Di-Anionic Alkyl Zincates as Transmetalators in Negishi Cross-Coupling

Generally, it is thought that the intermediate of the Negishi cross-coupling is a mono valent zincate complex [RZnX₂]^-. However, Organ et al. have reported that di-anionic alkyl zincates [RZnBr₃]^2- are the intermediate of alkyl-alkyl Negishi cross-coupling reactions using alkyl bromides.¹⁾ In this report, pure EtZnBr₃^2-• (Bu₄N⁺)₂ was prepared from tetrabutylammonium bromide and diethyl zinc following the Clyburne protocol,²⁾ and used for the cross-coupling in THF. The coupling took place in high yields without a salt additive and a high dielectric co-solvent. In addition, [RZnBr₂]^- was formed from [RZnBr₃]^2- with 1eq. of ZnBr₂ and used for the cross-coupling, which was stopped completely. These results provide evidence that the di-anionic zincates are the active transmetalating species.