Regioselectivity in electrophilic halogenation is conventionally understood in terms of differences in frontier orbital coefficients, atomic charges, or nucleophilicity parameters . However, the electronic environments at the C3-and C5-positions of Zincke imine 2e are remarkably similar in terms of Fukui f -coefficients (0.24 vs 0.25), natural charges (-0.20 vs. -0.22), and HOMO coefficients (both 0.26), necessitating an alternative rationale for the high levels of C3-selectivity in this transformation (Figure ). Computed energy profiles (Figure ) for 2e reacting with NCS, NBS, and NIS suggest an irreversible overall reaction with kineticallycontrolled regioselectivity in each case, with DDG ‡ values in quantitative agreement with experiment. Breaking the extended conjugation in the Zincke imine to form positively-charged scomplex Int-I is endergonic, a high-energy intermediate for which Grel values increase for C-Cl, C-Br and C-I formation, respectively (9.0, 13.7, 19.7 kcal mol -1 ). These differences result from two distinct selectivity-determining regimes: an irreversible C-Hal bond-forming step (TS-I) determines regioselectivity for chlorination and bromination, whereas C-I bond formation is reversible, and the second deprotonation step (TS-II) dictates the regioselectivity .