Pyrroles mesomeric structures

Copper Phthalocyanine Blue is the copper(II) complex of tetraazatetrabenzo-porphine. As shown below, the mesomeric structures indicate that all of the pyrrole rings simultaneously contribute to the aromatic system ... 

A careful analysis based on these experimental results excluded a chain-propagation process [33a]. On account of the 3-position of the methylthio or methoxy substituent in the thiophene or pyrrole rings, three isomeric dimers may be formed. The main reaction path can be deduced from the mesomeric forms of the radical cations (2)". The two most important mesomeric structures are those with the unpaired electron in... 

When the imidazole ring is considered to be something resembling a pyrrole-pyridine combination (1) it would appear that any electrophilic attack should take place preferably at C-5 (pyrrole-or, pyridine-j8). Such a model, though, fails to take account of the tautomeric equivalence of C-4 and C-5 (Section 4.06.5.1). The overall reactivities of imidazole and benzimidazole can be inferred from sets of resonance structures in which the dipolar contributors have finite importance (Section 4.06.2) or by mesomeric structures such as (2). These predict electrophilic attack in imidazole at N-3 or any ring carbon atom, nucleophilic attack at C-2 or C-1, and also the amphoteric nature of the molecule. In benzimidazole the acidic and basic properties, the preference for nucleophilic attack at C-2 and the tendency for electrophiles to react at the fused benzene ring can be readily rationalized. 

Structure of pyrrole resonance contribntors (mesomeric structures)... 

The amino form is usually much more favored in the equilibrium between amino and imino forms than is the hydroxy form in the corresponding keto-enol equilibrium. Grab and XJtzinger suggest that in the case of a-amino- and a-hydroxy-pyrroles, structure 89 increases the mesomeric stabilization and thus offsets the loss of pyrrole resonance energy, but the increase due to structure 90 is not sufficient to offset this loss. Similar reasoning may apply to furans and... 

The apparent fickleness of the acyl-pyrroles and -indoles in their reaction with carbanions to form new C—C bonds arises from the contribution made by the zwitterionic structure, e.g. (410b), to the resonance hybrid and the choice of the reaction conditions is critical for a successful nucleophilic reaction. Thus, formyl-pyrroles and -indoles do not normally undergo the Cannizzaro reaction nor do they form stable cyanohydrins or undergo benzoin-type reactions. However, surprisingly, 2-formylpyrrole reacts with arylaldehydes in the presence of potassium cyanide to yield (428), which is easily oxidized to (429) (B-77MI30505). It is noteworthy that the presence of an ester substituent adjacent to the formyl group modifies the mesomeric interaction to such an extent to allow the formation of (430) in low yield, as a result of an initial benzoin-type self-condensation (Scheme 76) (68BSF637). 

The basis and extent of their aromaticity is discussed in Chapter 1. In summary, the capacity for the lone pair on a particular heteroatom to be delocalised is inversely related to the electronegativity of the heteroatom. For instance, furan is the least aromatic of the trio because oxygen has the greatest electronegativity and hence mesomeric representations 2.4b-e make relatively less of a contribution to the electronic structure of furan than they do in the cases of pyrrole and thiophene. The order of aromaticity is furan < pyrrole < thiophene. We shall see later how this variation in aromaticity affects the reactivities of these three related heterocycles. 

Indole is a ten-7t electron aromatic system. As with pyrrole, delocalisation of the lone pair of electrons from the nitrogen atom is necessary for aromaticity. The single overall electronic structure of indole is not completely described by structure 7.1, because this implies localisation of the lone pair on the nitrogen atom. Mesomeric representation 7.1a makes a contribution to the electronic structure of indole, as to a lesser extent do mesomeric representations where the negative charge occurs on the benzenoid ring. 

The EPR spectra of the radical cations derived from pyrrole solutions can all be simulated by electronic structures in which the unpaired electron is located in an orbital with the nodal plane on the nitrogen and (2) showing large coupling constant values at the 2- and 5-positions and small values at the 3- and 4-positions <2000J(P2)905>. The EPR parameters of the radical cations from 2,5-dimethyl-l-phenylpyrroles and 3,4-bis(alkylthio)-2,5-dimethyl-l-phenylpyrroles (Table 30) denote a marked stabilization of the radical cations by the sulfanyl groups through mesomeric effects. 

Figure 3.11. Mesomeric Kekuli structures of various systems (a) pyridine, (b) pyrazoie, (c) pyrrole and their graphs S, 0 mi M.

The aromatic character of indolizine is expressed by three main mesomeric contributors, two of which incorporate a pyridinium moiety other structures (not shown) incorporating neither a complete pyrrole nor a pyridinium are less important. 

Rg. 1 Pyrrole, thiophene, aniline and ftiran chemical structures (A), c/s-polyacetylene chemical structure and the two limiting mesomeric forms, aromatic and quinoid, of the heteroaromatic conjugated conducting polymers (B). 

In most SgAr processes, indole reacts more slowly than pyrrole, but faster than benzo(b]furan. In contrast to pyrrole, substitution occurs in the 3-position.This might be due to different a-complex stabilization Attack of the electrophile (E ) on the 3-position leads to a low-energy iminium structure 3 conserving the benzene ii-sextet, attack on the 2-position, however, leads to a a-complex 4, for which mesomeric delocalization is possible only by participation of an orthoquinonoid structure ... 

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