The Barton–Zard reaction is a route to

pyrrole Pyrrole is a heterocyclic, aromatic, organic compound, a five-membered ring with the formula . It is a colorless volatile liquid that darkens readily upon exposure to air. Substituted derivatives are also called pyrroles, e.g., ''N''-methylpyrrol ...

derivatives via the reaction of a nitroalkene with an α-isocyanide under basic conditions. pp.43-4 It is named after

Derek Barton Sir Derek Harold Richard Barton (8 September 1918 – 16 March 1998) was an English organic chemist and Nobel Prize laureate for 1969. Education and early life Barton was born in Gravesend, Kent, to William Thomas and Maude Henrietta Barton ...

and Samir Zard who first reported it in 1985.

Mechanism

The mechanism consists of five steps: * Base catalyzed carbonyl

enolization In organic chemistry, enols are a type of functional group or intermediate in organic chemistry containing a group with the formula (R = many substituents). The term ''enol'' is an abbreviation of ''alkenol'', a portmanteau deriving from "-ene ...

of the α-

isocyanide An isocyanide (also called isonitrile or carbylamine) is an organic compound with the functional group –. It is the isomer of the related nitrile (–C≡N), hence the prefix is ''isocyano''.IUPAC Goldboo''isocyanides''/ref> The organic fragme ...

. * Michael-type addition between the α-

carbonyl enolate and the nitroalkene. * 5-endo-dig

cyclization A cyclic compound (or ring compound) is a term for a compound in the field of chemistry in which one or more series of atoms in the compound is connected to form a ring. Rings may vary in size from three to many atoms, and include examples where ...

(see:

Baldwin's rules Baldwin's rules in organic chemistry are a series of guidelines outlining the relative favorabilities of ring closure reactions in alicyclic compounds. They were first proposed by Jack Baldwin (chemist), Jack Baldwin in 1976. Baldwin's rules dis ...

). * Base catalyzed elimination of the nitro group. * Tautomerization leading to aromatisation.

Scope

The nitro compound may be aromatic rather than just an alkene. The reaction has been used for the synthesis of polypyrroles, including

porphyrins Porphyrins ( ) are heterocyclic, macrocyclic, organic compounds, composed of four modified pyrrole subunits interconnected at their α carbon atoms via methine bridges (). In vertebrates, an essential member of the porphyrin group is heme, whi ...

, as well as dipyrromethenes such as

BODIPY BODIPY is the technical common name of a chemical compound with formula , whose molecule consists of a boron difluoride group joined to a dipyrromethene group ; specifically, the compound 4,4-difluoro-4-bora-3a,4a-diaza-''s''-indacene in the ...

References

{{DEFAULTSORT:Barton-Zard synthesis Heterocycle forming reactions Name reactions