Lithium Organozincate Complexes LiRZnX2: Common Species in Organozinc Chemistry

We have used a combination of electrospray ionization (ESI) mass spectrometry, electrical conductivity measurements, and NMR spectroscopy to investigate the effect of LiCl on solutions of organylzinc halides RZnX (R = Bu, Bn, Ph; X = halogen) and dibutylzinc Bu2Zn in tetrahydrofuran. In the case of RZnX, the addition of LiX (X = Cl) leads to a steep rise of the ESI signal intensities of RZnX2- organozincate anions. At the same time, the electrical conductivities strongly increase and the NMR absorptions of the alpha-H atoms of BuZnX shift upfield. These results consistently point to the formation of lithium organozincates Li+RZnX2-. As the most common syntheses of RZnX reagents involve stoichiometric amounts of LiX salts, Li+RZnX2- complexes are supposedly widespread and may hold the key to understanding the marked effects of LiCl on the reactivity of organozinc halides. In contrast, we find Bu2Zn to have a much lower tendency to add LiCl and form ate complexes. This result is in line with the weak effect of LiCl on the reactivity of diorganozinc compounds reported in the literature.