Guided by the mechanistic insights described above, we performed additional screening of the asymmetric system in reference to the previous best result (Table , entry 1). First, we tested whether a more sterically demanding ligand could prevent catalyst deactivation by slowing down steps A and B of Scheme 4. Ligands were selected based upon previous reports by Baran and Table . Final optimization of the asymmetric system a Percent yield determined by GC-FID with a calibration curve. b Enantiomeric ratio (er) determined by HPLC with a chiral stationary phase. Peaks reported in order of elution. coworkers. 32 Similar to Baran's observations, we observed a balance between steric demands and reactivity. While both Fe(dpm)3 and Fe(dibm)3 increased yield, Fe(dibm)3 additionally provided a slight boost in enantioinduction (Table , entries 2-3). This result indicates that iron may be involved in the enantiodetermining step, which is consistent with our earlier hypothesis that it is an iron-thiol complex that engages the transient radical. See SI for additional optimization of the metal catalyst.