Chlorination results in a more stable intermediate than bromination and, in accordance with the Hammond postulate, proceeds through an earlier TS (as shown by smaller TS distortion energies in Figure and smaller pyramidalization values at the reacting carbon atom (Fig S18 )). Given the electronic similarity between C3-and C5-positions in the Zincke imine, and the absence of appreciable intermolecular non-covalent interactions (Fig S17 ), there is little regioisomeric preference in the early chlorination TS. Bromination proceeds through a later TS that incurs larger substrate distortion and strongly favors addition at C3 (ΔΔEdist = 4.6 kcal mol -1 . This difference in distortion energies reflects a more stable developing a,b-unsaturated iminium ion in TS-I-C3-NBS versus TS-I-C5-NBS (ΔΔG ‡ = 2.6 kcal mol -1 ). In contrast, the high regioselectivity of iodination (ΔΔG ‡ = 3.4 kcal mol -1 ) results from a rate-and selectivity-determining deprotonation (TS-II). Restoring planarity in TS-II-C5-NIS results in increased A 1,3 strain between the enamine carbon substituent and the iodide (Figure ). The bond and dihedral angles in the C5-iodinated product clearly reflects this developing strain (Figure ).