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 C–C and C–O 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 C–C bond is being formed to some extent, whereas an
electrostatic interaction is present between carbon and oxygen atoms
previous to the formation of the C–O bond. Indeed, the formation
of the second C–O bond only begins when the first C–C
bond is completely formed without formation of any intermediate. Once
the C–C bond is formed and before the C–O bond formation
starts the RMS gradient norm dips, approaching but not reaching 0,
giving rise to a hidden intermediate.