Homocoupling reactions arylation

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... 

Abstract This chapter highlights the use of iV-heterocyclic carbenes as supporting ligands in arylation reactions different than the more common cross-coupling reactions, including C-F bond activation, catalytic arylation, homocoupling, direct arylation and oxidative Heck reactions. 

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... 

It was shown a few years ago that a-haloesters can react with an aryl-nickel compound generated electrochemically in a divided cell (Eq. 8) [57]. The main drawback for this two-step method is the use of a large excess of PPhs to prevent the homocoupling reaction ... 

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 . 

These a-functionalized lithiosilanes are stable for up to 6 days at 0°C, when at least one substituent at silicon is an aryl group. Alkylated mono- or diaminochlorosilanes, however, undergo with lithium in THF homocoupling reactions to yield the corresponding symmetrical di- and tetraaminodisilanes (equation 40)89. 

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. 

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,... 

Scheme8.22. Cyclopalladation during Pd-mediated homocoupling of aryl iodides [156] and Heck reaction with norbornene [157]. Ar = 2-(tBu)C6H4.

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. 

Radical arylations of phenols differ in some respects from those of phenolates (Scheme 37). First, the decreased nucleophilicity of the phenol, such as 100, allows the use of unmasked aryl diazonium chlorides 101 as radical sources. Given that an efficient reductant is present in the reaction mixture and that the diazonium salt is added slowly, biphenyl alcohols 102 can be prepared in moderate to good yields [153,154]. In this way, the concentration of the salt 101 is kept low at any time and homocoupling reactions (addition of the aryl radical to diazonium ions) as well as azo coupling to the phenol 100 can be successfully overcome. 

The same author reported the cross-coupling reactions of aryl tellurides with olefins289 and the cross-coupling and homocoupling reactions promoted by palladium involving vinyl tellurides. Different solvents and reaction conditions, including a reoxidant, were used, leading to the products in variable yields. In Scheme 119, some of the obtained results are summarized. 

Recently, the catalytic use of Cu(I) salts was applied to the aryl-aryl and aryl-vinyl couplings and to the allylation of furans and thiophenes, even without the benefit of such activation. In parallel with the use of Cu(I) salts, the reactivity of stoichiometric amounts of Cu(II) was estabhshed for the homocoupling of vinyltins" °°. Copper nitrate was found to be particularly efficient, leading to the cyclotrimerization of l-bromo-2-stannylaUcenes . Lately, the use of catalytic amounts of CuCla was shown to be superior to the use of Cu(I) salts for achieving these homocoupling reactions . ... 

The Ullmann-type reaction that is homocoupling of aryl or vinyl halides is conveniently mediated by copper at high temperature. The copper powder serves as a zerovalent metal. The classical Ullmann reaction reported in 1901 has long been employed by chemists to generate a carbon-carbon bond between two aromatic nuclei. 

Indirect activation of the C-0 bond of the aryl alcohol is further developed by conversion to corresponding triflate. The C-0 bond in the aryl triflate has been achieved in the homocoupling reaction catalyzed by a palladium(O) or nickel(O) complex in the presence of an electron source (either a cathode or zinc power) (Eq.47) [110]. 

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. 

Pd-Catalyzed Aryl-Aryl Bond Formation via Oxidative Homocoupling Reaction. 170... 

Aryl triflates are also useful as electrophiles in the reaction of arylzinc compounds [48]. Pd(0) and Ni(0)catalysed cross-coupling and homocoupling reactions of aryl triflates were reviewed recently [49]. Aryl fluorosulfonates gave biaryls in this reaction too. 

Reductions. Nitrones and N-oxides are deoxygenated by (BnNEtjljMoS. Acyl azides give amides. Alkyl azides undergo a homocoupling reaction to form imines, whereas stabilized azides, such as acyl, sulfonyl, and aryl azides, undergo reductive elimination of... 

Homocoupling of aryl iodide in the presence of Cu (Ullmann reaction) proceeds via a radical mechanism (Scheme 4.44). 

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