Pyrrole is a colorless volatile liquid that darkens readily upon exposure to air, and is usually purified by distillation immediately before use.<ref>Template:Cite book</ref> Pyrrole has a nutty odor. Pyrrole is a 5-membered aromatic heterocycle, like furan and thiophene. Unlike furan and thiophene, it has a dipole in which the positive end lies on the side of the heteroatom, with a dipole moment of 1.58 D. In CDCl3, it has chemical shifts at 6.68 (H2, H5) and 6.22 (H3, H4). Pyrrole is an extremely weak base for an amine, with a conjugate acid pKa of −3.8. The most thermodynamically stable pyrrolium cation (C4H6N+) is formed by protonation at the 2 position. Substitution of pyrrole with alkyl substituents provides a more basic molecule—for example, tetramethylpyrrole has a conjugate acid pKa of +3.7. Pyrrole is also weakly acidic at the N–H position, with a pKa of 16.5.
As a hydrogen bonding Lewis acid it is classified as a hard acid and the ECW model lists its acid parameters as EA = 1.38 and CA = 0.68.
Pyrrole has aromatic character because the lone pairs of electrons on the nitrogen atom is partially delocalized into the ring, creating a 4n + 2 aromatic system (see Hückel's rule). In terms of its aromaticity, pyrrole's is modest relative to benzene but comparable to related heterocycles thiophene and furan. The resonance energies of benzene, pyrrole, thiophene, and furan are, respectively, 152, 88, 121, and 67 kJ/mol (36, 21, 29, and 16 kcal/mol).<ref>Template:March6th</ref> The molecule is flat.
Pyrrole was first detected by F. F. Runge in 1834, as a constituent of coal tar.<ref>Template:Cite journalTemplate:Open access See especially pages 67–68, where Runge names the compound Pyrrol (fire oil) or Rothöl (red oil).</ref> In 1857, it was isolated from the pyrolysate of bone. Its name comes from the Greek pyrrhos (Template:Lang, "reddish, fiery"), from the reaction used to detect it—the red color that it imparts to wood when moistened with hydrochloric acid.<ref name="Ullmann">Template:Ullmann</ref>
Pyrrole itself is not naturally occurring, but many of its derivatives are found in a variety of cofactors and natural products. Common naturally produced molecules containing pyrroles include vitamin B12, bile pigments like bilirubin and biliverdin, and the porphyrins of heme, chlorophyll, chlorins, bacteriochlorins, and porphyrinogens.<ref name="Jonas Jusélius and Dage Sundholm 2000 2145–2151"/> Other pyrrole-containing secondary metabolites include PQQ, makaluvamine M, ryanodine, rhazinilam, lamellarin, prodigiosin, myrmicarin, and sceptrin. The syntheses of pyrrole-containing haemin, synthesized by Hans Fischer was recognized by the Nobel Prize.
Pyrrole is a constituent of tobacco smoke and may contribute to its toxic effects.<ref>Template:Cite web</ref>
Pyrrole can also be formed by catalytic dehydrogenation of pyrrolidine.Template:Cn
Several syntheses of the pyrrole ring have been described.<ref name="Lubell">Template:Cite journal</ref> Three routes dominate,<ref name=Gil>Template:Cite book</ref> but many other methods exist.
Pyrroles bearing multiple substituents have been obtained from the reaction of münchnones and alkynes. The reaction mechanism involves 1,3-dipolar cycloaddition followed by loss of carbon dioxide by a retro-Diels–Alder process. Similar reactions can be performed using azalactones.
Pyrroles can also be prepared by silver-catalyzed cyclization of alkynes with isonitriles, where R2 is an electron-withdrawing group, and R1 is an alkane, aryl group, or ester. Examples of disubstituted alkynes have also been seen to form the desired pyrrole in considerable yield. The reaction is proposed to proceed via a silver acetylide intermediate. This method is analogous to the azide–alkyneclick chemistry used to form azoles.
Proline can be used as precursor of aromatic pyrroles in secondary natural products, as in prodigiosins.
File:Prodigiosin 1.pngFigure 1: Structure of Prodigiosin 1 highlighting the A, B, and C pyrrole rings
The biosynthesis of Prodigiosin<ref>Template:Cite journal</ref><ref name=Hu>Template:Cite journal</ref> involves the convergent coupling of three pyrrole type rings (labeled A, B, and C in figure 1) from L-proline, L-serine, L-methionine, pyruvate, and 2-octenal.
Ring A is synthesized from L-proline through the nonribosomal peptide synthase (NRPS) pathway (figure 2), wherein the pyrrolidine ring of proline is oxidized twice through FAD+ to yield pyrrole ring A.
Ring A is then expanded via the polyketide synthase pathway to incorporate L-serine into ring B (figure 3). Ring A fragment is transferred from the peptidyl carrier protein (PCP) to the Acyl Carrier Protein (ACP) by a KS domain, followed by transfer to malonyl-ACP via decarboxylative Claisen condensation. This fragment is then able to react with the masked carbanion formed from the PLP mediated decarboxylation of L-serine, which cyclizes in a dehydration reaction to yield the second pyrrole ring. This intermediate is then modified by methylation (which incorporates a methyl group from L-methionine onto the alcohol at the 6 position) and oxidation of the primary alcohol to the aldehyde to yield the core A–B ring structures.
Due to its aromatic character, pyrrole is difficult to hydrogenate, does not easily react as a diene in Diels–Alder reactions, and does not undergo usual olefin reactions. Its reactivity is similar to that of benzene and aniline, in that it is easy to alkylate and acylate. Under acidic conditions, pyrroles oxidize easily to polypyrrole,<ref>Template:Cite book</ref> Template:Citation neededand thus many electrophilic reagents that are used in benzene chemistry are not applicable to pyrroles. In contrast, substituted pyrroles (including protected pyrroles) have been used in a broad range of transformations.<ref name="Lubell"/>
Pyrroles react easily with nitrating (e.g. HNO3/Ac2O), sulfonating (Py·SO3), and halogenating (e.g. NCS, NBS, Br2, SO2Cl2, and KI/H2O2) agents.<ref>Template:Cite encyclopedia</ref> Halogenation generally provides polyhalogenated pyrroles, but monohalogenation can be performed. As is typical for electrophilic additions to pyrroles, halogenation generally occurs at the 2-position, but can also occur at the 3-position by silation of the nitrogen. This is a useful method for further functionalization of the generally less reactive 3-position.Template:Citation needed
N-Metalated pyrrole can react with electrophiles at the N or C positions, depending on the coordinating metal. More ionic nitrogen–metal bonds (such as with lithium, sodium, and potassium) and more solvating solvents lead to N-alkylation. Nitrophilic metals, such as MgX, lead to alkylation at C (mainly C2), due to a higher degree of coordination to the nitrogen atom. In the cases of N-substituted pyrroles, metalation of the carbons is more facile. Alkyl groups can be introduced as electrophiles, or by cross-coupling reactions.Template:Citation needed
Substitution at C3 can be achieved through the use of N-substituted 3-bromopyrrole, which can be synthesized by bromination of N-silylpyrrole with NBS.Template:Citation needed
Pyrroles with N-substitution can undergo cycloaddition reactions such as [4+2]-, [2+2]-, and [2+1]-cyclizations. Diels-Alder cyclizations can occur with the pyrrole acting as a diene, especially in the presence of an electron-withdrawing group on the nitrogen. Vinylpyrroles can also act as dienes.Template:Citation needed
Polypyrrole is of some commercial value. N-Methylpyrrole is a precursor to N-methylpyrrolecarboxylic acid, a building-block in pharmaceutical chemistry.<ref name="Ullmann" /> Pyrroles are also found in several drugs, including atorvastatin, ketorolac, and sunitinib. Pyrroles are used as lightfast red, scarlet, and carmine pigments.<ref>Template:Cite web</ref><ref>Template:Cite journal</ref>
