The carbamimidate nitrene precursor is key to the high ee exhibited in this system compared to more conventional sulfamates and carbamates. We performed density functional theory (DFT) calculations to investigate the origin of enantioselectivity and the effects of ligand (Figure ). In particular, we examined the difference between the modes of enantioinduction with L5, the most effective ligand with the carbamimidate nitrene precursor, and L1, which was the optimal ligand in asymmetric aziridinations of carbamates but gave low ee with N-Tces carbamimidates. The computations show that L5, with two indane arms and a cyclopropyl backbone, is substantially more rigid than L1. In the L5-supported Ag-carbamimidate nitrene complex (3a) the L5 ligand adopts a completely planar conformation, whereas the corresponding L1-supported complex has two conformers-a C2-symmetric conformer (4a) in which the oxazolines adopt an envelope conformation and the six-membered metallacycle is twisted but nearly planar, and a non-C2-symmetric conformer (4b) featuring a non-planar boat conformation of the sixmembered metallacycle. The boat conformer 4b is only 1.1 kcal/mol less stable than 4a because the steric repulsion between the ligand and the bucky Tces group on the carbamimidate is diminished in the non-planar geometry. On the other hand, with the more rigid L5-supported Ag-nitrene complex, the non-C2-symmetric boat conformer (3b) cannot be located. Constrained geometry optimization forcing the six-membered metallacycle into a boat conformation suggests the L5-supported 3b is about 11.9 kcal/mol less stable than the planar geometry 3a. Ligand rigidity also impacts the mode and effectiveness of enantioinduction in the stereoselectivitydetermining aziridination transition state (Figure , see Figure in the SI for less favorable transition state conformers). In the L5-supported transition states, the rigid ligand L5 maintains the C2-symmetric geometry with a planar six-membered metallacycle. In the most favorable transition state TS1 that leads to the observed major enantiomeric product (R)-2f, the Et-substituted alkenyl carbon is placed in a quadrant not occupied by the C2-symmetric L5 ligand. By contrast, in TS2, the transition state leading to the opposite enantiomer (S)-2f, the Et-substituted alkenyl carbon is placed in an occupied quadrant, leading to greater ligand-substrate repulsion. The steric repulsion in TS2 is evidenced by the relatively short C⋯H distance (2.78 Å) between an indane arm of the ligand and the Et on the alkene. Additionally, this repulsion results in a distorted asynchronous transition state geometry with a notably longer C-N distance (2.46 Å) with the Et-substituted alkenyl carbon than the other forming C-N bond (2.12 Å). TS2 is 1.9 kcal/mol less stable than TS1, which is consistent with experimentally observed 95% ee. By contrast, when the more flexible ligand L1 was used, the computed enantioselectivity (ΔΔG ‡ ) decreased to 1.4 kcal/mol. The L1 ligand in TS3 leading to the major enantiomeric product has a C2-symmetric planar geometry where the Etsubstituted alkenyl carbon is placed in the unoccupied quadrant. By contrast, L1 in TS4 leading to the disfavored enantiomer adopts the non-planar boat conformation. TS4 is only 1.4 kcal/mol less stable than TS3 because its non-planar geometry diminishes the steric repulsions between ligand and the bulky N-Tces group as well as the Et-substituted alkenyl carbon. The distance between the sulfonyl oxygen and the ligand (d(O⋯H)) is 2.73 Å in TS4, which is much longer than those in TS1-TS3 (<2.31 Å) with the C2symmetric planar ligand conformation. Taken together, the DFT calculations highlighted the importance of implementing a conformationally rigid BOX ligand (L5) to maintain the C2-symmetric steric environment. A more flexible ligand (L1) may distort to a non-C2-symmetric boat conformation to avoid steric repulsion with the bulky N-Tces group. This undesired conformational flexibility diminishes the stereochemical control in the aziridination transition states.