A DFT Theoretical Analysis of Aldehyde Condensation Pathways onto Methyllithium, Lithium Dimethylamide, and Their Aggregates
Résumé
A DFT analysis of the condensation of monomeric methyllithium and lithium dimethylamide (LMA), as well as their homo and hetero dimers, on formaldehyde and acetaldehyde is reported. A stable complex, exhibiting a directional interaction between a lone pair of the oxygen on the aldehyde and a lithium, is first found. At this stage, the aldehyde carbonyl and the Li−X (X = C or N) bonds lie in the same plane. To proceed, the condensation reaction has to go through a transition state that mainly consists of a rotation of the aldehyde plane, placing it perpendicular to the C−C or C−N forming bond. The reaction then leads, in a strongly exothermic final step, to the addition product that is a lithium alcoholate or α-amino alcoholate, associating into an hetero-aggregate with the remaining moiety of the initial dimer. From the relative heights of the activation barriers, it appears that, for the heterodimer MeLi−LMA, the formation of the C−N bond should be kinetically favored over the C−C one, while the lithium ethylate resulting from the C−C binding is the thermodynamic product. A decomposition of the activation energy barriers has been carried out in order to determine the physicochemical forces responsible for the variation of the condensation activation barriers with the structure of the final species formed. The results obtained are discussed in relation with corresponding experimental data.