Pyrrole: resonance structures

The heteroaromatic compounds can be divided into two broad groups, called n-excessive and n-deficient, depending on whether the heteroatom acts as an electron donor or an electron acceptor. Furan, pyrrole, thiophene, and other heterocyclics incorporating an oxygen, nitrogen, or sulfur atom that contributes two n electrons are in the rr-exeessive group. This classification is suggested by resonance structures and confirmed by various MO methods. ... 

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 regioselectivity observed in these reactions can be correlated with the resonance structure shown in Fig. 2. The reaction with electron-rich or electron-poor alkynes leads to intermediates which are the expected on the basis of polarity matching. In Fig. 2 is represented the reaction with an ynone leading to a metalacycle intermediate (formal [4C+2S] cycloadduct) which produces the final products after a reductive elimination and subsequent isomerisation. Also, these reactions can proceed under photochemical conditions. Thus, Campos, Rodriguez et al. reported the cycloaddition reactions of iminocarbene complexes and alkynes [57,58], alkenes [57] and heteroatom-containing double bonds to give 2Ff-pyrrole, 1-pyrroline and triazoline derivatives, respectively [59]. 

Alkylation of carbazole has not been studied systematically but isolated preparative examples suggest that N- alkylation can be carried out as for pyrroles and indoles. No complication from competing carbon alkylation occurs. N- Alkylation of the anion of isoindole does not seem to have been investigated. One might expect serious competition from C- alkylation because of the favorable benzenoid conjugation of the resonance structure with charge on carbon (equation 161). 

A perusal of the / values in Table 1 also shows that/aa is higher in the [3,2-6]-annelated systems than in the corresponding [2,3-6] isomers. For example / , in thieno[3,2-6]thiophene (3) and N-benzylthieno[3,2-6]pyrrole (18) is 1.55 and 1.3 Hz compared to 1.17 and 1.0 Hz for the corresponding [2,3-6 ]- annelated systems (7) and (19). This relatively high value for / - in the [3,2-6]-fused systems is a consequence of the transmission of the spin interactions between the protons through the v- framework rather than the cr-framework. In qualitative terms this means that several resonance structures can be written for such systems wherein electronic perturbations at C-2 are effectively transmitted to C-5 rather than C-6 (Scheme 4). 

Indole is planar with 10 7t-electrons in a completely conjugated system. The ring is classified as a 7t-excessive he tero aromatic compound because of the electron-donating character of the pyrrole-type nitrogen atom. The 71-system is relatively electron-rich, particularly at C-3, as represented by resonance structure (lb). 

There is an interesting difference in the cydization of the closely related pyrrole derivatives 7 and 9. A reaction of N-vinylpyrrole 7 with hydrogen chloride in ether yielded a dark blue immonium salt, represented by resonance structures 8a and 8b. The N-ethyl analog (9), on the other hand, gave the 1-amino-3//-pyrrolizine (10) under the same conditions. Hydrolysis of 8 did not give the expected pyrrolizinone(12) but stopped half way to give adduct 11.19... 

The cation derived from protonation of pyrrole at C-2 is a hybrid of the following resonance structures. 

Reaction of 4-nitro-2,l,3-benzoselenadiazole 143 with ethyl isocyanoacetate in the presence of 1,8-diazabicy-clo[5.4.0]undec-7-ene (DBU) in tetrahydrofuran (THF) at room temperature gave the pyrrole-fused product 146 in 56% yield as the sole product (Scheme 9) <1996J(P1)1403>. Reaction of 5-nitro-2,l,3-benzoselenadiazole 147 with ethyl isocyanoacetate under similar reaction conditions gave the pyridimine iV-oxide-fused product 150 in 28% yield as the sole product. Proposed mechanism for the formation of pyrrole and pyrimidine rings involves initial attack of the ethyl isocyanoacetate anion at the /3-position to the nitro groups forming the anionic intermediates 145 and 148 and the resonance structure intermediate 149. The reactivity and chemoselectivity were explained by the steric effect in the intermediates. 

Draw one or more resonance structures of pyrrole (Section 7.4.3) that show why the carbon chemical shifts have the relative order they exhibit. 

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. 

Attempts to correlate reaction mechanisms, electron density calculations and experimental results have met with only limited success. As mentioned in the previous chapter (Section 4.06.2), the predicted orders of electrophilic substitution for imidazole (C-5 > -2 > -4) and benzimidazole (C-7>-6>-5>-4 -2) do not take into account the tautomeric equivalence of the 4- and 5-positions of imidazole and the 4- and 7-, 5- and 6-positions of benzimidazole. When this is taken into account the predictions are in accord with the observed orientations of attack in imidazole. Much the same predictions can be made by considering the imidazole molecule to be a combination of pyrrole and pyridine (74) — the most likely site for electrophilic attack is C-5. Furthermore, while sets of resonance structures for the imidazole and benzimidazole neutral molecules (Schemes 1 and 2, Section 4.06.2) suggest that all ring carbons have some susceptibility to electrophilic attack, consideration of the stabilities of the expected tr-intermediates (Scheme 29) supports the commonly observed preference for 5- (or 4-) substitution. In benzimidazole attack usually occurs first at C-5 and a second substituent enters at C-6 unless other substituent effects intervene. 

The contribution of the resonance forms XXI, XXII, XXIII, and XXIV to the structure of the anions is frequently overlooked, yet many base-catalyzed condensation reactions of phenol and pyrrole undoubtedly proceed through these resonance structures at the moment reaction occurs. The condensation of phenol with aqueous formaldehyde, the Kolbc synthesis (p. 197), and the Reimer-Tiemann reaction (p. 202) are striking examples of reactions which occur through the seemingly less important carbanion structure of the resonance hybrid. (See p. 133.)... 

Woodward has pointed out that porphyrins contain pyrrole units that are on average one half of an electron away from a stable 6-ji-electron configuration [Woodward (214)]. From the presumed tendency to become aromatic pyrrole units one can deduce a metalloporphyrin resonance structure like (VIII), where the methine bridge carbons have lost n-... 

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

Use SpartanView to compare electrostatic potential maps and N-CHO bond distances in iV-formyl pyrrole and AMormylpyrrolidine. Account for any differences using resonance structures.. ... 

The role of heteroatoms in ground- and excited-state electronic distribution in saturated and aromatic heterocyclic compounds is easily demonstrated by a comparison of a number of heteroaromatic systems with their perhydro counterparts. In Jt-excessive heteroaromatic systems, because of their resonance structures, their dipole moments are less in the direction of the heteroatom than in the corresponding saturated heterocycles furan (1, 0.71 D) vs. tetrahydrofliran (2, 1.68 D), thiophene (3, 0.52 D) vs. tetrahydrothiophene (4, 1.87 D), and selenophene (5, 0.40 D) vs. tetrahydroselenophene (6, 1.97 D). In the case of pyrrole (7, 1.80 D), the dipole moment is reversed and is actually higher than that of pyrrolidine (8, 1.57 D) due to the acidic nature of the pyrrole ring (the N-H bond) In contrast, the dipole moment of n-deficient pyridine (9, 2.22 D) is higher than that of piperidine (10, 1.17 D). In all these compounds, with the exception of pyrrole (7), the direction of the dipole moment is from the ring towards the heteroatom [32-34]. 

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