Aryllithium reagents biaryls

Arylcopper intermediates can be generated from organolithium compounds, as in the preparation of cuprates.95 These compounds react with a second aryl halide to provide unsymmetrical biaryls in a reaction that is essentially a variant of the cuprate alkylation process discussed on p. 680. An alternative procedure involves generation of a mixed diarylcyanocuprate by sequential addition of two different aryllithium reagents to CuCN, which then undergo decomposition to biaryls on exposure to oxygen.96 The second addition must be carried out at very low temperature to prevent equilibration with the symmetrical diarylcyanocuprates. 

The homo-coupling Ullmann reaction can be performed at room temperature or bellow by metallation of suitable substrates such as relatively acidic arenes or aryl hahdes with the -BuLi (or other strong bases) followed by addition of Cu(II) salts [28], Aryllithium reagent undergoes transmetallation with the Cu(II) salts to form diarylcopper(II) which is prone to rapid reductive elimination of the biaryl with extrusion of a copper nrirror. Benincori and coworkers [28] described an efficient procedure for the homo-coupling reaction of bromide 21 via diarylcopperfll) 22 under very mild reaction conditions, where the 3,3 -bibenzo[b]thiophene (23) was obtained in 56% yield. Scheme 4. 

Aryllithium reagents derived from simple arenes react with aryl halides in the presence of different nickel or palladium catalysts to give biaiyls in moderate to good yields. Thus the reaction of 2-lithiatcd 1-mcthylindolc 117 and 4-iodoanisolc (39) resulted in the respective biaryl 118 in 68% yield [12], Scheme 3. 

Although the SM reactions of haloferrocenes proceed rather slow, they are clean and chemoselective, providing the production of arylferrocenes which would be hardly accessible by other strategies. Several papers regarding improved and alternative technics for performing the Suzuki-Miyaura reactions have been published. A useful procedure involves transmetallation of (half amount) simple aryl halides with only 0.5 eq. of -BuLi, followed by quenching the resulted aryllithium reagent with trimethyl borate [88], Thus obtained arylboronic ester is coupled under the standard SM conditions method A) with an excess of parent aryl halide to furnish the symmetrical biaryl in fair yields. For example, 2-bromoanisole (218) was coupled by this way to afford 2,2 -dimethoxybiphenyl (92) in 56% yield [88], Scheme 36. 

Figure 1.48 A cross-section of results for the Pd-catalyzed synthesis of hindered biaryls using aryllithium reagents as reported by Feringa and coworkers [182].

Biaryl Synthesis with the Hindered Aryllithium Reagent, 2,6-Dimethoxyphenyllithium Catalytic Synthesis of 1,3-Dimethoxy-2-(1-naphthyl)benzene ... 

Unsymmetrical coupling of vinylic, alkynyl, and arylmercury compounds was achieved in moderate-to-good yields by treatment with alkyl and vinylic dialkylcopper reagents (e.g., PhCH=CHHgCl -t- Mc2CuLi PhCH=CHMe). Unsymmetrical biaryls were prepared by treating a cyanocuprate ArCu(CN)Li (prepared from ArLi and CuCN) with an aryllithium Ar Li. ... 

Similar variants of Ullmann-type reactions based on intra- as well as intermolecular homo-couplings of various arylmetallic reagents, beside aryllithiums, also aryl Grignard reagents, arylzincs etc. with copper(II) salts give intermediate diarylcopper(Il) reagents which subsequently produce respective biaryls, usually in good yields, see Chapter 7. 

Variuos arylmetallic reagents as formal aryl-carbanion donors can be oxidatively coupled to give symmetrical biaryls. Some of these reactions have been described at a relevant place in the text. For example, the homo-coupling of aryllithiums with copper(II) chloride is well known modem alternative to the Ullmann reaction, see Chapter 2 [10,11]. Herein, analogous reactions with other arylmetallic reagents are described. Scheme 5. 

Somewhat different approach to the synthesis of orf/jo-substituted biaryls was reported by Julia and coworkers [48]. They found that tert-butylsulfonyl group can be replaced by aryl-carbanions originated from Grignard reagents in Ni(acac)2-catalysed process with formation of biaryls in moderate to high yields. Aryl-tert-butylsulfones are readily obtained by quenching the aryllithiums with di-tert-butyl disulfide, followed... 

Unsymmetrical biaryls can be piepared by reaction of aryl Grignard reagents or aryllithiums with aryl halides in the presence of an arylpalladium intermediate which serves as a catalyst. The arylpalladium catalyst is generated from the halide by oxidative addition. 

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