Aryl derivatives homocoupling reactions

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). 

Interestingly, the chemoselectivity of this catalytic system changed dramatically when using aryl oxazolines along with substituted allyl acetate 104. Thus, oxidative homocoupling reactions occurred under these reaction conditions (Scheme 9.37) [50]. Imidazole, pyrazole or thiazole derivatives could also be efEciently homocoupled. 

There has been a review of palladium-catalysed carbonylative coupling reactions of aryl halides with carbon nucleophiles in the presence of carbon monoxide. It has been shown that rhodium is an efficient catalyst for the homocoupling reaction of arylzinc compounds in the presence of 1 atm of carbon monoxide to give diaryl ketones. Under similar conditions, palladium and nickel catalysts yield biaryls. The beneficial catalysis by rhodium is likely to derive from the ease of migration of the aryl ligand to carbon monoxide in the rhodium(III) intermediate. A rhodium catalyst has also been used in the formation of indole-3-carboxylates by reaction of indoles with alcohols in the presence of carbon monoxide. The catalytic cycle. Scheme 5, is likely to involve metallation of the indole at the 3-position, followed... 

In the presence of a strong base e.g., NaH or BuOK) and in acetonitrile, (3-thienylmethyl)triphenylphosphonium salts undergo a homocoupling reaction to form ethenyldithiophenes that is faster than a Wittig reaction with aromatic ketones (Scheme 70). The coupling reaction was also extended to other (hetero)aryl derivatives. ... 

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... 

The use of imidazolium salts for in situ catalyst formation was shown to be optimal for the coupling of TMS-protected alkynes even with sterically demanding aryl bromides and avoids the formation of homocoupling-derived products. For this reaction, Nolan reported that the activation of chlorobenzene by this catalytic system was possible in moderate yield [125] (Scheme 6.41). 

A second example from the same group is the synthesis of an elaborate diethynyltriphenylene derivative (Scheme 7 Table 8,entries 12,13) [58].Zn/Pd-promoted homocoupling of a 4-iodo-l,2-dialkoxybenzene furnishes the desired tetraalkoxybiphenyl, an electron-rich aromatic system. Iron trichloride-catalyzed Friedel-Crafts arylation of the biphenyl derivative with dimethoxy-benzene furnishes an unsymmetrical triphenylene derivative. Deprotection, oxidation, and subsequent Diels-Alder reaction with cyclohexadiene is followed by catalytic hydrogenation and reoxidation. TMS-CC-Li attack on the quinone delivers the alkyne modules, treatment with SnCl2 aromatizes the six-mem-bered ring, while KOH in MeOH removes the TMS groups cleanly to give the elaborate monomer. 

Additionally, Suzuki-Miyaura cross coupling reactions can be performed on similar supported iodides. Aryl bromides could be employed in this reaction as well. After optimization of conditions, it turned out that a hydroxyl-derived IL was the best solvent for this reaction. Namely, reaction completion was obtained after 12 h at room temperature in [N11130H][NTf2] in the presence of 1% Pd(OAc)2 and using potassium carbonate as abase. Under these conditions, less than 1% of homocoupling product is observed and easily eliminated by washing with diethyl ether prior to transesterification with methanol. Overall, biphenyls were isolated under analytically pure form in 90-95% yields [126],... 

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... 

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