Download (6Mb) - Etheses - Saurashtra University

Chapter-5
Synthesis of highly substituted pyrroles
5.1 Introduction
Pyrrole is an important π-excessive aromatic heterocycle because this ring
system is incorporated in as a basic structural unit in porphyrins;porphin (heam) and
chlorine (chlorophyll) and corrins (vitamin B12) (Figure-1). More over the presence
of pyrrole ring in system in porphobilinogen (intermediate in biosynthesis of
porphyrins and vitamin B12), biliverdin and bilirubin (pyrrole-based bile pigments)
and pyrrolnitrin (with antibiotic activity) has provided an impetus to the pyrrole
chemistry.
Figure-1
Pyrrole has a planar pentagonal structure with four carbon atoms and nitrogen
sp 2 -hybridized. Each ring atom forms two sp 2 -sp 2 σ -bonds to its neighbouring ring
atoms and one sp 2 -sσ -bond to a hydrogen atom. The remaining unhybridized p-
orbitals, one on each ring atom (with one electron on each carbon and two electron on
nitrogen), are perpendicular to the plane of σ–bonds and overlap form a π-molecular
system with three bonding orbitals. The six π-electrons form an aromatic sextet which
is responsible for aromaticity and renders stability to the pyrrole ring. The C α -N and
C α -C β bonds are shorter than normal single bonds, where as C α -C β bonds are normal
than normal double bonds. The molecular dimentions of the pyrrole reflect cyclic
delocalization with the environment of lone pair of electrons on the nitrogen atom.
Department of Chemistry, Saurashtra University, Rajkot-360005 158