Formation of methylmagnesium or coordinated ylide? Competition between decarboxylation of acetate and betaine ligands in CH3CO 2MgO2CCH2X(CH3)2 (where X = NCH3 and S)

Abstract

Decarboxylation of the ligands in the magnesium cations [CH 3CO2MgO2CCH2X(CH3) 2]+ (where X = NCH3, la; and S, 1b) can give rise to either an organometallic cation [CH3MgO2CCH 2X(CH3)2]+, 2, or a coordinated ylide [CH3CO2MgCH2X(CH3) 2]+, 3. Collision-induced dissociation of a precursor complex in which the acetate ligand was 13C labeled allowed the direct determination of the relative ratio of formation of 2 and 3 via a comparison of the losses of 13CO2 and CO2. For the nitrogen betaine, la, both 2a and 3a are formed, in a ratio of ca 1:3, respectively. In contrast, for the sulfur betaine, lb, the coordinated ylide, 3b, is almost exclusively formed. These striking differences are confirmed b..