absorption spectra of azulene, 4-azaazulene, and 5-azaazulene, given in Figure 2.28. 

Figure 2.28. Absorption spectra of azulene, 4-azaazulene, and 5-azaazulene (by permission from Meth-Cohn et al., 1985).

Phenanthrene o-quinodimethane, 55 Phenyl-2-aza-l,3-pentadiene, 258 6-Phenyl-5-azaazulene, 260... 

The intramolecular aza-Wittig reaction has been described previously as a convenient route for the synthesis of azepines, and, in a bimolecular equivalent, the reaction of 2-formyl-6-dimethyl-aminofulvene with phenyl-substituted vinyliminophosphoranes in a one-pot reaction offers a facile synthesis of phenyl-substituted 5-azaazulene (201) <91TL6727>. 

The cyclopentazepine systems are often1-5 referred to as 4-aza-, 5-aza-, and 6-azaazulene, respectively. Confusingly, the cyclopent[a]azepine 8a, which is more correctly known as 1ff-pyrrolo[l,2-a]azepine, has been referred to as 4-azaazulene6 and as 3a-azaazulene.7 Only the carbonyl derivative 8b of this nonconjugated system is included in this section since it can be represented as the fully conjugated, aromatic, dipolar structure 9.6... 

Cyclization of 8-hydrazinO 3-phenyl-l-azaazulenes 69 with triethyl orthoformate gave 4,5-triazabenz[c(f]azulene derivative 70 which was easily hydrolyzed on silica gel and gave (2-chloro-8-imino-3-phenyl-l-aza-l,8-dihydoazulen-l-yl) formaldehyde oxime (87H767) (Scheme 17). 

The reactions of 2-hydrazino-l-azaazulenes and EMME in boiling ethanol for 0.5-1.0 hr gave 2-(l-azaazulen-2-yl)hydrazinomethylenemalonates (1%) in 92-96% yields (88BCJI440). 

The cyclization of (l-azaazulen-2-yl)hydrazinomethylenemalonates (1409) in boiling te/7-butylbenzene for 30-40 min afforded l,2,5a-triaza-cyclohept[u]azulen-5-ones (1410) in 74-93% yields (88BCJ1440). Whereas the methyl derivative of 1409 (R = Me) could be cyclized in 56% yield in chloroform by treatment with silica gel at room temperature for 7 days, the ethoxycarbonyl derivative (1409, R = COOEt) could not. 

In the case of five-membered nitrogen heterocycles, the fused systems are frequently referred to as azaazulenes" [see 81H(I5)547, 8IYGK690]. 

In 2,6-disubstituted 1,3-diazaazulenes, such as 14a-c (Scheme 5 62BCJ1188), nucleophilic substitution occurs preferentially at C-6, rather than at C-2. Attack at C-2, if possible at all, demands harsher conditions. In 2,8-dichloro-l-azaazulenes only chlorine atoms at position 8 are susceptible to hydrolysis (61YZ1799 62YZ418 77BCJ1184), and in 2-ethoxy-l,3-diazaazulenes 17 and 18 only the substituents at C-6 are attacked (68CPB1308). The reason is that the electron densities at the seven-membered-ring positions 6 and 8 are lower than at C-2. 

Recently, cyclohepta[6]pyrrol-4-one derivatives were obtained as byproducts in the 1-azaazulene synthesis from 2-chlorotropone and 1-(diphenylphosphinyl)azaallyl anions [93H(36)2247],... 

Pyrrolotropone is formed by a thermal retro-Grignard reaction from a 4-aryl-4-hydroxycyclohepta[i]pyrrole derivative (72CB1224). Finally, cycloadditions of 8-styryl-l-azaazulenes with dimethyl acetylenedicarboxy-late and subsequent oxidation yield 1,7-ansa-bridged pyrrolotropones (80BCJ1406). 

Treatment of the Michael adducts 94 with a strong base such as potassium ferf-butoxide yielded the 2-azaazulenes 131 in 48-70% yield, respectively (Scheme 43) [60]. 

Scheme 43. Synthesis of the 2-azaazulenes 131 from the Michael adducts 94 of DMPA-H to 1-Me [60]...

The Beckmann and Schmidt reactions are applicable to the synthesis of azepines fused to other heterocyclic and carbocyclic systems, such as pyridoazepinones (70JA203, 76AP550, 78JHC249), indoloazepinones (78CB1780) and the 14v aromatic azabenzazulen-nones (204) (80BCJ3232). The general chemistry of azaazulenes has been reviewed <81H(15)547>. 

Wittig reactions with pyrrole-2-aldehyde led to the esters (79) which were cyclisized to 3a-azaazulen-4-ones (80).104,105 4-Methylene-3a-aza-azulenes (81) have been obtained from 80 with stabilized phos-phoranes.36 Reaction of dimethyl acetylenedicarboxylate with 81 could not be achieved. A similar cycloaddition was successful in the synthesis of cycl[3,3,3]azines (2) (Section V). 

The azaazulene 81a reacted with oxalyl chloride in benzene to give 82, which is insoluble in all conventional solvents.106... 

Benzocondensed azulenes can be treated in the same way. Correspondingly, numerous pseudoazulenes car, be constructed from azaazulenes or... 

Cycloaddition reactions have been carried out with corresponding bases of Vilsmeier salts of pyrroles. Thus reaction of 153 with dimethyl acetylenedicar-boxylate gave pyrrolizines 154 and 3a-azaazulenes 155 in a nonsynchronous reaction. The structure of 3H-pyrrolizine 154a was proved by X-ray analysis.96 In a closely related reaction, pyrrolizinones 156 and 157 were obtained from the corresponding Vilsmeier salts with phenyl isocyanate.97 Reaction of 2-p-nitrobenzoylindolin-3-one (158) with methyl acetoacetate gave the benzopyrrolizinone (159).5 7... 

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