92 Indoles
Oxidation and reduction
Indoles are easily autoxidised in the reactive heterocyclic ring. Conversion of 3-substituted indoles into their cor-
responding oxindoles (page 94) can be brought about using dimethylsulfoxide in acid – a small concentration of
-protonated indole is the key to the sequence (compare with page 86).
Treatment of indole with sodium in liquid ammonia (Birch reduction) causes reduction of the benzene ring only; it
is signifi cant that the products are now dialkyl-pyrroles. Metal (or metal hydride)-acid combinations reduce only the
heterocyclic ring; the process involves attack on a small amount of 3H-indolium salt. The 2,3-dihydroindole products
are known as indolines but, chemically speaking, behave as anilines.
Pericyclic reactions
Indoles, as aromatic molecules, only rarely act as partners in electrocyclic processes – special situations are needed to
force the 2,3-double bond to participate in cycloadditions. The 2,3-double bond in simple indoles will take part in
cycloaddition reactions with dipolar 4 components, and with electron-defi cient dienes (i.e. inverse electron demand),
especially when the two reacting components are tethered together.
When electron-withdrawing substituents are present on nitrogen and/or at C-3, even electron-rich dienes react, for exam-
ple 3-nitro-1-phenylsulfonylindole takes part in dipolar cycloaddition with azomethine ylides and undergoes Diels–Alder
addition across the 2,3-double bond with 1-acylaminobuta-1,3-dienes (the fi nal product is formed via successive losses of
nitrous acid and then the amine giving a carbazole). Both 2- and 3-vinylindoles will act as dienes in Diels–Alder additions.