Aryllithium homo-couplings

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. 

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. 

Arylzincs are converted to symmetrical biaryls by copper(II) halides [16], as well as by iV-chlorosuccinimide [17]. The method is more important than the homo-coupling of aryllithiums since, in a number of cases, higher yields were obtained, presumably because of more effective transmetallation of intermediate diarylzincs to copper(Il). In this maimer, tetra-ort/zo-bromobiphenyl (434) was lithiated with n-BuLi, and subsequently converted to the zinc species 435, stable at low temperatures. The latter is reacted with copper(ll) chloride to biphenylene 436 in 72% yield, accompanied with formation of intermolecular coupling product 437 in very small amount [16], Scheme 8. In contrast, the direct homo-coupling of bis-lithiated compound 434 with copper(Il) chloride furnished the compound 436 in only 15% yield, with predominant formation of intermolecular coupling product 437 in 61% yield [16],... 

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