Indoles

This chapter focuses on indoles. Fusion of a benzene ring onto the C2/C3 positions of pyrrole formally produces the corresponding benzopyrrole known as indole. Indole is a ten π-electron aromatic system. As with pyrrole, delocalization of the lone pair of electrons from the nitrogen atom is necessary for aromaticity. A consequence of this delocalization is that the lone pair is not available for protonation under moderately acidic conditions. Pyrrole is therefore just like indole, another weakly basic heterocycle. Another similarity to pyrrole is that being an ‘electron-rich’ heterocycle indole easily undergoes aromatic electrophilic substitution, and is also rather unstable to oxidative conditions. However, an important difference emerges here, in that whereas pyrrole preferentially reacts with electrophiles at the C2/C5 positions, indole substitutes selectively at the C3 position. The chapter then looks at the synthesis, electrophilic substitution, and anion chemistry of indoles.