Aryl homo-coupling reactions

Early development of the homo-coupling reactions of aryl halides involves the use of stoichiometric amounts of air-sensitive Ni(0) complexes.54 The reaction could also be realized with a catalytic amount of Ni(0) complexes formed in situ when a stoichiometric amount of Zn was present. Besides aryl iodides, tosylates,... 

The rate-determining step in the homo-coupling reaction of aryl halides could be the oxidative step or the reduction of Ni(II) to Ni(I) step. 

NMR spectroscopy, 93. See also Proton NMR integrations Hoechst continuous process, 548 Homo-coupling reactions, aryl halide, 486-487 Homopolymers, 7 Hot-cast prepolymer method, 211 Hot phosgenation, 222 Houvink-Sakurada equation, 286 HTMAB. See Hexadecyltrimethylammonium bromide (HTMAB)... 

Hiyama and Mori have reported that homo-coupling reactions of vinyl-, aryl-, and alkynylsilanes are effectively promoted by an equimolar amount of CuCl and air (Scheme 10.218) [565]. The reaction mechanism probably involves formation of organocopper species by Si-Cu transmetalation and subsequent oxidative dimerization by air. The strong tendency of CuCl to activate alkynylsilanes has been used for coupling reactions with 1-chloroalkynes (Scheme 10.218) [566]. 

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... 

The homo-coupling Ullmann reaction can be performed at room temperature or bellow by metallation of suitable substrates such as relatively acidic arenes or aryl hahdes with the -BuLi (or other strong bases) followed by addition of Cu(II) salts [28], Aryllithium reagent undergoes transmetallation with the Cu(II) salts to form diarylcopper(II) which is prone to rapid reductive elimination of the biaryl with extrusion of a copper nrirror. Benincori and coworkers [28] described an efficient procedure for the homo-coupling reaction of bromide 21 via diarylcopperfll) 22 under very mild reaction conditions, where the 3,3 -bibenzo[b]thiophene (23) was obtained in 56% yield. Scheme 4. 

The successful unsymmetrical coupling could be carried out if the difference of the reaction temperatures for homo-coupling reactions is more than approximately 20 C. Then, the unsymmetrical coupling reaction has to be performed by slow addition of more reactive aryl halide to the solution of less reactive aryl halide and copper powder at the temperature slightly higher than is the homo-coupling reaction temperature for the more reactive aryl halide. Under these reaction conditions more reactive aryl halide forms the arylcopper(l) compound in the fast process, which is subsequently cross-coupled with the less reactive aryl halide present in excess. This conditions suppress the homo-coupling of both aryl halides by ... 

Cohen has shown that copper(l) salts readily induce the decomposition of the aryldiazonium salt to generate the aryl radical which subsequently reacts with Cu(l) to give an arylcopper(ll) species [99,100]. This reacts with another aryl radical to produce the key diarylcopper(III) intermediate. The latter undergoes reductive elimination of the biaryl with regeneration of Cu(l), similar to other copper-based homo-coupling reactions, see Chapter 3, [100] Scheme 26. 

Homo-coupling reactions of aryl halides to biaryls catalysed by nickel complexes... 

Table 7. Homo-coupling reaction of aryl iodides to biaryls using Norman s Pd(OAc)2 / EtjN method [4]...

Method C involves the palladium-catalysed coupling of two different aryl halides with Hiinig s base as an ultimate reductant. Selectivity for cross- vs. homo-coupling reactions has been achieved in the following instances [17,71] ... 

Furthermore, besides activated niekel, eobalt, and copper (see Chapter 2), indium has been proved as an effective active metal for the coupling of aryl iodides. Thus by heating an equimolar amount of indium and aryl iodides in refluxing DMF, symmetrical biaryls were furnished with high to excellent yields [78]. The indium-promoted homo-coupling reaction tolerates the presence of free hydroxy and carboxylie acid groups well. For example, 2-iodobenzoie acid (104) was converted to diphenic acid (57) in 75% yield. Scheme 22. 

In similar manner, hexaalkyldistannanes allow the homo-coupling reactions of aryl iodides, bromides [104] and triflates [105] to the symmetrical biaryls. First, the aryltrialkylstannane is generated by reaction of the aryl halides with Me6Sn2 or -BugSn2, followed by the in situ Stille reaction with an excess of electrophilic reactant, since both reactions are catalysted by the same complexes and proceed at comparable reaction rates. For instance, 2-naphthyl triflate (165) was coupled to 2,2 -binaphthalene (166) in 69% yield [105], respectively, Scheme 22. 

The only side-reaction is the homo-coupling reaction of the parent diarylmercury which occurs as a result of oxidative addition of diarylmercury to palladium(O) followed by oxidative demercuration reaction. However, the selectivity can be enhanced by adding a nucleophilic catalyst, e.g. iodide ion. The role of iodide ion is apparently crucial as it significantly reduces the amount of symmetrical biaryl from the oxidative demercuration process. Since diarylmercurials act as two aryl-groups donors, only a half equivalent of aryl iodide is required for the cross-coupling reaction. 

Table 7. Synthesis of symmetrical biaryls via in situ generation of arylboronic acids from aryl iodides and bromides followed by the homo-coupling reaction with silver(I) nitrate [19,20]...

Homo-coupling reaction of aryl Grignard reagents with FeC Preparation of biphenyl (8) [12/... 

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