In order to identify which of the three pathways is operative, we modeled each pathway leading to the major enantiomer of the cis-epoxide 24a for both the boron-and the aluminum catalysts. For both catalyst systems, among the two modes of Lewis acid activation (BLA and LA) it was found that the BLA pathway was favored over the LA pathway (See SI for details). Therefore, our next step was to ) and diazoacetamide 1a in the BLA pathway, which is the lowest energy stationary point in either pathway for both catalysts. The preferred pathway for each catalyst is highlighted in yellow and the selectivity-determining step (SDS) is indicated. It is worth noting that a qualitatively similar free energy profile, where the ring-closing step was enantiodetermining, was previously established via experimental and computational studies of the Brønsted acid catalyzed aziridination reaction of imines and diazoacetates developed in our lab. Selectivity-determining transition structures for the (S)-VANOL borate catalyzed epoxidation reaction. Based on our evaluation of the mechanism of the title reaction, in order to probe the origin of enantioselectivity and thereby the origin of enantiodivergence in these systems, we need to evaluate the relative energies of TSRC leading to both enantiomers of cis-24a for both catalyst systems. We will first discuss our results from the reaction with the boron catalyst. We have already identified the lowest energy TS leading to the major enantiomer (R,R) of cis-24a via the BLA mechanism (discussed in Figure ). The fine details of this TS -TSRC-B-BLA-maj -is shown Figure . It is worth noting here that it was during the extensive conformational search performed to locate TSRC-B-BLA-maj that we identified existence of the LA pathway (discussed in Scheme 4). Also shown in Figure is the lowest energy TS leading to the (R,R)-24a in the LA pathway (TSRC-B-LA-maj), which is 4.9 kcal/mol higher in energy than TSRC-B-BLA-maj. Following this, we performed a thorough conformational search to locate TSRC leading to the minor enantiomer (S,S) of cis-24a to enable a theoretical prediction of the enantioselectivity observed in this reaction. Shown in Figure is the lowest energy transition structures leading to (S,S)-24 in the BLA (TSRC-B-BLA-min) and LA (TSRC-B-LA-min) pathways. The BLA pathway is found to be slightly favored over the LA pathway for the minor enantiomer by 1.8 kcal/mol. Therefore, the predicted enantioselectivity for the reaction of boron catalyst is obtained by a comparison of the relative energy of the lowest energy TSs leading to the major enantiomer (TSRC-B-BLA-maj resulting in (R,R)-24a highlighted in green in Fig. ) and the minor enantiomer (TSRC-B-BLA-min resulting in (S,S)-24a highlighted in red in Fig. ). The energy difference between these two TSs is 3.2 kcal/mol, a value that is in excellent agreement with the > 99% ee observed experimentally for this reaction. Analysis of these highlighted TSs reveal that both these TSs have identical bond-forming (rO-C = 2.12 Å) and bond-breaking (rC-N = 1.79 Å) distances for the SN2-like ring-closing event. Additionally, both these TSs are characterized by a short Hbonding interaction between the free phenolic OH of the VANOL ligand and one of the other oxygen atoms bound to boron -the interaction that is responsible for BLA activation. However, these TSs are fundamentally different in the nature of catalyst-substrate interactions that are involved in transition state stabilization -TSRC-B-BLA-maj is stabilized by a short, strong CH … O interaction (1.99 Å) between the acidic CH of 1a and one of the catalyst oxygen atoms and an intramolecular H-bonding interaction between the NH of 1a and the aldehyde oxygen (2.23 Å); on the other hand, TSRC-B-BLA-min is stabilized by a strong Hbonding interaction between the NH of 1a and one of the catalyst oxygen atoms (1.88 Å) and a possible CH-pi interaction between the acidic CH of 1a and the aromatic rings of the VANOL ligand.