The highly trans‐stereoselective reaction of oxyphosphonium enolate (OPE) with N‐benzylidene‐4‐methoxybenzenesulfonamide (N‐BMS) in toluene, at room temperature leading to trans‐aziridine‐2‐carboxylate (trans‐Az 3), was theoretically studied using molecular electron density theory (MEDT) at the B3LYP/6‐31G(d) computational level to shed light on the energy transformation, selectivities, and mechanistic aspects. This domino process is initialized by adding of N,N,N′,N′,N″,N″‐hexamethylphosphanetriamine (HMPT) to methyl benzoylformate to generate OPE. Subsequently, nucleophilic addition of OPE to the C–N double bond in N‐BMS leads to the intermediate IN‐Ta. Finally, trans‐Az 3 is formed through a 3‐exo‐tet ring closure step as a result of the nucleophilic attack of the negatively charged nitrogen atom on the carbon atom bearing –OP (NMe2)3 in IN‐Ta. Analysis of the relative Gibbs free energies shows that the ring closure step is the rate‐determining step. By an investigation of the conceptual density functional theory reactivity indices, OPE and N‐BMS are classified as a strong nucleophilic and as a strong electrophilic species, respectively, which indicates that the additional step of OPE to N‐BMS should display a high‐polar character. An analysis of the calculated electrophilic and nucleophilic Parr functions at the reactive sites of reagents clarifies the regioselectivity observed experimentally along the C1–C2 bond formation process.

The highly trans stereoselective formation of cyclopropane derivative trans‐CP 6 via the reaction of ammonium salt (AS 2) with methyl vinyl ketone (MVK) in the presence of Na2CO3 was theoretically investigated within Molecular Electron Density Theory (MEDT) at the B3LYP/6‐31G(d) computational level to probe energetics, selectivities, and molecular mechanism. This domino reaction starts by nucleophilic substitution reaction between 1,4‐diazabicyclo[2.2.2]octane (DABCO) and phenacyl chloride (PC) to form AS 2 followed by a proton abstraction from AS 2 in the presence of Na2CO3 to yield ammonim ylide (AY 3). Subsequently, AY 3 nucleophilically attacks to the MVK to produce gauche betaine‐like intermediate IN‐Tg 2, which is converted into anti betaine‐like intermediate IN‐Ta through a C–C single bond rotation. Finally, trans‐CP 6 is generated along a ring‐closure step. Analysis of the relative Gibbs free energies shows that nucleophilic attack of AY 3 to MVK is the rate‐determining step (RDS). An analysis of the density functional theory (DFT)‐based reactivity indices permits that AY 3 and MVK are, respectively, classified as a strong nucleophile and as a strong electrophile, in clear agreement with the high polar character of the additional step while exploring of the electrophilic as well as nucleophilic Parr functions computed at the reactive sites of reagents elucidates the regioselectivity within the C1–C4 single bond formation process.