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

A range of mononuclear organomagnesates [RMgX2](-) were generated in the gas phase by decarboxylation of the magnesium carboxylate precursors [RCO2MgX2](-) (R = Me, Et, Pr, iPr, tBu, vinyl, allyl, HC equivalent to 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 equivalent to 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 beta-hydride transfer. Which of these competing reactions dominates depends on the nature of R and X. A conjugatively stabilized R-center dot allows the observation of [MgX2](center dot-), whereas the presence of a beta-hydride generates [HMgX2](-). Weaker Mg-X bonds (e g, Br and I) promote the formation of X- upon CID.