Ullmann Reaction

There are two different transformations referred as the Ullmann Reaction. The "classic" Ullmann Reaction is the synthesis of symmetric biaryls via copper-catalyzed coupling. The "Ullmann-type" Reactions include copper-catalyzed Nucleophilic Aromatic Substitution between various nucleophiles (e.g. substituted phenoxides) with aryl halides. The most common of these is the Ullmann Ether Synthesis.

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Mechanism of the Ullmann Reaction

Biaryls are available through coupling of the aryl halide with an excess of copper at elevated temperatures (200 °C). The active species is a copper(I)-compound which undergoes oxidative addition with the second equivalent of halide, followed by reductive elimination and the formation of the aryl-aryl carbon bond.

The organocopper intermediate can be generated at a more moderate 70 °C using a novel thiophenecarboxylate reagent. The reaction otherwise follows the same reaction path as above.

Another possibility is the use of Cu(I) for the oxidative coupling of aryllithium compounds at low temperatures. This method can also be used to generate asymmetric biaryls, after addition of the appropriate halide.

Ullmann-type reactions proceed through a catalytic cycle, and in one mechanism the copper is postulated to undergo oxidation to Cu(III). As some Cu(III) salts have been prepared, the suggestion for the mechanism is intriguing (see also Chan-Lam Coupling):

    

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Recent Literature

An Improved Cu-Based Catalyst System for the Reactions of Alcohols with Aryl Halides
R. A. Altman, A. Shafir, P. A. Lichtor, S. L. Buchwald, J. Org. Chem., 2008, 73, 284-286.

1,1,1-Tris(hydroxymethyl)ethane as a New, Efficient, and Versatile Tripod Ligand for Copper-Catalyzed Cross-Coupling Reactions of Aryl Iodides with Amides, Thiols, and Phenols
Y.-J. Chen, H.-H. Chen, Org. Lett., 2006, 8, 5609-5612.

N,N-Dimethyl Glycine-Promoted Ullmann Coupling Reaction of Phenols and Aryl Halides
D. Ma, Q. Cai, Org. Lett., 2003, 5, 3799-3802.

A General and Mild Ullmann-Type Synthesis of Diaryl Ethers
H.-J. Cristau, P. P. Cellier, S. Hamada, J.-F. Spindler, M. Taillefer, Org. Lett., 2004, 6, 913-916.

Copper-Catalyzed Coupling of Alkylamines and Aryl Iodides: An Efficient System Even in an Air Atmosphere
F. Y. Kwong, A. Klapars, S. L. Buchwald, Org. Lett., 2002, 4, 581-584.

Regioselective Copper-Catalyzed Amination of Bromobenzoic Acids Using Aliphatic and Aromatic Amines
C. Wolf, S. Liu, X. Mei, A. T. August, M. D. Casimir, J. Org. Chem., 2006, 71, 3270-3273.

Synthesis of Pyrroles via Copper-Catalyzed Coupling of Amines with Bromoenones
Y. Pan, H. Lu, Y. Fang, X. Fang, L. Chen, J. Qian, J. Wang, C. Li, Synthesis, 2007, 1242-1246.

General and Highly Efficient Synthesis of 2-Alkylideneazetidines and β-Lactams via Copper-Catalyzed Intramolecular N-Vinylation
H. Lu, C. Li, Org. Lett., 2006, 8, 5365-5367.

Preference of 4-exo Ring Closure in Copper-Catalyzed Intramolecular Coupling of Vinyl Bromides with Alcohols
Y. Fang, C. Li, J. Am. Chem. Soc., 2007, 129, 8092-8093.

A General Method for the Formation of Aryl-Sulfur Bonds Using Copper(I) Catalysts
C. G. Bates, R. K. Gujadhur, D. Venkataraman, Org. Lett., 2002, 4, 2803-2806.