Table . Optimization of the enantioselective semipinacol rearrangement Previously we have shown that this approach is viable for conducting Wagner-Meerwein rearrangements that involve a 1,2-aryl migration. These studies unveiled a phenonium ion as a probable intermediate with subsequent regioselective nucleophilic fluorination finalizing the relocation process. Thus, given this reactivity, we postulated that a hydro-semipinacol rearrangement involving a 1,2-aryl shift was viable. The p-fluoro substituted substrate 1a was chosen for optimization due to the ease of quantitative analysis by F NMR spectroscopy. During these preliminary investigations, it was revealed that the yield of product (R)-2a was dependent on the amount of air in the reaction vessel and significant quantities of 4,4'-difluorobenzophenone were identified when the flask was not properly inerted. However, the presence of air did not impact enantioselectivity and this byproduct likely arises from the interception of the carbon-centered radical generated after the MHAT step by molecular oxygen. Based on our prior work we conducted a cobalt catalyst screen using N-fluoro-2,4,6-trimethylpyridinium tetrafluoroborate (Me3NFPY•BF4) as an oxidant and 1,1,3,3-tetramethyldisiloxane as the hydrogen atom source in chlorobenzene (Table ). The commercially available cobalt catalyst (R,R)-Co(II)-1, built from optically pure trans-1,2-diaminocyclohexane, afforded the desired product 2a albeit with a low enantiomeric ratio (Table , entry 1). Switching to (R,R)-Co(II)-2 with a 1,2-diphenyl ethane backbone led to an improved yield and enantioselectivity (Table , entry 2). Given the ease of modifying the 1,2-diphenyl motif, we then explored alternative salen ligands of this type. From this, it was clear that increasing the steric bulk of the aromatic rings was more beneficial than introducing electron-donating groups (Table , entries 3-5). Ultimately, 1,2-bis(mesityl)-containing catalyst, (R,R)-Co(II)-5, was superior affording the ketone (R)-2a in 95% yield with 99 : 1 er. We also examined conducting the reaction at room temperature rather than at 0 °C, but a slight decrease in enantioselectivity was observed (Table , entry 6). Lowering the catalyst loading to 3 mol% or increasing the reaction concentration to 0.2 M were both detrimental to yield within the same two-hour timeframe (Table , entries 7-8). Exploring a range of alternative solvents with (R,R)-Co(II)-5 demonstrated the importance of the reaction medium with large variations in efficiency and levels of asymmetric induction (Table , entries 9-12).