Structure of Five-Membered Heteroaromatic Systems

It is most important to recognise that the nitrogen lone pair in pyrrole forms part of the aromatic six-electron system. 

3- and 1,2-azoles, five-membered rings with two hetero atoms, present a fascinating combination of hetero atom types - in all cases, one hetero atom must be of the five-membered heterocycle (pyrrole, thiophene, furan) type and one of the imine type, as in pyridine imidazole with two nitrogen atoms illustrates this best. Contributor 39 is a particularly favourable one. 

---

The vibration spectra of five-membered heteroaromatic systems also give information about their electronic structure, because the positions and intensities of the bands depend on the electron distribution in the molecule. 

One such problem is the possibility of the existence and aromatic character of analogues of five-membered heteroaromatics with one heteroatom and those of azoles in which one or several ring C atoms are substituted by a metal atom, e.g. structures (289)-(305) (M = Ge, Sn, Pb X = O, S, NR). The other specific problems involve antiaromaticity of 871-electron carbenoids (306) and homoaromaticity of 67t-electron systems with a tetravalent metal (307). 

The assessment of aromatic character from structural criteria would appear to be a very valid approach. Aromatic -electron delocalization requires planarity of the aromatic molecule and leads to a typical carbon-carbon bond length intermediate between that of a single bond and a formal olefinic double bond. Indeed qualitative orders of the aromaticity of five-membered heteroaromatics have been derived from a consideration of the relative degree of diene character of the conjugated system as assessed from structural determinations of bond lengths (see, for example, Sections III,C, 1 and 5). 

The UV spectra of the simple five-membered heteroaromatic systems all show just one medium-to-strong low-wavelength band with no fine structure. Their absorptions have no obvious similarity to that of benzene, and no detectable n —> tt absorption, not even in the azoles, which contain a pyridine-like nitrogen (Tables 1.3 and 1.4). 

Red crystals formed from the reaction of 2-nitrothiophen with aliphatic secondary amines in the cold are tentatively assigned the structure R2NCH=CHCH=C(N02) 2Sj a very similar cleavage reaction with amines involving a stable five-membered heteroaromatic system, 2-bromo-5-nitrothiazole, was reported some time ago. "... 

Pseudoazulenes possess a heteroaromaticity produced essentially by the [4n + 2k electron system extending over the ring periphery. The trans-annular bond between the five- and six-membered ring enforces coplanarity thus, the pseudoazulenes can also be considered as bicyclic analogs of the heteronins.2 Dipolar structures (Eq. 7) contribute to the resonance stabili-... 

---