Aryl halide, homocoupling

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... 

He, H.S., Zhang, C., Ng, C.K.-W. and Toy, P.H. (2005a) Polystyrene-supported triphenylarsines useful ligands in palladium-catalyzed aryl halide homocoupling reactions and a catalyst for alkene epoxidation using hydrogen peroxide. Tetrahedron, 61(51), 12053-57. 

Total basicity is measured by standard acid-base titration techniques. The activity divided by the total basicity should be greater than 90%. If it is not, then the Grignard reagent should be checked for unreacted alkyl or aryl halide, homocoupled product, hydrolysis products, and oxidation products. 

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. 

In addition, arylthiophene 70 was obtained by a one-pot Suzuki coupling of p-methoxyiodobenzene and 3-bromothiophene via an in situ boronate formation using one equivalent of the thermally stable diborane 69 [55], This method avoids the isolation of boronic acids and is advantageous when base-sensitive groups such as aldehyde, nitriles and esters are present. However, the cross-coupling yields are low when both aryl halides are electron-poor because of competitive homocoupling during the reaction. 

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

As mentioned in Section 8.2, organopalladium(II) complexes can react with organyl halides to yield products of cross-coupling. The formation of large amounts of symmetric biaryls as a result of homocoupling of the aryl halide is often observed during... 

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. 

Kuroboshi, M. Waki, Y. Tanaka, H. Palladium-catalyzed tetrakis(dimethylamino)ethy-lene-promoted reductive coupling of aryl halides./. Org. Chem. 2003, 68, 3938-3942. Luo, F.-T. Jeevanandam, A. Basu, M. K. Efficient and high-turnover homocoupling reaction of aryl iodides by the use of palladacycle catalysts. A convenient way to prepare poly-p-phenylene. Tetrahedron Lett. 1998, 39, 7939-7942. 

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. 

Wang L, Zhang Y, Liu L et al (2006) Palladium-catalyzed homocoupling and cross-coupling reactions of aryl halides in poly(ethylene glycol). J Org Chem 71(3) 1284-1287... 

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