On The Remarkably Different Role of Salt in the Cross-Coupling of Arylzincs From That Seen With Alkylzincs

The role of halide salt additives has been investigated in the Negishi reaction involving aryl zinc reagents. Diarylzincs readily transmetallate to Pd in relatively non‐polar media (e.g., THF) with zero salt present and coupling proceeds. Arylzinc halides (ArZnX) fail to couple in THF without salt, but do couple with it. However, unlike alkylzincs that form higher‐order zincates in order to facilitate transmetallation, all that is required with arylzincs in an increase in solvent dielectric as even ZnX2 works as an additive, which completely terminates alkylzinc coupling. Pass the salt! Alkyl‐ and arylzinc halides both require salt (e.g., LiBr, MgCl2) in order to undergo the Negishi cross‐coupling reaction—but not for the same reason: Whereas alkylzinc halides must form high‐order zincates to undergo transmetallation, in the aryl case the role of the salt is to increase solvent dielectric. Conversely, the analogous diarylzinc much more readily undergoes coupling without salt and in low dielectric solvents.