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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 EtZnBr32-• (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.
