Coordination of BF3 LA to the oxygen of formaldehyde drastically accelerates both reactions given the high electrophilic character of the BF3:formaldehyde complex. As a consequence, these reactions present a very low activation enthalpy, less than 2.2 kcal·mol-1, thus becoming competitive. In dioxane, the P-AE reaction is slightly favoured because of the larger polar character of the corresponding transition state (TS) structure.
Understanding the domino reactions between 1-diazopropan-2-one and 1,1-dinitroethylene. A molecular electron density theory study of the [3+2] cycloaddition reactions of diazoalkanes with electron-deficient ethylenes
How does the Global Electron Density Transfer Diminish Activation Energies in Polar Cycloaddition Reactions? A Molecular Electron Density Theory Study
ChemistrySelect 2016, 1, 6026 - 6039.
Electrophilic activation of CO2 in cycloaddition reactions towards a nucleophilic carbenoid intermediate: new defying insights from the Molecular Electron Density Theory.