Pyrroles 81Pyrrolyllithiums can also be generated via metal-halogen exchange processes.
Palladium(0)-catalysed reactions
Pyrrole halides, boronic acids and stannanes have been employed in palladium(0)-catalysed cross-couplings, as illus-
trated by the synthesis of some 2- and 3-substituted pyrroles shown below.
Oxidation and reduction
Simple pyrroles are rather easily oxidised, usually with degradation of the ring, although sometimes maleimides can
be isolated.
An important area where technology and heterocyclic chemistry overlap is that of electroactive organic materials.
Pyrrole can be oxidatively polymerised either electrochemically or chemically (e.g., using iron(III) chloride), to give
(mainly) 2,5-coupled polymers. The initial neutral polymers are non-conducting but on further oxidation are con-
verted partially into cation radicals or dications, with incorporation of counterions from the reaction medium – a
process known as ‘doping’ – giving conducting materials (see page 193).
Reduction of pyrroles to pyrrolidines with hydrogen can be effected using various catalysts. Partial reduction using
a hydride reagent in the presence of acid, gives a 2,5-dihydropyrrole, via an -protonated intermediate. This is better
carried out using 1-phenylsulfonylpyrrole, as shown.