Biaryls, formation arylzinc compounds

The slow addition of the arylzinc compound to a solution containing both an aryl hahde, Ar X, and the palladium complex furnished the biaryl in good to excellent yields. The very reactive Pd(0) complex was likely formed in situ by reduction of the starting palladium(II) complex by ArZnX. The reactions were very rapid (ca 1 or 2 h) compared to most usual Pd-catalyzed reactions involving ArZnX (ca 24 h). The reduction of Pd(II) could account for the formation of a small amount of Ar-Ar (2-5%) in the last non-electrochemical step while no homocoupling of Ar X occurred. 

Figure 5.20. Biaryl formation from resin-bound aryl bromides and arylzinc compounds [32,204], and by oxidative coupling of phenols [205],...

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... 

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],... 

---