Barton–Zard reaction

The Barton–Zard reaction is a route to pyrrole derivatives via the reaction of a nitroalkene with an α-isocyanide under basic conditions. It is named after Derek Barton and Samir Zard who first reported it in 1985.
[Image:Barton-Zard reaction.svg|center|450px]

Mechanism

The mechanism consists of five steps:

Base catalyzed carbonyl enolization of the α-isocyanide.
Michael-type addition between the α-isocyanide carbonyl enolate and the nitroalkene.
5-endo-dig cyclization.
Base catalyzed elimination of the nitro group.
Tautomerization leading to aromatisation.

[Image:Barton-Zard mechanism corrected.svg|center|650px]

The nitro compound may be aromatic rather than just an alkene. The reaction has been used for the synthesis of polypyrroles, including porphyrins, as well as dipyrromethenes such as BODIPY.