Multi-configuration spin-coupled description of organometallic reactions: a comparative study of the addition of RMBr (M = Mg and Zn) to acetone

The large differences in the reactivity between methyl and allyl Grignard reagents and allylzinc bromide in their addition reactions to acetone are theoretically studied. The chemical nature of the transition states is revealed through multi-configuration spin-coupled (MC-SC) calculations. The additional spin-coupled configuration proves to be instrumental in the description of the electronic rearrangements that lead to the reaction. The resulting singly occupied overlapping orbitals indicate clearly that in all cases the transition states present a pronounced carbanion character. The greater reactivity of Grignard reagents is associated with a product-like nature of the transition state, while the zinc reagent presents a reactant-like transition state. Through the analysis of the wave functions around the transition state geometries, we are able to provide unique insights into the electronic inner workings of the alkylation process.