Boronic acids homocoupling

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. 

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. 

Although catalytic amounts of Ag20 have been found to promote the palladium-catalyzed coupling of aryl boronic acids and terminal alkynes, the authors in this case do not attribute the desired reactivity to the formation, and subsequent transmetalla-tion, of a silver acetylide. Rather, it is proposed that the Ag20 activates the alkynylpalladium complex to allow transmetallation from the boronic acid, and that any competing formation of the silver acetylide results in a homocoupling of the alkynes (Scheme 1.55).123... 

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

Completely aromatic, hyperbranched polyphenylenes were synthesized as monodendrons from AB2 type monomers by Kim and Webster [111, 112]. These dendrimers were prepared either by the homocoupling of 3,5-dibromophenyl boronic acid under modified Suzuki conditions, or by aryl-aryl coupling reactions involving 3,5-dihalo-phenyl Grignard reagents in the presence of Ni(II) catalysts as shown in Scheme 7. 

Suzuki methodology, with a 3-halotriazolopyridine and an aryl or heteroaryl boronic acid, sodium carbonate and tetrakistriphenylphosphine as catalyst, the synthesis of 3-aryl derivatives 6b-g, i, j, ha ve been described (Figure 2). In these reactions a secondary compound was also formed, the 33 -bitriazolopyridine 37 (Figure 10), as a consequence of a homocoupling Ullman reaction. From 7-bromo-3-methyl-triazolopyridine, the synthesis of some 7-aryl derivatives 7a-g in very good yields (60-90%) was also reported (Figure 2) (06TL8101). 

In the 2-position, the Suzuki coupling of WBoc-pyrrole-2-boronic acid with phenyl iodide gave a moderate yield of the 2-phenyl product 20 (Scheme 11). With bromobenzene the yield was low. The best results were obtained with rr-deflcient arenes. The same coupling with indole derivatives gave even lower yields of arylated products. Homocoupling of the heterocycle is responsible for the major by-product. The boronic acid group... 

Recently, homocoupling of aryl boronic acids was reported to give good yields of the corresponding biaryls (Table 6). This paper included studies of effects of different bases and oxidants on the yield of homocoupling. ... 

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... 

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... 

Wacker-Tsuji oxidation of olefins (eq 23) and Pd-mediated homocoupling of aryl boronic acids (eq 24) were completed on the interior of PDMS thimbles. In each of these reactions, the catalyst was PdCl2 and a polar protic solvent was used on the interior of the thimbles. The products were fiuxed to the exterior of the thimbles by the addition of CH2CI2 or hexanes to the exterior. Isolated yields were 56-93%. 

Ag20 was also found to promote cross-coupling between aryl-boronic acids and terminal alkynes (eq 27). Reactions worked well with both electron-donating and electron-withdrawing groups present on the boronic acid. Alkylboronic acids, on the other hand, were inert under these conditions and furnished only the homocoupled product. Electron-deficient alkynes, which under standard conditions fail to react, produced the desired coupled products in good yields. 

The Suzuki coupling has been successfully employed in the homocoupling of aryl derivatives, and two examples are shown in Scheme 12.33. As outlined in Equation 12.33-1, the reaction has been applied to the preparation of binaphthyls [114], Wong prepared thiophene derivatives via borox-ine 153. The phenanthrene derivative 155 serves as ligand for palladium and helped to regenerate the catalytically active Pd(0) species (Eq. 12.33-2) [115], In both cases, the boronic ester is converted to the corresponding boronic acid under the basic reaction conditions. As the installation of the... 

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