Ullmann reaction

The Ullmann reaction or Ullmann coupling is a coupling reaction between aryl halides with copper. The reaction is named after Fritz Ullmann.



A typical example is the coupling of o-chloronitrobenzene to 2,2'-dinitrobiphenyl with a copper - bronze alloy.



The traditional version of the Ullmann reaction requires harsh reaction conditions, and the reaction has a reputation for erratic yields. Since its discovery some improvements and alternative procedures have been introduced.

The reaction mechanism of the Ullmann reaction is extensively studied. Electron spin resonance rules out a radical intermediate. The oxidative addition / reductive elimination sequence observed with palladium catalysts is unlikely for copper because copper(III) is rarely observed. The reaction probably involves the formation of an organocopper compound (RCuX) which reacts with the other aryl reactant in a nucleophilic aromatic substitution. Alternative mechanisms do exist such as σ-bond metathesis.

The Ullmann reaction is limited to electron deficient aryl halides and requires harsh reaction conditions. In organic synthesis this reaction is often replaced by palladium coupling reactions such as the Heck reaction, the Hiyama coupling and the Sonogashira coupling

In a variation of the Ullmann reaction, 2-bromovinylstyrene is reacted with imidazole in an ionic liquid to an n-vinylimidazole. The reaction requires (L)-proline catalysis.