Turning our attention back to the resting intermediate, A4, we delved deeper into the subsequent stages of catalysis, specifically the phase where the olefin insertion leads to aziridine formation. Because A4 is saturated, it releases CsOAc and reverts to the 16-electron complex A3, which can then react with an olefin. Due to the asymmetric structure of A3, it presents two distinct vacant sites for the olefin and this leads to an enantioselective C-N bond formation ( S A5 and R A5). This observation aligns with the experimental results indicating a 19~24 times higher yield of the (S)-product compared to the (R)-product (Figure , Entry 3, 4). Notably, our calculations estimate the energy difference between two transition states, S A5-TS and R A5-TS, to be 1.9 kcal/mol. Moreover, this migratory insertion (first C-N bond formation) was found to have the highest activation energy through the catalytic cycle. It likely serves as both the enantio-and rate-determining transition step.