Arylboronic homocoupling

Very recently, well-defined complexes with general formula [PdCl(T -Cp) (NHC)] were synthesised and tested for the homocoupling of non-electrodeficient arylboronic acids at room temperature with good results (Scheme 7.7) [51]- This new class of catalysts were synthesised from commercially available NHC palladium(II) chloride dimers and are air-stable. 

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

A. Base-free oxidative homocoupling of arylboronic esters. Tetrahedron Lett. 2003, 44, 1541-1544. 

Note The efficiency of the catalyst cannot be evaluated in the absence of a cross-coupling product since homocoupling of the arylboronic acid usually occurs in the absence of a defined catalyst. 

Nanoparticlulated gold supported by nanocrystalline or mesostruc-tured nanocrystalline ceria catalysts represents an alternative to catalysts for selective aerobic oxidation of aliphatic and aromatic aldehydes which is much better than the gold supported by the precipitated ceria (Corma and Domine, 2005). The ceria or yttria supported Au are also active and extremely selective for the homocoupling of arylboronic acids, and the activity is directly correlated with Au(lll) (Carrettin et al., 2005). 

Biaryls. Aryllead triacetates couple with arylboronic acids by (dbaljPdj-CHCIj, Cul, and NaOMe. The critical role of NaOMe is in the formation of ArPbfOMe), to facilitate oxidative addition with Pd(0), and thence the coupling. Note that (dbaljPdj-CHClj catalyzes the homocoupling of ArPbCOAclj at room temperature. ... 

The coupling of arylboronic acids with acid chlorides is the basis of a ketone synthesis. It is found that the Suzuki coupling in an ionic liquid has several advantages reduced catalyst concentration, no homocoupling, and reaction in the air. ... 

Finally, the Suzuki-Miyaura reaction, which has become one of the most versatile and important reactions for the construction of C-C bonds, could be performed in a continuous flow process under ligand-free conditions [43]. The PASSfiow reactor is suited for the coupling of 4-iodoacetophenone and other aryl halides with arylboronic acid to yield the corresponding diaryls within 3 h without formation of homocoupling by-products (Scheme 15). After collecting... 

The reaction is also compatible with the presence of an ortho substituent in the arylboronic acid, while arylboronic acid with two ortho substituents was not reactive. By-products from aryl scrambling and biaryl from homocoupling were not found. The reaction proceeds as previously indicated up to the formation of the biphenylylpalladium species and is terminated by the Suzuki coupling as in Eq. 45. 

The bundle of available synthetic methods today includes the Suzuki cross coupling of arylboronic acids or esters with chloro-, bromo-, iodo-, or tosylaryls (see Scheme 5.3), Yamamoto homocoupling of chloro-, bromo-, or iodoaryls, and StiUe cross coupling of trialkylstaimylaryls with haloaryls. 

The first report of the synthesis of )S-keto sulfoxides using a palladium-catalyzed carbonylative Suzuki reaction of a-bromosulfoxide with arylboronic acids appeared in 2005 [28]. Asensio and his associates described the synthesis of 12 different /f-keto sulfoxides in moderate to excellent yields using Pd(PPh3)4 and CsF as the base in the presence of 1 bar CO (Scheme 4.13). The carbonylative coupling reaction is strongly favored over the competing non-carbonylation and homocoupling processes. However, the chemoselectivity drops and by-products... 

A very minor amount of homocoupling biaryl is derived during the reduction of a palladium(II) or nickel(II) halide complex with aryl-boronic acid (Eq. 23) or by the metathetic reaction shown in Eq. 48. However, a large number of homocoupling products of arylboronic acids are reported in literature. The mechanism proceeding through oxidative addition of the C-B bond to palladium(O) is recently proposed as the route to homocoupling (Eq. 54). The oxidative addition of the C-B... 

Careful consideration must be given to oxygen during experiments involving the cross-coupling reaction. When the reaction mixture is exposed to air, arylboronic acid readily produces a homocoupling biaryl 10 in the presence of a palladium(O) catalyst and base (Eq. 55). The... 

A third potential side reaction is the homo-coupling of arylboronic acid to give the biaryl product (Section 5.9.3) [23a]. This has also been reported recently by Demir et al. [53]. DMF is the preferred solvent to generate more homocoupled biaryl products. 

Selective C-H bond arylation of electron-rich arenes with aiyl boronic acids proceeds to give biaryls (Scheme 15.18). It is noteworthy that the homocoupling of arylboronic acids does not occur. Although the 0)grgen atmosphere is not required for the coupling, the rate increases significantly... 

Oxidative Dimerization of Arylboronic Acids. Cu(OAc)2 was shown to mediate dimerization of various arylboronic acids forming symmetric biaryls in good yields. The oxidative homocoupling proceeds smoothly at rather elevated temperatures with Cu(OAc)2 present in catalytic or stoichiometric amounts. In an earlier case air was employed as an oxidant. The mechanism presumably involves transmetallation of arylboronic acids by copper followed by dimerization of the organocopper intermediate, followed by reductive elimination to give the product. ... 

Ceria- or yttria-supported Au catalysts are also active and extremely selective for the homocoupling of arylboronic acids, and... 

This procedure allows the synthesis of arylboronic esters in good yields while tolerating various ftinctional groups. A by-product is the homocoupling product (see 4.18). In Scheme 5-164, some successful substrates are depicted. 

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