Ullmann reaction halide reactivity

Additional studies showed that iodoferrocene was approximately as reactive as l-iodo-2-nitrobenzene under Ullmann conditions. A mixed Ullmann reaction involving these two reactive aryl halides produced 2-nitrophenylferrocene. Ullmann condensations of iodoferrocene with various sodium phenoxides and sodium arenethiolates likewise led to ferrocenyl aryl ethers and sulfides, respectively (84, 85). 

Active copper powder. A highly reactive copper powder can be prepared by reduction of Cut with potassium naphthalenide in DME (8 hours). The slurry should be used immediately. It is particularly effective for Ullmann reactions high yields of biaryls are obtained even at 85°. It also promotes cross-coupling with allyl halides. ... 

Ullmann reaction. Brown and Robin have found that two different aryl halides hindered by bulky substituents in the orrto-positions can be coupled by a inoditied Ullmann reaction. A large excess of copper powder (not activated) is necessary and the temperature should be lower than that which leads to self-coupling of the less reactive aryl halid. At optimal temperatures, usually 190-240the reaction can tie completed in 20-30 minutes. In favorable cases yields of biphenyls as high as 757o can be obtained, example ... 

The yields of symmetrical biaryls in the Ullmann reaction may vary from approximately a few to more than 90%, depending on the reactant structure and reaction conditions [1-6]. However, in many examples the yields of biaryls were moderate to high. Theoretically, the Ullmann reaction requires two equivalents of the copper metal per one equivalent of the biaryl formed, thus two equivalents of copper(I) halide are generated. Practically, several equivalents, three to tenfold excess, of the copper metal are used in order to reach high conversion of the aryl halides [3,4,5]. The reactivity of the aryl halide with the copper metal in homo-coupling reactions is... 

For the highly reactive copper metal forms, e.g. Rieke copper, oxidative addition to aryl halide occurs readily at room temperature or below [27]. At these temperatures oxidative addition of the resultant arylcopper(l) compound to the second molecule of aryl halide, or decomposition to biaryl proceeds very slowly or not at all. The arylcopper(I) compounds can then be homo-coupled or react with the aryl halides to form biaryl as the temperature is raised. In the classical Ullmann reaction employing... 

A halogen atom directly attached to a benzene ring is usually unreactive, unless it is activated by the nature and position of certain other substituent groups. It has been show n by Ullmann, however, that halogen atoms normally of low reactivity will condense with aromatic amines in the presence of an alkali carbonate (to absorb the hydrogen halide formed) and a trace of copper powder or oxide to act as a catalyst. This reaction, known as the Ullmant Condensation, is frequently used to prepare substituted diphenylamines it is exemplified... 

The use of dimethylformamide (b.p. 153°) as a solvent and diluent oftai increases the yield materially. The vigour of the exothermic reaction which occurs with a relatively reactive aryl halide is moderated and, furthermore, the dimethylformamide is easily removed from the reaction product since it is water soluble. Aryl hah des which are inert under the usual Ullmann conditions do not react in the presence of dimethylformamide. 

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