Homocoupling, of aryl halides

Traditionally, the synthesis of symmetrical biaryls was routinely accomplished using the Ullmann reaction. Recently, palladium-catalyzed homocoupling of aryl halides has also been demonstrated to rival the utility of the Ullmann coupling. As illustrated in Scheme 21, using Pd(OAc)2 as the... 

The homocoupling of aryl halides and triflates can be made catalytic in nickel by using zinc as a reductant for in situ regeneration of the active Ni(0) species. 

The homocoupling of aryl halide to diaryl compounds, known as Ull-mann coupling, is a synthetically useful reaction and has wide applications in material research. Such couplings have been studied in aqueous conditions. In 1970, arylsulfinic acids were coupled with Pd(II) in aqueous solvents to biaryls (Eq. 6.25).53 However, the reaction required the use of a stoichiometric amount of palladium. In the presence of hydrogen gas, aryl halides homocoupled to give biaryl compounds in moderate yields (30-50%) in an aqueous/organic microemulsion (Eq. 6.26).54... 

Homocoupling of aryl halides in the presence of Cu or Ni or Pd to afford biaryls. 

Tetrakis (dimethylamino) ethylene—Pd promoted reductive homocoupling of aryl halides. Synlett 2002, 637-639. 

Hennings, D. D. Iwama, T. Rawal, V. H. Palladium-catalyzed (Ullmann-type) homocoupling of aryl halides a convenient and general synthesis of symmetrical biaryls via inter- and intramolecular coupling reactions. Org. Lett. 1999, 1, 1205-1208. 

Penalva, V. Hassan, J. Lavenot, L. Gozzi, C. Lemaire, M. Direct homocoupling of aryl halides catalyzed by Pd. Tetrahedron Lett. 1998, 39, 2559-2560. 

NiCRA-bpyA NiCRA complex containing 2,2 -bipyridine (bpy) effects homocoupling of aryl halides (Ullmann coupling), often in high yield, which can be improved in some cases by addition of KI or Nal. When used in a catalytic amount, reduction to an alkane is the main side reduction. 

Although copper-catalyzed homocoupling of alkynes and Ni(0)-catalyzed homocoupling of aryl halides were well known, Cadiot and Chodkiewicz gave first-time synthetically more useful copper-catalyzed cross-coupling of alkynes. 

Other couplings. Homocoupling of aryl halides to provide biaryls using... 

The homocoupling of aryl halides, like the previously mentioned reaction, is caused by oxygen contaminant in the reaction medium. It may occur in spite of the site isolation effect caused by the polymer. Vigorous exclusion of oxygen in the coupling medium suppressed this side reaction efficiently. 

Biaryls. Homocoupling of aryl halides has been effected by Pd-C in an aqueous media (PEG-400, NaOH, H2O) under hydrogen" or in air in the presence of Zn and 18-crown-6. The considerable improvement in yield observed with added crown ether is intriguing. 

Nagaki A, Uesugi Y, Tomida Y et al (2011) Homocoupling of aryl halides in flow space integration of lithiation and FeCl3 promoted homocoupling. Beilstein J Org Chem 7 1064-1069... 

Alternatively, under strong reductive conditions, e.g. electrochemical reduction, arylpalladium(II) halide (XIII) is reduced to arylpalladium(0) which, upon the second oxidative addition, gives diarylpalladium(II) complex, also capable of undergoing reductive elimination of biaryl similarly to XIV [10]. Isopropanol [50-52], amines [4,17,52-54], hydrazine [55], tetrakis(dimethylamino)ethylene [18], hydroquinone [56], tetrabutylammonium ftuoride [57], formates [58-60], zinc [61-64], and molecular hydrogen [65] have been used as stoichiometric reductants. Electro-reductive homocoupling of aryl halides was also reported [10]. The most important methods for... 

The nickel-catalyzed homocoupling of aryl halides has a long history, and the development of chemoselective heterocouplings of this type constitutes an active and evolving area of research. A key study from Semmelhack in 1971 illustrated the homocoupling of haloarenes after exposure to Ni(cod)2. Kumada then illustrated that the process could be rendered catalytic in nickel by the use of zinc dust as the stoichiometric reductantJ ... 

The homocoupling of aryl halides was also studied in the presence of various palladium systems, and dimer (1) was found to be the best precursor. 7 For example, 4,4 diacetylbiphenyl was obtained in the presence of 2.5 mol % of 1 in 75% yield (eq 82). The use of tetrabutylammonium fluoride was crucial for the good outcome of the reaction. 

Polymer Forming Homocoupling Reactions. The high yield homocoupling of aryl halides and sulfonates has been extensively applied to the synthesis of aromatic polymers. Two types of aromatic polymers are prepared by this method. TTie first type contains heteroatom linkages between aromatic groups and the second type contains only aromatic groups in the backbone. 

The aryl-aryl bond is common in natural products and biologically active materials [6]. In addition, polyaryls have been extensively studied because of their potential applications in optoelectronics [7]. These carbon-carbon bonds are frequently synthesized by homocouplings of aryl halides mediated by transition-metal reagents or catalysts. Several excellent comprehensive reviews on these reactions have recently appeared [8,9]. Among different kinds of transition metal reagents... 

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