Quasi-aromatic

The reactivity of the various azolides as well as the order of reactivities within this group can be explained on the basis of the quasi-aromatic character of the azole rc-system the lone electron pairs on the acyl-substituted nitrogens N(l) are part of the cyclic tc-system of the azole units, leading to a partial positive charge on N(l) that interferes with the normal carboxamide resonance and exerts an electron-withdrawing effect on the... 

Quasi-aromatic structures are also known in which the stabilised cyclic species is an ion, e.g. the cycloheptatrienyl (tropylium) cation (15, cf. p. 106), the cyclopentadienyl anion (16, cf. p. 275), both of which have 67te (n = 1), and even more surprisingly the cyclopropenyl cation (17, cf. p. 106) which has 2ne (n = 0) ... 

The operation of (d) is seen in cyclopentadiene (14) which is found to have a pKa value of 16 compared with 37 for a simple alkene. This is due to the resultant carbanion, the cyclopentadienyl anion (15), being a 6n electron delocalised system, i.e. a 4n + 2 Hiickel system where n = 1 (cf. p. 18). The 6 electrons can be accommodated in three stabilised n molecular orbitals, like benzene, and the anion thus shows quasi-aromatic stabilisation it is stabilised by aromatisation ... 

This too is a Hiickel 4n + 2 p electron system (n = 2, this time) and shows quasi-aromatic stability stabilisation by aromatisation has again taken place, remarkably this time in a doubly charged carbanion (18). 

The quasi-aromatic azulenes dissolve in 50-60% sulfuric acid, a property used in their isolation. The sulfuric acid solutions are yellow to orange rather than blue like the parent hydrocarbon, and they are... 

S)-Aziridine-2-carboxylic acid (1), also named azyline (Azy)J4L42] exhibits an electron-withdrawing or quasi-aromatic effect on the nitrogen and is therefore susceptible to an attack by nucleophiles. [41 43] Unprotected aziridine-2-carboxylic acid (1) is very labile, but the... 

The iron(II) diimine complex (30) displays unusual stability towards acids, alkali, reducing and oxidizing agents and has been termed quasi-aromatic .44 Complex (30) undergoes bromination in acetic acid to give the dibromo analogue (31) (equation 20).45... 

Little data has been reported on x-ray structural methods. In a study of benzo[l,2-c 3,4-c ]bis-thiadiazole and a monoselenium analogue, S—N bond distances of the order of 1.62 A were found and attributed to the heterocyclic rings being quasi aromatic, but with severe distortion of the aromatic character of the carbocyclic ring <84ZN(B)485>. 

The fact that coordination of the diolefin occurs by displacing the arene also favors the hypothesis that the diolefin coordinates in the cis conformation by the two double bonds. Displacement reactions of this type are well known in organometallic chemistry (See e.g. (2)) and, in the cases already clarified, it has been shown that diolefin coordinates in the cis conformation by the two double bonds. This is not surprising if we consider that the diolefin, coordinated in this way to a transition metal, has a quasi-aromatic character owing to the complete delocalization of the 7r-electrons. 

In the case of the acetylacetonates, a drift of electrons from the ligand moiety to the central metal ion is presumably facilitated by the presence of a "quasi-aromatic" system of conjugated unsaturation within the ligand. 

The synthesis of azidocycloimonium fluoborates, compounds which possess a quasi-aromatic heterocyclic nucleus, has been reported by BalU and Kersting . Halogen atoms adjacent to quaternary nitrogens in heteroaromatic salts such as 154 157 undergo replacement by azide ion at low temperatures to produce the resonance stabilized azidinium salts (158). On the basis of infrared studies and reactivity towards nucleophiles, Balli has suggested that these salts are best considered as iV-diazonium compounds... 

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