Arylzinc reagents, Negishi coupling

Additionally, the Negishi cross-coupling of arylzinc reagents and aryl iodides using Pd(dba)2 and the cationic phosphine ligand 4 was performed in a biphasic system of an imidazolium salt 1 and toluene [34]. 

Teraryls are readily synthesized from l,4-his(iodozincio)benzene by consecutive Negishi coupling reactions. Such is feasible because of the different reactivity of the two types of arylzinc reagents, products of the first coupling are less reactive toward the Pd catalyst. ... 

Later, in 2014 McCann and Organ investigated the role of halide additives in the Negishi reaction catalyzed by 29 involving sp -hybridized zinc reagents. Diarylzinc compounds were found to couple in low dielectric solvents (i.e., THF) with zero salt present whereas arylzinc halides failed to couple in THF alone, requiring the addition of salt. Further, unlike alkyl-alkyl coupling there was no evidence to support the... 

The use of the well-defined PEPPSI-IPr precatalyst led to a significant improvement with respect to the rate and the substrate scope of the Negishi alkyl-alkyl coupling reactions (Scheme 3.62). This precatalyst can also be used to promote the cross-couphng of alkyl or aryl halides and sulfonates with arylzinc chloride reagents at room temperature by judicious choice of the solvent and additive (LiCl or LiBr) [236]. 

---