Both enantio-and regioselectivity achieved by using L1 are well reproduced with this mechanism. As shown in Fig. , TS-R-allylic-RA and TS-S-benzylic-RA are 3.9 kcal/mol and 5.8 kcal/mol less stable than TS-S-allylic-RA, respectively. These calculated results agree well with experimental findings on enantioselectivity (96:4 er) and regioselectivity. The enantioselectivity likely arises from stronger steric repulsions in transition state structures leading to unfavored products compared with TS-S-allylic-RA. Indeed, changing the ligand from L1 to a simplified ligand L4 leads to decreased enantioselectivity (Table. S1-S4), and the calculated results show a consistent trend (Fig. ), which further supports the reliability of the proposed mechanism. Our DFT simulations also explained the importance of BArF‾ as the counter ion for separating Cu I center and Cl‾ ion. In Fig. , if the Cu I center is coordinated by a Cl‾ ion, starting from Catalyst-Cl, the overall reaction to afford 3a is still downhill. However, the transition state TS-S-Cl necessitates an energy barrier of 18.2 kcal/mol for reductive elimination to afford 3a while recycling Catalyst-Cl. The significantly high energy barrier handicaps the overall reaction so that the yield is low (16%) without NaBArF, as shown in Fig. . Besides the proposed reaction pathway, other possible coordination structures are also tested (Fig. ). However, energy changes for forming such structures are significantly more positive compared with that of TS-III-OSS.