Formation of Organozincate Anions in LiCl-Mediated Zinc Insertion Reactions

Tetrahydrofuran solutions of the products formed in LiCl-mediated zinc insertion reactions into various organic halides RHal were analyzed by anion-mode electrospray ionization (ESI) mass spectrometry. In all cases, organozincate anions were observed. The reactions with RHal, Hal = Br and I, yielded predominantly mononuclear complexes, such as ZnRHal 2 − and ZnRHalCl −, whereas for the reaction with benzylchloride abundant polynuclear organozincates, such as Zn 2 Bn 2 Cl 3 − and Zn 3 Bn 3 Cl 4 −, were detected. The equilibria governing the stoichiometry and aggregation state of these complexes appear to be mainly controlled by the nature of the halide ions present in solution. It seems likely that the formation of organozincate complexes also changes the reactivity of the organozinc species, thus offering a rationale for the recently found pronounced effect of LiCl in organozinc chemistry. Additional preliminary studies suggest that organozincate anions as well as organozinc cations may moreover form in the absence of LiCl.