Pyrrole

Pyrrole, or pyrrol, is a NH.^[1] Substituted derivatives are also called pyrroles. For example, C₄H₄NCH₃ is N-methylpyrrole. Porphobilinogen is a trisubstituted pyrrole, which is the biosynthetic precursor to many natural products.^[2]

Pyrroles are components of more complex macrocycles, including the chlorophyll, and porphyrinogens.

Properties

Pyrrole has very low basicity compared to electrons of the nitrogen atom in the aromatic ring. Pyrrole is a very weak base with a pK_aH of about -4. Protonation results in loss of aromaticity and is therefore unfavorable.

Synthesis

Many methods exist for the organic synthesis of pyrrole and its derivatives. Classic named reactions are the Knorr pyrrole synthesis, the Hantzch pyrrole synthesis and the Paal-Knorr synthesis.

The starting materials in the Piloty-Robinson pyrrole synthesis are 2 equivalents of an ammonia to the pyrrole.

In one modification microwave irradiation^[6]:

In the second step a [3,3]sigmatropic reaction takes place between two intermediates.

Reactivity

Both NH and CH protons in pyrroles are moderately acidic and can be deprotonated with strong acyl halide) reveals which sites were deprotonated based on which ring positions actually react as nucleophiles. The product distribution of such a reaction can often be complex and depends on the base used (especially the counterion, such as lithium from butyllithium or sodium from sodium hydride), existing substitution of the pyrrole, and the electrophile.

Pyrrole undergoes porphyrins.

Pyrrole compounds can also participate in Diels-Alder) reactions under certain conditions, such as Lewis acid catalysis, heating, or high pressure.

Commercial Uses

In a 1994 report released by five top cigarette companies, pyrrole is one of the 599 additives to cigarettes. Its use or purpose, however, is unknown, like most cigarette additives. ^[9]

See also

• arsenic analog.
• oxygen instead of the nitrogen.
• benzene ring.
• phosphorus analog.
• Polypyrrole
• Pyroluria
• Pyrroline, a partially saturated analog with one double bond
• Pyrrolidine, the saturated hydrogenated analog
• Simple aromatic rings
• atom.

References

1. ^ # Loudon, Marc G. (2002). "Chemistry of Naphalene and the Aromatic Heterocycles.", Organic Chemistry, Fourth Edition, New York: Oxford University Press, 1135-1136. ISBN 0-19-511999-1. 
2. ^ Nelson, D. L.; Cox, M. M. "Lehninger, Principles of Biochemistry" 3rd Ed. Worth Publishing: New York, 2000. ISBN 1-57259-153-6.
3. ^ The aromatic pathways of porphins, chlorins and bacteriochlorins Jonas Juse lius and Dage Sundholm Phys. Chem. Chem. Phys., 2000, 2, 2145-2151 doi:10.1039/b000260g Open access
4. ^ Piloty, O. Chem. Ber. 1910, 43, 489.
5. ^ Robinson, R.; Robinson, G. M. J. Chem. Soc. 1918, 43, 639-644.
6. ^ Microwave-Assisted Piloty-Robinson Synthesis of 3,4-Disubstituted Pyrroles Benjamin C. Milgram, Katrine Eskildsen, Steven M. Richter, W. Robert Scheidt, and Karl A. Scheidt J. Org. Chem.; 2007; 72(10) pp 3941 - 3944; (Note) doi:10.1021/jo070389+
7. ^ Comparison of Benzene, Nitrobenzene, and Dinitrobenzene 2-Arylsulfenylpyrroles Jose R. Garabatos-Perera, Benjamin H. Rotstein, and Alison Thompson J. Org. Chem. 2007, 72, 7382-7385 doi:10.1021/jo070493r
8. ^ The 2-Raney nickel
9. ^ quitsmoking.about.com Link