Pyrrole delocalization

Pyrrole has a planar, pentagonal (C2 ) stmcture and is aromatic in that it has a sextet of electrons. It is isoelectronic with the cyclopentadienyl anion. The TT-electrons are delocalized throughout the ring system, thus pyrrole is best characterized as a resonance hybrid, with contributing stmctures (1 5). These stmctures explain its lack of basicity (which is less than that of pyridine), its unexpectedly high acidity, and its pronounced aromatic character. The resonance energy which has been estimated at about 100 kj/mol (23.9 kcal/mol) is intermediate between that of furan and thiophene, or about two-thirds that of benzene (5). 

Pyrrole, furan, and thiophene, on the other hand, have electron-rich aromatic rings and are extremely reactive toward electrophilic aromatic substitution— rnore like phenol and aniline than benzene. Like benzene they have six tt electrons, but these tt electrons are delocalized over five atoms, not six, and ar e not held as strongly as those of benzene. Even when the ring atom is as electronegative as oxygen, substitution takes place readily. 

Examine pyrrole s highest-occupied molecular orbital (HOMO) to see if your can predict the most favorable protonation site. Which of the pyrrole s conjugate acids (N protonated, C2 proto noted, C3 proto noted pyrrole) is lowest in energy Examine electrostatic potential maps to see if the lowest-energy form is also that in which the positive charged is best delocalized. Rationalize your result using resonance arguments. What should be the favored substitution product ... 

Inspection of the three cations shows that (13) and (14) would be expected to be quito active as electrophilic reagents by reason of delocalization of the positive charge by mesomerism leading to the transfer of electrophilic character to the carbon atom. Cation (12), on the other hand, would show electrophilic reactivity at carbon only by induction. Since neutral pyrrole is so susceptible to electrophilic attack, it is extremely likely that it would react with one or other of the three cations. 

Judging by the predominance of a-substitution in electrophilic attack of pyrrole, one would say that of the cations (13) and (14), the former would greatly predominate. In support of this there is the argument that the charge in (13) is more delocalized (3 canonical... 

The porphyrin ring system (the parent compound 1 is also known as porphin) consists of four pyrrole-type subunits joined by four methine ( = CH-) bridges to give a macrotetracycle. The macrocycle contains 227i-electrons from which 1871-electrons form a delocalized aromatic system according to Huckel s 4n + 2 rule for aromaticity. The aromaticity of the porphyrin determines the characteristic physical and chemical properties of this class of compounds. The aromatic character of porphyrins has been confirmed by determination of their heats of combustion.1"3 X-ray investigations4 of numerous porphyrins have shown the planarity of the nucleus which is a prerequisite for the aromatic character. 

Analysis suggested that cyclic delocalization could, however, occur in 17 and 18 to a lesser extent than in pyrrole and benzene, respectively [97], This suggests low aromaticity of 17 and 18. Donation from sp lone pairs (Scheme 9) weakens the N-N sp -sp ) single bonds in the cyclic conjugated hydronitrogens and polynitrogens. 

Carbon and nitrogen are the most common elements from the first row of the periodic table to form aromatic compounds, characterized by cyclic electron delocalization. The bonding of these elements in the conjugated systems shows a large variety. Carbon can be a divalent (carbene), sp carbon with one jT-electron, but also sp carbon can be part of hyperconjugate aromatic systems, provided that it is properly substituted. The pyrrole- and pyridine-type nitrogens also allow the formation of cyclic electron delocalization in a large variety of aromatic systems. 

A typical example of steric control over spin delocalization has been described for the cation-radical of 3,4-bis(thioisopropyl)-2,5-dimethyl-l-phenylpyrrole (Domingo et al. 2001). Scheme 3.15 depicts this sitnation. In this cation-radical, one thioisopropyl group is almost coplanar with the pyrrole ring, whereas the other one occupies an orthogonal position. Accordingly, the ESR spectra established an eqnilibrinm between the symmetrical and asymmetrical conformations of the cation-radical. This equi-librinm is shifted toward the asymmetrical form at low temperatmes. The main feature of the equilibrium is the widening of spin delocalization, which includes not only the pyrrole ring but also one donor sulfur atom at the expense of the other sulfur atom. The steric control predetermines the discrimination of the other sulfur atom in the spin-delocalization process. 

Pyridine, like benzene, is an aromatic system with six jt electrons (see Section 11.3). The ring is planar, and the lone pair is held in an sp orbital. The increased s character of this orbital, compared with the sp orbital in piperidine, means that the lone pair electrons are held closer to the nitrogen and, consequently, are less available for protonation. This hybridization effect explains the lower basicity of pyridine compared with piperidine. Pyrrole is also aromatic, but there is a significant difference, in that both of the lone pair electrons are contributing to the six-jr-electron system. As part of the delocalized Jt electron system, the lone pairs are consequently not available for bonding to... 

This acid-base equilibrium involves the interconversion of two distinct delocalized systems, a pyrrole and a dihydrodiazepinium cation. Pyrrole... 

Formally, pyrrole can be described as azacyclopentadiene, i.e. cyclopen-tadiene in which a CH2 unit has been replaced by a NH group. However, this translates into a classical structure that does not adequately describe the compound, for pyrrole has aromatic character, even though there are only five atoms in the ring The aromaticity arises because the nitrogen atom contributes two electrons and the four carbons one electron each to form a delocalized sextet of 7t-electrons. In valence bond terms the structure of pyrrole can be represented as a resonance hybrid (Scheme 6.1)... 

Pyrrole generally reacts with electrophiles (E ) (including the proton) at C-2 in preference to C-3. This selectivity can be explained by more extensive delocalization of the positive charge in the o intermediate for C-2 substitution than is the case in the equivalent intermediate for attack at C-3 (Scheme 6.2). 

Attack at nitrogen is inhibited because no delocalization of charge is possible in the intermediate (Scheme 6.3). A-Alkylation and -acylation of pyrrole are best carried out indirectly using the anion of pyrrole (see Section 6.1.3). 

Pyrrole and its simple derivatives do not react easily as dienes. Pyrrole itself only combines with dimethyl acetylenedicarboxylate (DMAD, dimethyl but-2-ynedicarboxylate) under high pressure and then it is by C-2 substitution. However, A-acylpyrroles, such as A-acetyl- and N- tert-butoxycarbonyl)pyrrole, do undergo Diels-AIder addition reactions. Here, internal resonance within the acyl group reduces the availability of the lone-pair electrons, formally on nitrogen, to delocalize into the ring, thus making the carbon unit more diene-like (Scheme 6.12). 

Like pyrrole, its resonance description requires the delocalization of one of the lone pair electrons of the oxygen atom with the cyclopentadiene unit (Scheme 6.21). 

The definition of aromaticity conceived by Hiickel strictly applies to monocyclic ring systems, but indole, constructed from the fusion of benzene and pyrrole, behaves as an aromatic compound, like quinoline and isoquinoline. The ring fusion, however, affects the properties of both components. This is reflected in the valence bond description of indole, shown in Scheme 7.1, where one canonical representation shows electron density shared between N-1 and C-3 in the pyrrole unit (implying enamine character). Note that although other canonical forms can be drawn, where the lone-pair electrons are delocalized into the benzenoid ring, their energy content is relatively high and they are of limited importance. 

The term charge tranter refers to a succession of interactions between two molecules, ranging from very weak donor-acceptor dipolar interactions to interactions that result in the formation of an ion pair, depending on the extent of electron delocalization. Charge transfer (CT) complexes are formed between electron-rich donor molecules and electron-deficient acceptors. Typically, donor molecules are p-electron-rich heterocycles (e.g., furan, pyrrole, thiophene), aromatics with electron-donating substiments, or compounds... 

Note that dipoles are generated in pyrrole and furan because of delocalization of electrons from the heteroatoms. 

Pyrrole is a nitrogen-containing unsaturated five-membered heterocyclic aromatic compound. It shows aromaticity by delocalization of a lone pair of... 

Indole contains a benzene ring fused with a pyrrole ring at C-2/C-3, and can be described as benzopyrrole. Indole is a ten tt electron aromatic system achieved from the delocalization of the lone pair of electrons on the nitrogen atom. Benzofuran and benzothiaphene are very similar to benzopyrrole (indole), with different hetero-atoms, oxygen and sulphur respectively. 

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