Using DFT calculations, we identified tetrahedral adduct transition state structures (S)-11aa and (R)-11aa and tetrahedral intermediate collapse transition state structures (S)-12aa and (R)-12aa that give access to enantiomers (S)-4aa and (R)-4aa (Figure ). We note that these structures were obtained after thorough consideration and a comprehensive search of all accessible conformers and rotamers for the aforementioned structures. The calculated addition and elimination transition state barriers (ΔG ⧧ (add) = 25.0 kcal/mol versus ΔG ⧧ (col) = 26.6 kcal/mol, respectively) should be surmountable under the reaction conditions. While both addition (ΔΔG ⧧ (S)vs(R) = 1.4 kcal/mol) and elimination (ΔΔG ⧧ (S)vs(R) = 1.4 kcal/mol) show a slight preference for the observed (S)-enantiomer, the energy barrier for the tetrahedral collapse is higher by ΔΔG ⧧ (add)vs(col) = 1.6 kcal/mol. At this point, we would like to address that upon addition of the electrophile, a second stereocenter is generated in the tetrahedral adduct. For instance, starting material (S)-1a reacting with electrophile 2a could form two tetrahedral adducts 10aa with (S, R) and (S, S) configuration for the sulfur and carbon centers, respectively. However, only the (S, R)-adduct is accommodated in the tetrahedral collapse transition state (S)-12aa due to formation of a 6-membered ring intermediate and proper alignment of the trifluoroethoxy group for elimination by proton transfer from quinuclidine. We suspect that the tetrahedral adduct formation is reversible and thus the other (S, S)adduct can revert to starting materials that eventually proceed through the collapse transition state (S)-12aa. Furthermore, the necessity to have a good leaving group seems to be in accordance with our electrophile scope evaluation. Thus, we conclude that the collapse of the tetrahedral intermediate is most likely the rate-and selectivity-determining step.