Application of Pyrrole

Pyrrole is isoelectronic with a cyclopentadienyl anion but is electrically neutral because of the higher nuclear charge on nitrogen. The heteroatoms, N, O, and S, in the ring can be either neutral or carry a positive charge. In fact, heteroatoms participate in a cyclic conjugated system through their unshared or lone pair of electrons present in an orbital perpendicular to the plane of the ring. In the conjugated five-membered heterocyclic ring system all the ring atoms are sp2 hybridized and form 3-equivalent coplanar hybrid orbitals, and the p orbital, which is perpendicular to the hybrid orbital, is occupied by a lone pair of electrons. In the case of pyrrole the three hybrid orbitals form three covalent bonds: two with vicinal carbon atoms and a third with hydrogen atoms. The unshared pair of electrons present in the p orbital of nitrogen participates in the formation of a delocalized 6π electron system responsible for its aromatic character.

Occurrence in nature

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. 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.

Application

Heterocyclic compounds are widely distributed in nature and play a vital role in the metabolism of all living cells. Numerous heterocyclic compounds both natural and synthetic are pharmacologically active and are in clinical use. Pyrrole has a wide range of applications as pharmaceuticals,agrochemicals, and veterinary products. They are also useful as antioxidants, developers, sensitizers, corrosion inhibitors, and copolymer dyes.They have been used as precursors in the synthesis of important natural products. Reserpine is an indole alkaloid, which has been used for the treatment of psychotic behavior and also for controlling high blood pressure. Ajmaline and strychnine are indole alkaloids, in which the former is used as an an tiarrhythmic agent, while latter is a highly toxic pesticide. Saxitoxin, a neurotoxin, is a marine natural product, characterized by guanidinium moieties.

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