In the present work, the electron density flows involved throughout the progress of the four reaction pathways associated with the intramolecular [3 + 2] cycloaddition of cyclic nitrones Z-1 and E-1 are analyzed using the bonding evolution theory. The present study highlights the nonconcerted nature of the processes, which can be described as taking place in several stages. The first stage consists in the depopulation of the initial C N and C C double bonds to render the N lone pair and the corresponding C N and C C single bonds, and these electronic flows initiate the reactions. The C C and C O sigma bond formations take place later on, once the transition states have been overcome. Along the bridged pathways, the C C bond formation process precedes the O C bond formation event, although, along the fused paths, the O C bond formation process occurs first and the formation of the C C bond is the last electronic flow to take place. Finally, curly arrow representations accounting for the timing of the electron flows are obtained from the bonding evolution theory results. K E Y W O R D S bonding evolution theory, curly arrows, cyclic nitrones, intramolecular [3 + 2]cycloaddition

The molecular mechanism of the reactions between four carbonyl oxides and ammonia/water are investigated using the M06‐2X functional together with 6‐311++G(d,p) basis set. The analysis of activation and reaction enthalpy shows that the exothermicity of each process increased with the substitution of electron donating substituents (methyl and ethenyl). Along each reaction pathway, two new chemical bonds C−N/C−O and O−H are expected to form. A detailed analysis of the flow of the electron density during their formation have been characterized from the perspective of bonding evolution theory (BET). For all reaction pathways, BET revealed that the process of C−N and O−H bond formation takes place within four structural stability domains (SSD), which can be summarized as follows: the depopulation of V(N) basin with the formation of first C−N bond (appearance of V(C,N) basin), cleavage of N−H bond with the creation of V(N) and V(H) monosynaptic basin, and finally the V(H,O) disynaptic basin related to O−H bond. On the other hand, in the case of water, the cleavage of O−H bond with the formation of V(O) and V(H) basins is the first stage, followed by the formation of the O−H bond as a second stage, and finally the creation of C−O bond.