Understanding Bond Formation in Polar One-Step Reactions. Topological Analyses of the Reaction between Nitrones and Lithium Ynolates

The mechanism of the reaction between nitrones and lithium ynolates has been studied using DFT methods at the M06-2X/cc-pVTZ/PCM=THF level. After the formation of a starting complex an without energy barrier, in which the lithium atom is coordinated to both nitrone and ynolate, the reaction takes place in one single kinetic step through a single transition structure. However, the formation of CC and CO bonds takes place sequentially through a typical two-stage, one-step process. A combined study of noncovalent interactions (NCIs) and electron localization function (ELFs) of selected points along the intrinsic reaction coordinate (IRC) of the reaction confirmed that, in the transition structure, only the CC bond is being formed to some extent, whereas an electrostatic interaction is present between carbon and oxygen atoms previous to the formation of the CO bond. Indeed, the formation of the second CO bond only begins when the first CC bond is completely formed without formation of any intermediate. Once the CC bond is formed and before the CO bond formation starts the RMS gradient norm dips, approaching but not reaching 0, giving rise to a hidden intermediate