Halides 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. 

Perfluoroalkylzinc iodides, prepared in situ from iodides and ultrasonically dispersed Zn, are coupled with allylic halides via an allylic rearrangement[271]. The Pd-catalyzed homocoupling of allylic acetate in the presence of Zn to give a mixture of regioisomers 416 and 417 may proceed via in situ formation of allylzinc species[272,273]. 

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. 

Homocoupling of alkyl halides in aqueous media can be mediated by manganese/cupric chloride to give the dimerization products in good yield. Cross-coupling can also be controlled to give the desired... 

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. 

Application in organic synthesis of pentacoordinated triorganodifluorosilicate anions, such as [Bu4N][Ph3SiF2] 825, have been extended to palladium(0)-catalyzed cross coupling reactions (solvents DMF, TFIF, dioxane) with arene halides (Scheme 111).825 This method is tolerant to various palladium(O) catalysts and provides excellent yields of mainly heterocoupled products and only small amounts of homocoupled byproducts. 

Non-steroidal anti-inflammatory a-arylpropionic acids were also prepared from the corresponding benzylic chlorides and CO2 using as catalyst Ni-dppe or Ni-dppp in the presence of COD (Table 16) [103]. The use of the catalyst in this reaction is not absolutely required but its use limits the homocoupling reaction which would be the main process at high concentration of the benzylic halide and low pressure of CO2 [104]. 

Functionalized organozinc halides are best prepared by direct insertion of zinc dust into alkyl iodides. The insertion reaction is usually performed by addition of a concentrated solution (approx. 3 M) of the alkyl iodide in THF to a suspension of zinc dust activated with a few mol% of 1,2-dibromoethane and MeaSiCl [7]. Primary alkyl iodides react at 40 °C under these conditions, whereas secondary alkyl iodides undergo the zinc insertion process even at room temperature, while allylic bromides and benzylic bromides react under still milder conditions (0 °C to 10 °C). The amount of Wurtz homocoupling products is usually limited, but increases with increased electron density in benzylic or allylic moieties [45]. A range of poly-functional organozinc compounds, such as 69-72, can be prepared under these conditions (Scheme 2.23) [41]. 

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

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