The Role of LiBr and ZnBr₂ on the Cross‐Coupling of Aryl Bromides with Bu₂Zn or BuZnBr

The impact of LiBr and ZnBr 2 salts on the Negishi coupling of alkylZnBr and dialkylzinc nucleophiles with both electron‐rich and ‐poor aryl electrophiles has been examined. Focusing only on the more difficult coupling of deactivated (electron‐rich) oxidative addition partners, LiBr promotes coupling with BuZnBr, but does not have such an effect with Bu 2 Zn. The presence of exogenous ZnBr 2 shuts down the coupling of both BuZnBr and Bu 2 Zn, which has been shown before with alkyl electrophiles. Strikingly, the addition of LiBr to Bu 2 Zn reactions containing exogenous ZnBr 2 now fully restores coupling to levels seen without any salt present. This suggests that there is a very important interaction between LiBr and ZnBr 2 . It is proposed that Lewis acid adducts are forming between ZnBr 2 and the electron‐rich Pd 0 centre and the bromide from LiBr forms inorganic zincates that prevent the catalyst from binding to ZnBr 2 . This idea has been supported by catalyst design as chlorinating the backbone of the NHC ring of Pd‐PEPPSI‐IPent to produce Pd‐PEPPSI‐IPent Cl catalyst now gives quantitative conversion, up from a ceiling of only 50 % with the former catalyst.