Pi-Excessive Aromatic Heterocycles

The most important ring systems have N, O, or S as the heteroatom, and the discussion here will be centered on each of these systems as a family. The ring with phosphoms as the heteroatom remained unknown until relatively recently (1959) in the long saga of heterocyclic chemistry, but it has been gaining in importance because of the special chemistry associated with phosphorus, this system will be considered separately. 

The hydrogen atom on nitrogen is known to be in the plane of the ring from X-ray diffraction studies, which means that the hybridization on nitrogen is sp rather than sp as is t5 ically found in amines. The latter hybridization would put nitrogen into a tetrahedral configuration 

Fundamentals of Heterocyclic Chemistry Importance in Nature and in the Synthesis of Pharmaceuticals, By Louis D. Quin and John A. lyrell Copyright 2010 John Wiley Sons, Inc. 

Resonance energies have been determined for the three heterocycles thiophene has the highest value (29.1 kcal/mol), which is similar to that for pyridine (27.9 kcal/mol), whereas pyrrole has the value 21.6 kcal/mol, and furan 16.2 kcal/mol. These values clearly are consistent with the view that these rings possess the extra stabilization associated with the presence of high electron delocalization. 

6-membered rings, and it accounts for the unique properties associated with each family. As an example, the 5-memhered rings are easily substituted by the attack of electrophiles (see section 7.1.2), whereas the 6-membered rings are highly resistant to these agents. 

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AROMATICITY AND OTHER SPECIAL PROPERTIES OF HETEROCYCLES PI-EXCESSIVE RING SYSTEMS AND MESOIONIC RING SYSTEMS... 

The indole family is one of the most important of all heterocyclic families, and the chemistry of this system is vast. Many natural products and synthetic medicinals contain this nucleus. We are restricted here to matters of aromaticity, and we And that we are dealing with a stable ring system that exhibits all the properties expected from being pi-excessive. Calculated electron densities are shown in Figure 7.5, where it is found that all carbons have densities greater than 1.00. 

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