Gas-Phase Formation and Fragmentation Reactions of the Organomagnesates [RMgX₂]⁻

Abstract

A range of mononuclear organomagnesates [RMgX₂]⁻ were generated in the gas phase by decarboxylation of the magnesium carboxylate precursors [RCO₂MgX₂]⁻ (R = Me, Et, Pr, <i>i</i>Pr, <i>t</i>Bu, vinyl, allyl, HCC, Ph, PhCH₂, PhCH₂CH₂; X = Cl, Br, I). The gas-phase formation and unimolecular reactivity of these organomagnesates were examined using a combination of experiments carried out in linear ion trap and triple-quadrupole mass spectrometers and DFT calculations. Halide loss was found to directly compete with decarboxylation in the formation of mononuclear [RMgX₂]⁻. However, sterically unhindered, stable R^– substituents and strong Mg–Cl bonds can be employed to facilitate the decarboxylation reaction at the expense of the halide loss channel. Thus, in the case of R = HCC, PhCH₂, decarboxylation is the main fragmentation pathway. The resultant mononuclear organomagnesates [RMgX₂]⁻ were mass-selected, and their unimolecular chemistry was examined. Four competing fragmentations were observed: bond homolysis, bond heterolysis, halide loss, and β-hydride transfer. Which of these competing reactions dominates depends on the nature of R and X. A conjugatively stabilized R^• allows the observation of [MgX₂]^•–, whereas the presence of a β-hydride generates [HMgX₂]⁻. Weaker Mg–X bonds (e.g., Br and I) promote the formation of X^– upon CID