Biaryl homocoupling

Biaryls. Homocoupling of ArZnI employs NCS as an oxidant in the presence of a Pd(0) catalyst. Yields are good. 

Biaryls. Homocoupling of substituted arenes to biphenyls occurs on treatment with Hg(II)-Ce(IV) salts, for example, Hg(OAc)2 and Ce(OTf)4. 

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. 

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... 

A drawback of the Heck-type reaction is that it is not strictly regioselective [119]. Depending on the substituents >1% of 1,1-diarylation is observed. Soluble 2,5-dialkoxy-PPVs 63 or 2-phenyl-PPV PPPV 93, without 1,1-diarylated moieties, were synthesized by Heitz et al. in a Suzuki-type cross coupling of substituted 1,4-phenylenediboronic acids and fran5-l,2-dibromoethylene, catalyzed by Pd(0) compounds [120]. However, about 3% of biaryl defect structures are observed in the coupling products (M up to 12,000), resulting from homocoupling of boronic acid functions. 

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... 

Kappe and co-workers developed two general microwave methods for the synthesis of symmetrical (hetero)-biaryls 110 from 4-chloroquinolin-2(177)-one 111 <06JOC1707>. The two methods include either a Pd(0)- or Ni(0)-mediated homocoupling, as exemplified below. 

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

Synthesis of biaryls by Raney Ni-catalysed homocoupling of diaryltellurium dichlorides and aryltellurium trichlorides... 

Symmetrical biaryls are important intermediates for synthesising agrochemicals, pharmaceuticals and natural products (1). One of the simplest protocols to make them is the Ullmann reaction (2), the thermal homocoupling of aryl chlorides in the presence of copper iodide. This reaction, though over a century old, it still used today. It has two main disadvantages, however First, it uses stoichiometric amounts of copper and generates stoichiometric amounts of CuL waste (Figure 1, left). Second, it only works with aryl iodides. This is a problem because chemicals react by their molarity, but are quantified by their mass. One tonne of iodobenzene, for example, contains 620 kg of iodo and only 380 kg of benzene . 

The slow addition of the arylzinc compound to a solution containing both an aryl hahde, Ar X, and the palladium complex furnished the biaryl in good to excellent yields. The very reactive Pd(0) complex was likely formed in situ by reduction of the starting palladium(II) complex by ArZnX. The reactions were very rapid (ca 1 or 2 h) compared to most usual Pd-catalyzed reactions involving ArZnX (ca 24 h). The reduction of Pd(II) could account for the formation of a small amount of Ar-Ar (2-5%) in the last non-electrochemical step while no homocoupling of Ar X occurred. 

Biaryls and 1,3-dienes can be synthesized by the Pd-promoted oxidative homocoupling of different substrates. The homocoupling of the arylstannane 172 and the alkenylstannane 175 gives the biaryl 173 and 1,3-diene 176 using a catalytic amount of Pd(OAc)2 and 02 in the presence of the iminophosphine ligand 174, or in the absence of ligand [107a]. 

A classic example is the homocoupling of aryl iodides in the presence of copper, known as the Ullmann reaction (Figure 1.17). This reaction gives symmetric 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... 

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. 

G. B. High yields of symmetrical biaryls from Pd catalysed homocoupling of arylboronic acids under mild conditions. Synlett 1997, 131-132. 

JA3684), and homocouplings of methoxybenzenes and methoxynaph-thalenes to biaryls 103 (01T345) (Scheme 32). 

Au111 supported on nanocrystalline ceria efficiently catalyses the homocoupling of boronic acids to give quantitative yields of biaryls in the presence... 

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