Ullmann reaction

With over a century of history, the Ullmann reaction has been one of the first to use a transition metal, primarily copper, in its higher oxidation states.

Despite the significant implications of biaryl coupling in industries, the Ullmann reaction was plagued by a number of problems in its early development.

However, in modern times the Ullmann reaction has revived interest due to several advantages of copper over other catalytic metals.

The reaction likely involves the formation of an organocopper compound (RCuX) which reacts with the other aryl reactant in a nucleophilic aromatic substitution.

[8] This groundbreaking result was the first to show that a transition metal could help perform an aryl carbon-carbon bond formation.

A typical example of classic Ullmann biaryl coupling is the conversion of ortho-chloronitrobenzene into 2,2'-dinitrobiphenyl with a copper - bronze alloy.

These problems were solved in 2015 with the design of special oxalic diamine ligands, making the Ullmann reaction viable for industrial application.

[20] Nelson and collaborators worked on the synthesis of asymmetric biaryl compounds and obtained the thermodynamically controlled product.

Unsymmetrical Ullmann reactions are rarely pursued but have been achieved when one of the two coupling components is in excess.