Are sp Lithiated Carbons More Nucleophilic than sp² or sp³ Ones? A Comparative DFT Study of the Condensation of Propynyllithium Aggregates on Formaldehyde

The interaction between 1-propynyllithium, taken as a model of sp organolithium compound, and formaldehyde has been investigated with DFT theoretical methods. The unsolvated monomer, homogeneous dimer, trimer, tetramer, and hexamer have been considered, as well as the mixed aggregates with lithium dimethylamide in various oligomeric forms. In most cases, the separate entities, their docking complexes, the transition states, and the condensation products have been characterized. Overall, the general reaction scheme remains the same whatever the hybridization and the aggregation. However, the dimeric sp nucleophiles are expected to be more reactive (at least in kinetic conditions), while monomeric sp³ entities would be the best nucleophiles in kinetic and thermodynamic conditions, even if its docking is the least exothermic. This work also suggests that the aggregation plays a relatively limited role on the model reaction.