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
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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