Stable organozincs for Negishi

14 September 2012 - Orgo

Organozinc reagents knochel 2012  The Knochel lab is reporting air-stable organozinc compounds for Negishi coupling reactions (DOI). Here are the details. Key is the transmetallation of an organomagnesium compound with a zinc salt. The Grignard reagent is prepared via a turbo Grignard. The success of the Mg - Zn transmetallation is found to depend on the nature of the zinc counter ion. A pivaloyl ion is effective because zinc is coordinated to the two oxygen atoms. The organozinc compound is air-stable for up to 4 hours and can even be used with technical-grade THF.
This stability was tested in a Negishi coupling with Pd(OAc)2/X-Phos. The lithium chloride is essential for the reactivity of the TMPMgCl reagent and for the dissolution of Zn(OPiv)
in THF. For the remainder of the reaction its presence is a nuisance due to its hydrophilicity.