Pyrroles delocalized lone electron pairs

Figure 25 1 Orbital pictures of (A) pyrrole and (B) furan p< = O), and thiophene = S). The het-erocitom in each is sp hybridized and bears one delocalized lone electron pair.

Pyrrole, furan, and thiophene contain delocalized lone electron pairs... 

In principle, the ring system where PH replaces NH should be capable of having aromatic character because the lone electron pair on P could be delocalized into the ring as in pyrrole. The parent phosphole molecule, however, is unstable and has only been detected spectroscopically at —90°C, but the P-methyl derivative is stable and distillable, and many other substituted phospholes are known. The field is growing regularly and deserves brief consideration here. The literature has been reviewed extensively. ... 

Pyrrole, furan, and thiophene are l-hetero-2,4-cydopentadienes. Each contains a butadiene unit bridged by an -hybridized heteroatom bearing lone electron pairs. These systems contain delocalized tt electrons in an aromatic six-electron framework. This section considers the structures and methods of preparation of these compounds. 

In Summary The donation of the lone electron pair on the heteroatom to the diene unit in pyrrole, furan, and thiophene makes the carbon atoms in these systems electron rich and therefore more susceptible to electrophilic aromatic substitution than the carbons in benzene. Electrophilic attack is frequently favored at C2, but substitution at C3 is also observed, depending on conditions, substrates, and electrophiles. Some rings can be opened by hydrolysis or by desulfurization (for thiophenes). The diene unit in furan is reactive enough to undergo Diels-Alder cycloadditions. Indole is a benzopyrrole containing a delocalized tt system. 

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... 

Delocalization of the chromophore is as indicated in XVII. Porphyrins and porphinoids closely resemble the 18-membered 18 7t-electron [18]an-nulenes, avoiding energetically unfavorable conjugation via pyrrolic nitrogen lone pairs. If interactions of this type are necessary for structural reasons [as in... 

Pyrrole is cyclic and planar, with a total of four ji electrons from the two 7i bonds. Is the non-bonded electron pair localized on N or part of a delocalized ji electron system The lone pair on N is adjacent to a double bond. Recall the following general rule from Section 16.5 ... 

Electrostatic potential maps, shown in Figure 17.7 for pyridine and pyrrole, confirm that the nonbonded electron pair in pyridine is localized on N, whereas the lone pair in pyrrole is part of the delocalized n system. Thus, a fundamental difference exists between the N atoms in pyridine and pyrrole. 

When the coupling is across a tricoordinate carbon atom involved in a delocalized 7t-electron system, the absolute value of 2/(15N-C-H) may increase to as much as 16 Hz. (le) In the review period, the signs of such coupling constants have been determined as negative for formamide, (144) pyrrole, (146, 154, 155) pyrazole, thiazole, pyridine and pyridazine (155) as well as for some azinium ions. (151) The latter afford another example of the decrease in the absolute value of the coupling constant upon protonation of the nitrogen lone pair (Table XXXIII) [79], [80]. 

Pyrrole, furan, and thiophene are five-membered-ring heterocycles. Each has three pairs of delocalized rr electrons Two of the pairs are shown as rr bonds, and one pair is shown as a lone pair on the heteroatom. Furan and thiophene have a second lone pair that is not part of the rr cloud. These electrons are in an sp orbital perpendicular to the p orbitals. Pyrrole, furan, and thiophene are aromatic because they are cyclic and planar, every carbon in the ring has a p orbital, and the tt cloud contains three pairs of tt electrons (Sections 15.1 and 15.3). 

While pyrrole has 6 Ti-electrons, indole, with 10 7t-electrons, is also an electron-rich aromatie heteroeycles. Indole is not as reactive as pyrrole because its electrons are more delocalized. An indole lone pair of electrons takes part in the delocalization essential to indole s aromaticity. Therefore, the indole nitrogen atom (p/fa = -3.5/ is not basic Indole loses its aromaticity when protonated. The protonation takes place predominantly at the C3 position to form the 3//-indolium ion, which combines with another molecule of indole to give oligomers. 

For example, pyridine and pyrrole are both aromatic, but the nonbonded electron pair on the N atom in these compounds is located in different orbitals. Recall from Section 17.8C that the lone pair of electrons in pyridine occupies an sp hybridized orbital, perpendicular to the plane of the molecule, so it is not part of the aromatic system, whereas that of pyrrole resides in a p orbital, making it part of the aromatic system. The lone pair on pyrrole, therefore, is delocalized on all of the atoi -m j ri jj e much weaker base than pyridine. 

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. 

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. 

This contrasts to pyrrole in which the lone pair on the only nitrogen atom is needed to complete the six aromatic 7t electrons and is therefore delocalized around the ring. Protonation, if it occurs at all, occurs on carbon rather than on nitrogen since the cation is then delocalized. But the cation is no longer aromatic (there is a saturated CH2 group interrupting the conjugation) and so pyrrole is not at all basic (p-fCaH about -A). 

If you examine this structure you will see that there is definitely a pyrrole ring but that the pyridine ring is not all there. Of course, the lone pair and the tt electrons are all delocalized but this system, unlike indole and quinoline, is much better regarded as a ten-electron outer ring than as two six-electron rings joined together,... 

Heterocycles containing oxygen, nitrogen, or sulfur—atoms that also have at least one lone pair of electrons—can also be aromatic. With heteroatoms, we must always determine whether the lone pair is localized on the heteroatom or part of the delocalized ji system. Two examples, pyridine and pyrrole, illustrate these different possibilities. 

Methoxatin has an aromatic pyridine ring (don t count the lone pair as it is in an sp orbital) and in aromatic pyrrole ring (do count the lone pair as it is in a p orbital) but the middle ring cannot be even six electrons even if you try delocalization (example given). 

In N-substituted derivatives, the nature of the electronic environment of the pyrrole-type nitrogen is different from that of its pyridine-type counterpart. Each of these pyrrole-type nitrogens is bound covalently to three neighboring atoms, and supplies two electrons to the delocalized 7r-bond system. In this case, the component, which is oriented radially with the bond N-R (R = H, alkyl, metal), is dominated by a o n-c- transition. The 22 component, which is dominated by a aN R--7r transition, is approximately tangential to the aromatic ring. In other words, the tangential component ( t) is dominated by an h-tt (lone-pair) transition for a pyridine-type nitrogen. 

A number of transition metal complexes containing weakly basic (5-member ring) HDN-related ligands are known. The authenticated bonding modes of pyrrole (Pyr) and pyrrolyl ions (Pyl) -or their alkylated analogues- in mononuclear metal complexes are summarized in Fig. 6.1. Pyrrole is a 5-member aromatic heterocycle in which the lone pair is delocalized over the n system of the ring, and it is therefore an electron rich molecule that reacts readily with electrophiles but is not susceptible to nucleophilic attack. 

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