To support the proposed mechanism and pursue more elusive details for the enantioselective hydrophosphination of cyclopropenes, we performed a density functional theory (DFT) analysis on the title reaction of 3-methyl-3phenylcyclopropene (1a) and diphenylphosphine (2a) catalyzed by Cu-i Pr-Duphos complex (4). DFT computations were performed utilizing ωB97XD/def2TZVP PCM(toluene)// B97D/def-2SVP level of theory as implemented in Gaussian 16. Thermal corrections were computed using Grimme's quasi-rigid rotor harmonic oscillator approximation. IRC calculations were performed to confirm that transition structures (TSs) connected minima along the potential energy surface. A thorough exploration of the catalyst conformational space was performed using CREST. In addition, a detailed exploration of TS conformations was performed for the selectivity-determining step (see SI page S33 for details) Our computational study sought to identify both the turnover-limiting and stereoselectivity-determining steps of the catalytic cycle and to explain the origins of experimentally observed stereoselectivity. The potential energy surface for the lowest energy pathway resulting from our investigation is shown in Figure . The reaction is initiated via the coordination of diphenyl phosphine 2a to Cu-Duphos to give the Cu-HPPh2 + complex 5aH + . Deprotonation of this cationic complex 5aH + by DBU is a low barrier step (TSDep, ∆G ‡ = 4.8 kcal/mol) that leads to the reversible formation of Cu-phosphido intermediate 5a (chosen as the reference structure in the reaction coordinate). Following deprotonation, 5a binds cyclopropene 1a via 𝜋coordination transition structure TSCoord (∆G ‡ = 15.1 kcal/mol) to generate Cu-alkene complex IntCoord. The subsequent 1,2migratory insertion into the cyclopropene 𝜋-bond (TSMI) has a free energy barrier of 19.2 kcal/mol relative to 5a and represents a highly exothermic step in the pathway, which results in a stable, significantly lower energy copper coordinated cyclopropyl phosphine intermediate 6a -residing 12.5 kcal/mol below the monomeric resting state 5a. The reaction pathway then proceeds through a facile stereoretentive protodemetalation (TSPDM, ∆G ‡ = -6.3 kcal/mol relative to 5a) -a copper mediated protonation from DBU occurs syn to the diphenylphosphine substituent -to concomitantly regenerate 4 and afford the synhydrophosphinated cyclopropene product 3aa (transferred proton shown in blue). This protodemetalation step (TSPDM) proceeds through a unique three-center, two-electron bond transition structure (C-Cu bond-breaking is 2.11 Å and C-H bond forming distance is 1.42 Å), consistent with the exclusive syn addition observed when the reaction is performed with d-2a (Figure and Figure2C) (vide supra). Incidentally, several alternative pathways for protodemetalation were also explored computationally (See SI page S34). A syn-protodemetalation TS analogous to TSPDM, whereby copper mediates the proton transfer from a protonated PPh2 on the adjacent carbon, was found to be 25 kcal/mol higher in energy than TSPDM. Analysis of the potential energy surface indicates that TSMI is the enantio-and diastereoselectivity-determining step in the hydrophosphination reaction. To investigate catalystsubstrate interactions that dictate the enantioselectivity in this reaction, a conformational search was conducted on TSMI for transition structures that lead to the formation of both the major and minor enantiomers of 3aa. The lowest energy transition structure which leads to the major enantiomer (Figure , TSMI-Major) is favored by 2.1 kcal/mol with respect to the lowest energy structure for the minor enantiomer (Figure , TSMI-Minor). At 298 K, a ∆∆G ‡ of 2.1 kcal/mol corresponds to a predicted er of 97.5:2.5, which is in excellent agreement with the experimental er of 98:2 (∆∆G ‡ of 2.3 kcal/mol) for the title reaction.