Structures of pyrrole and

Figure 1.3.1 Structures of pyrrole and porphyrin showing their general naming/ numbering schemes...

The classical structures of pyrrole, furan and thiophene (31) suggest that these compounds might show chemical reactions similar to those of amines, ethers and thioethers (32) respectively. On this basis, the initial attack of the electrophile would be expected to take place at the heteroatom and lead to products such as quaternary ammonium and oxonium salts, sulfoxides and sulfones. Products of this type from the heteroaromatic compounds under consideration are relatively rare. 

Bullock E (1958) The structure of pyrrole salts and the basic strengths of some simple pyrroles. Can J Chem 36, 1686-1690. 

Formally, pyrrole can be described as azacyclopentadiene, i.e. cyclopen-tadiene in which a CH2 unit has been replaced by a NH group.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)... 

An alternative approach to the experimental estimation of REs utilizes equilibrium (protonation) data rather than thermochemical data, the idea being that comparisons of the basicities of pyrrole and its benzo fused analogues with those of non-aromatic systems which form cations of 7r-electron structure similar to the aromatic compounds should furnish a measure of the loss of RE accompanying protonation of the aromatic system (76T1767, 72CI(L)335). Thus, for the a-protonation of N-methylpyrrole, the model non-aromatic system was chosen as (20). Combination of pKa values for the protonation of the aromatic and non-aromatic molecules, taking into account the intrinsic resonance stabilization of the... 

This review, which covers the literature up to mid-1968, presents the knowledge, which is available to date, concerning the molecular and electronic structure of pyrrole. It also surveys the spectroscopic and nonspectroscopic physical properties of pyrrole and its simple derivatives. During the past 25 years, as a result of developments and improvements in instrument design, the literature on the physicochemical properties of organic compounds has expanded rapidly. Mainly as a result of the instability or nonavailability of many compounds, however, the study of pyrroles has lagged somewhat behind... 

Over the past 20 years and, in particular since 1955, many theoretical studies of the electronic structure of pyrrole using the molecular orbital approach with varying degrees of refinement have been reported. The 7r-electronic structure of pyrrole has been extensively discussed in terms of both the simple Hiickel molecular orbital (LCAO) theory37- 41-55-65 and the more sophisticated self-consistent field molecular orbital method (SCFMO method).18- 66-77 Extended... 

The electrochemical polymerization of thiophene is apparently rather similar to that of pyrrole and studies have been reported by Tourillon and Gamier133) and by Kaneto et al.134,135). Early studies are reviewed by Tourillon136). The oxidation potential of the monomer is significantly higher (1.6 V v SCE) than that of pyrrole and it might be expected that the more reactive cations would lead to greater structural irregularity in the polymer, which appears to be the case. 

Hennig, C., Hallmeier, K. H., Bach, A., Bender, S., Franke, R., Hormes, J., and Szargan, R. (1996). Influence of substituents on the N K X-ray absorption near-edge structure of pyrrole derivatives. Spectrochim. Acta 52,1079-1083. 

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. 

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. 

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