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.
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):
Synthesis of Axially Chiral 2,2′-Bisphosphobiarenes via a Nickel-Catalyzed Asymmetric Ullmann Coupling: General Access to Privileged Chiral Ligands without Optical Resolution
Z. Zuo, R. S. Kim, D. A. Watson, J. Am. Chem. Soc., 2021, 143, 1328-1333.
Copper-Catalyzed Ullmann-Type Coupling and Decarboxylation Cascade of Arylhalides with Malonates to Access α-Aryl Esters
F. Cheng, T. Chen, Y.-Q. Huang, J.-W. Li, C. Zhou, X. Xiao, F.-E. Chen, Org. Lett., 2022, 24, 115-120.
Oxalic Diamides and tert-Butoxide: Two Types of Ligands Enabling Practical Access to Alkyl Aryl Ethers via Cu-Catalyzed Coupling Reaction
Z. Chen, Y. Jiang, L. Zhang, Y. Guo, D. Ma, J. Am. Chem. Soc., 2019, 141, 3541-3549.
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.
Environmentally Friendly and Recyclable CuCl2-Mediated C-S Bond Coupling Strategy Using DMEDA as Ligand, Base, and Solvent
G. Shen, Q. Lu, Z. Wang, W. Sun, Y. Zhang, X. Huang, M. Sun, Z. Wang, Synthesis, 2022, 54, 184-198.
CuI/2-Aminopyridine 1-Oxide Catalyzed Amination of Aryl Chlorides with Aliphatic Amines
W. Liu, J. Xu, X. Chen, F. Zhang, Z. Xu, D. Wang, Y. He, X. Xia, X. Zhang, Y. Liang, Org. Lett., 2020, 22, 7486-7490.
A Facile and Practical Copper Powder-Catalyzed, Organic Solvent- and Ligand-Free Ullmann Amination of Aryl Halides
J. Jiao, X.-R. Zhang, N.-H. Chang, J. Wang, J.-F. Wei, X.-Y. Shi, Z.-G. Chen, J. Org. Chem., 2011, 76, 1180-1183.
"On Water" Promoted Ullmann-Type C-N Bond-Forming Reactions: Application to Carbazole Alkaloids by Selective N-Arylation of Aminophenols
G. Chakraborti, S. Paladhi, T. Mandal, J. Dash, J. Org. Chem., 2018, 83, 7347-7359.
CuI-Catalyzed Coupling Reaction of β-Amino Acids or Esters with Aryl Halides at Temperature Lower Than That Employed in the Normal Ullmann Reaction. Facile Synthesis of SB-214857
D. Ma, C. Xia, Org. Lett., 2001, 3, 2583-2586.
Room-Temperature Amination of Chloroheteroarenes in Water by a Recyclable Copper(II)-Phosphaadamantanium Sulfonate System
U. Parmar, D. Somvanshi, S. Kori, A. A. Desai, R. Dandela, D. K. Maity, A. R. Kapdi, J. Org. Chem., 2021, 86, 8900-8925.
A Novel Synthesis of Disubstituted Quinazoline Diones with Differential N-Substitution via a Copper-Catalysed Cross-Coupling of Acyl Ureas
E. Durham, D. Perkins, J. S. Scott, J. Wang, S. Watson, Synlett, 2016, 27, 965-968.