Structure of Six-Membered Heteroaromatic Systems

Because inductive and mesomeric effects work in the same sense in pyridine, there results a permanent dipole towards the nitrogen atom. It also means that there are fractional positive charges on the carbons of the ring, located mainly on the a- and 7-positions. It is because of this general electron-deficiency at carbon that pyridine and similar heterocycles are referred to as electron-poor , or sometimes 7r-deficient . A comparison with the dipole moment of piperidine, which is due wholly to the induced polarisation of the cr-skeleton, gives an idea of the additional polarisation associated with distortion of the 7r-electron system. 

The structures of the diazines (six-membered sytems with two nitrogen atoms in the ring) are analogous, but now there are two nitrogen atoms and a corresponding two lone pairs as an illustration, the main canonical contributors (11-18) to pyrimidine are shown below. 

Pyridines with an oxygen at either the 2- or 4-position exist predominantly as carbonyl tautomers, which are therefore known as pyridones (see also section 1.5). 

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Further interesting examples of six-membered heteroaromatic systems including boron atoms are boranthracene derivatives, the basic structure of which consists of a 9,10-dihetero-substituted anthracene skeleton. The details of the structural analysis of such compounds are reported in a series of papers <1995ZNB1476, 1995SM1109,... 

An empirical rule defining the minimum requirements for activity in this type of compound is illustrated by structure VI which shows that the -C=N-N C- moiety must be in a six membered heteroaromatic system with the hydrazino group (or its hydrazone) attached to one of the carbons. 

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-... 

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