AbstractThe 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.