Gas-phase synthesis and reactivity of the organomagnesates [CH3MgL2]- (L = Cl and O2CCH3): from ligand effects to catalysis.

Multistage mass spectrometry experiments combined with density functional theory (DFT) calculations were used to examine the gas-phase synthesis and ion-molecule reactions of the organomagnesates CH(3)MgL(2) (L = Cl and O(2)CCH(3)). Neutral species containing an acidic proton (HX) react with the CH(3)MgL(2) ions via addition with concomitant elimination of methane to form XMgL(2) ions. Kinetic measurements combined with DFT calculations revealed reduced reactivity of CH(3)Mg(O(2)CCH(3))(2) toward water, caused by the bidentate binding mode of acetate, which induces overcrowding of the Mg coordination sphere. The CH(3)MgL(2) ions reacted with (i) aldehydes with enolizable protons via enolization rather than the Grignard reaction and (ii) CH(3)CO(2)H to complete a catalytic cycle for the decarboxylation of acetic acid. Other electrophilic reagents such as pivaldehyde, benzaldehyde, methyl iodide, and trimethylborate are unreactive. DFT calculations on the competition between enolization and the Grignard reaction for CH(3)MgCl(2) ions reacting with acetaldehyde suggest that while the latter has a smaller barrier, it is entropically disfavored.