Yin's related study, the addition of a base (e.g., Barton's base) to a mixture of Cu(CH3CN)4PF6, bidentate phosphine ligand, 9l and secondary phosphine results in a complicated and uncharacterizable mixture. In stark contrast, we found that addition of various bases (e.g., DBU, t BuOK, and Et3N) to a mixture of Cu(CH3CN)4PF6, (R,R)-i Pr-DuPhos, and diphenylphosphine results in immediate and selective formation of a new species on the basis of 31 P NMR. This species was isolated and further characterized by 1 H NMR, 31 P NMR and mass spec analysis and determined to be a Cuphosphido dimer 7 (Figure ). In this dimer, the lone pair of the X-type phosphido ligand of 5 acts as an L-type ligand to form a η 2 bridge to another unit of Cu-DuPhos mono-chelate 4 via a three-center four-electron bond. We propose this species to be the catalyst resting state. In line with this observation, our kinetics studies, and similar observations made by Appel and coworkers while studying copper hydride catalysis, we propose the DBU acts as not only a base but also an L-type ligand. The DBU undergoes ligand substitution with the dimeric resting state 7 to liberate the catalytically active monomeric Cu-phosphido 5. This hypothesis is further supported by P NMR studies which reveal decomposition of the dimer 7 with concomitant formation of an unidentified species in the presence of large excesses of DBU. Performing the transformation with isotopically labelled d-2a revealed that the hydrophosphination proceeds via a formal synaddition of the P-D bond across the cyclopropene double bond (Figure ). With this mechanism in mind, we examined the possibility of setting a phosphorus stereocenter via a dynamic kinetic asymmetric transformation (DyKAT). As outlined in the proposed mechanism, pyramidal inversion of the secondary phosphine is impractically slow at room temperature while epimerization of Cu-phosphido 5 with 5' occurs rapidly. We subjected unsymmetrically substituted phosphine 2k to the reaction conditions to test this hypothesis and observed a 3:1 dr for the cyclopropyl phosphine products and an er of 96:4 and 88:12 for the major and minor diastereomer, respectively, (Scheme 1). Based on our prior results, we assume effective desymmetrization of the cyclopropene occurs with relatively low control over the configuration of the phosphorus stereocenter; these results are in line with a recent report from Glueck using a similar catalyst for asymmetric alkylation of secondary phosphines. 15d,15e On the basis of 31 P NMR studies, Glueck and coworkers were able to observe both Cu-phosphido diastereomers and measure the relative diastereomeric ratio (3:1 dr). They found that this Cu-phosphido diastereoselectivity correlates to the observed enantioselectivity for the alkylation step (3:1 er). In our case, we observe a single dimeric resting state, which thwarted attempts at a similar analysis for the diastereoselectivity of Cu-phoshido formation.