Dissociation of Mg(ii) and Zn(ii) complexes of simple 2-oxocarboxylates – relationship to CO2 fixation, and the Grignard and Barbier reactions

Abstract

Three deprotonated 2-oxocarboxylic acids, glyoxylate, pyruvate, and 2-oxobutyrate (RCOCO₂⁻, R = H, CH₃, CH₃CH₂) have been associated with MgCl₂ and ZnCl₂ to generate [RCOCO₂MCl₂]⁻ (M = Mg, Zn) complexes. Upon collision-induced dissociation these complexes all undergo efficient eliminations of CO₂ and CO, via an intermediate [RCOMCl₂]⁻ product, to ultimately give [RMCl₂]⁻ products. The pyruvate and 2-oxobutyrate complexes also undergo efficient elimination of HCl to produce the enolate-metal complexes [H₂CCOCO₂MCl]⁻ and [H₃CHCCOCO₂MCl]⁻. These enolate complexes have binding motifs reminiscent of the active centres in some CO₂-fixating enzymes and the CO₂ reactivity of these enolate complexes was therefore investigated, but only adduct formation could be observed. Quantum chemical calculations predict the magnesium complexes to decarboxylate without reverse barriers to carboxylation, and the zinc complexes to decarboxylate with considerable reverse barriers. The subsequent CO loss occurs with reverse barriers in all cases. The HCl loss is also predicted to occur overall without reverse barriers for both metals.