|UN number||1992, 1993|
|Jmol-3D images||Image 1
|Molar mass||67.09 g mol−1|
|Density||0.967 g cm–3|
|Melting point||−23 °C (−9 °F; 250 K)|
|Boiling point||129 to 131 °C (264 to 268 °F; 402 to 404 K)|
|Vapor pressure||7 mmHg at 23 °C|
|Viscosity||0.001225 Pa s|
heat capacity C
|1.903 J k–1 mol k–1|
|Std enthalpy of
|108.2 kJ mol–1 (gas)|
|Std enthalpy of
|2242 kJ mol–1|
|Flash point||33.33 °C (91.99 °F; 306.48 K)|
|Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)|
|(what is: / ?)|
Pyrrole is a heterocyclic aromatic organic compound, a five-membered ring with the formula C4H4NH. It is a colourless volatile liquid that darkens readily upon exposure to air. Substituted derivatives are also called pyrroles, e.g., N-methylpyrrole, C4H4NCH3. Porphobilinogen, a trisubstituted pyrrole, is the biosynthetic precursor to many natural products such as heme.
- 1 Properties
- 2 History
- 3 Occurrence in nature
- 4 Synthesis
- 5 Reactions and reactivity
- 6 Commercial uses
- 7 Analogs and derivatives
- 8 See also
- 9 References
- 10 External links
Pyrrole is a colourless volatile liquid that darkens readily upon exposure to air, and is usually purified by distillation immediately before use. 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 a chemical shifts at 6.68 (H2, H5) and 6.22 (H3, H4). Pyrrole is weakly basic, with a conjugate acid pKa of -3.8. The most thermodynamically stable pyrrolium cation 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 17.5.
Pyrrole was first detected by F. F. Runge in 1834, as a constituent of coal tar. In 1857, it was isolated from the pyrolysate of bone. Its name comes from the Greek pyrros (“fiery”), from the reaction used to detect it—the red color that it imparts to wood when moistened with hydrochloric acid.
Occurrence in nature
Pyrroles are found in a variety of biological contexts, as parts 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. Other pyrrole-containing secondary metabolites include PQQ, makaluvamine M, ryanodine, rhazinilam, lamellarin, prodigiosin, myrmicarin, and sceptrin. Pyrroles are also found in several drugs, including atorvastatin, ketorolac, and sunitinib.
Biosynthesis of pyrroles
The de novo biosynthesis of pyrrole rings begins with α-aminolevulinic acid (ALA), which is synthesized from glycine and succinyl CoA. ALA dehydratase catalyzes the condensation of two ALA molecules via a Knorr-type ring synthesis to form porphobilinogen (PBG). This later reacts to form, for example, the macrocycles heme and chlorophyll.
Pyrrole can also be formed by catalytic dehydrogenation of pyrrolidine.
Several syntheses of the pyrrole ring have been described.
Hantzsch pyrrole synthesis
Knorr pyrrole synthesis
The Knorr pyrrole synthesis involves the reaction of an α-amino ketone or an α-amino-β-ketoester with an activated methylene compound. The method involves the reaction of an α-amino-ketone (1) and a compound containing a methylene group α- to (bonded to the next carbon to) a carbonyl group (2).
Paal-Knorr pyrrole synthesis
Van Leusen reaction
The Van Leusen reaction can be used to form pyrroles, by reaction of tosylmethyl isocyanide (TosMIC) with an enone in the presence of base, in a Michael addition. A 5-endo cyclization then forms the 5-membered ring, which reacts to eliminate the tosyl group. The last step is tautomerization to the pyrrole.
The Barton-Zard synthesis proceeds in a manner similar to the van Leusen synthesis. An isocyanoacetate reacts with a nitroalkene in a 1,4-addition, followed by 5-endo-dig cyclization, elimination of the nitro group, and tautomerization.
Piloty–Robinson pyrrole synthesis
The starting materials in the Piloty–Robinson pyrrole synthesis, named for Gertrude and Robert Robinson and Oskar Piloty, are 2 equivalents of an aldehyde and hydrazine. The product is a pyrrole with substituents at the 3 and 4 positions. The aldehyde reacts with the diamine to an intermediate di-imine (R–C=N−N=C–R). In the second step, a [3,3]-sigmatropic rearrangement takes place between. Addition of hydrochloric acid leads to ring-closure and loss of ammonia to form the pyrrole. The mechanism was developed by the Robinsons.
Various methods for the synthesis of pyrroles via two-component cycloaddition reactions are known. For example, dipolar cycloaddition of a oxido-oxazolium with an alkyne, followed by a retro-Diels-Alder reaction to expel CO2 provides pyrrole. Similar reactions can be performed using azalactones.
Pyrroles can 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 Ag-acetylide intermediate. This method is analogous to the azide-alkyne click chemistry used to form azoles.
Reactions and reactivity
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 polymerize easily, and thus many electrophilic reagents that are used in benzene chemistry are not applicable to pyrroles.
Reaction of pyrrole with electrophiles
Pyrroles generally react with electrophiles at the α position (C2 or C4), due to the highest degree of stability of the protonated intermediate.
Pyrroles react easily with nitrating (e.g. HNO3/Ac2O), sulfonating (Py-SO3), and halogenating (e.g. NCS, NBS, Br2, SO2Cl2, and KI/H2O2) agents. Halogenation generally provides polyhalogenated pyrroles, but monohalogenation can be performed. As is typical for electrophilic additions to pyrroles, halogenation generally occurs at the α position, but can also occur at the β position by silation of the nitrogen. This is a useful method for further functionalization of the generally less reactive β position.
Acylation generally occurs at the 2-position, through the use of various methods. Acylation with anhydrides and acid chlorides can occur without a catalyst; alternatively, a Lewis acid may be used. 2-Acylpyrroles are also obtained from reaction with nitriles, by the Houben–Hoesch reaction. Pyrrole aldehydes can be formed by a Vilsmeier–Haack reaction. N-acylation of simple pyrrole does not occur.
Electrophilic alkylation of simple pyrrole is uncommon. Alkylation to form enones at C2 has been seen.
Reaction of deprotonated pyrrole
The NH proton in pyrroles is moderately acidic with a pKa of 16.5. Pyrrole can be deprotonated with strong bases such as butyllithium and sodium hydride. The resulting alkali pyrrolide is nucleophilic. Treating this conjugate base with an electrophile such as methyl iodide gives N-methylpyrrole. N-metallated pyrrole can react with electrophiles at the N or C positions, depending on the coordinating metal. More ionic N-metal bonds (such as with Li, Na, and K) 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, metallation of the carbons is more facile. Alkyl groups can be introduced as electrophiles, or by cross-coupling reactions.
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.
Pyrroles can undergo reductions to pyrrolidines and to pyrrolines. For example, Birch reduction of pyrrole esters and amides produced pyrrolines, with the regioselectivity depending on the position of the electron-withdrawing group.
Pyrroles with N-substitution can undergo cycloaddition reactions such as (4+2), (2+2), (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 N. Vinylpyrroles can also act as dienes.
Pyrroles can react with carbenes, such as dichlorocarbene, in a (2+1) cycloaddition. With dichlorocarbene, a dichlorocyclopropane intermediate is formed, which breaks down to form 3-chloropyridine (Ciamician-Dennstedt Rearrangement).
Pyrrole is essential to the production of many different chemicals. N-methylpyrrole is a precursor to N-methylpyrrolecarboxylic acid, a building-block in pharmaceutical chemistry. Although there is a claim that pyrrole is used as an additive to cigarettes, it is typically listed as a constituent of tobacco smoke and not as an ingredient.
Analogs and derivatives
Structural analogs of pyrrole include:
- Pyrroline, a partially saturated analog with one double bond
- Pyrrolidine, the saturated hydrogenated analog
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- General Synthesis and Reactivity of Pyrrole
- Synthesis of pyrroles (overview of recent methods)
- Substitution reaction mechanisms of nitrogen-containing heteroaromatics
- Jolicoeur, Benoit; Chapman, Erin E.; Thompson, Alison; Lubell, William D. (2006). "Pyrrole protection". Tetrahedron 62 (50): 11531. doi:10.1016/j.tet.2006.08.071.