The use of asymmetric α,α-diarylallylic alcohols presented a unique opportunity to ascertain the migratory preference for electronically distinct aromatic rings in this Co-catalyzed hydro-semipinacol rearrangement. To do this, we performed the cobalt-catalyzed transformation using achiral Co(II)-6 catalyst on a set of racemic substrates (±)-3 with one phenyl ring unsubstituted and systemically varying the electronic character of the other aryl ring (Figure ). Examination of the relative isomeric product ratios [(±)-4 : (±)-5] by analysis of the crude reaction mixture by NMR spectroscopy revealed electron-rich arenes migrated preferentially. Indeed, the inclusion of this data in a Hammett plot gave an excellent linear correlation (R 2 = 0.986) with s values to give a r value of -2.57. The sign and magnitude of the r value suggests that the 1,2-migration occurs via a phenonium ion intermediate and provides further evidence that the organocobalt(IV) intermediate is akin to a free carbocation. Based on this outcome, we rationalized that a sufficiently electron-deficient arene would remain stationary even when competing with an unsubstituted phenyl ring. As anticipated, substrate (±)-3f bearing a 3-fluoro-4-trifluoromethyl substituted aromatic system afforded product (±)-5f in 88% yield as a single observable regioisomer with exclusive phenyl migration (Figure ). Having established the polar behaviour of the rearrangement we wanted to determine the oxidant responsible for the radicalpolar crossover event that generates the alkylcobalt(IV) intermediate. A control reaction verified that a stoichiometric quantity of Co(II)-6 in the absence of N-fluoro-2,4,6trimethylpyridinium tetrafluoroborate was not able to execute the rearrangement process returning only unreacted starting material. However, using 2 equivalents of a Co(III)-6 complex without any additional oxidant was able to convert tertiary alcohol (±)-3c into 0.4 : 1 mixture of ketones (±)-4c and (±)-5c (Figure ). This product ratio reflected the same outcome obtained under catalytic conditions used for the Hammett study implying the aryl migration is cationic, not radical in nature. Therefore, this is consistent with a cobalt(III) complex being responsible for the oxidation of the alkylcobalt(III) intermediate. Given previously devised catalytic cycles for similar cobalt-catalyzed reactions, in-depth mechanistic studies by others, and our findings, we tentatively propose a mechanism for our enantioselective hydro-semipinacol rearrangement (Figure ). Wilson and Holland have shown that N-fluoropyridinium salts react with cobalt(II)-salen complexes to generate a fluoride-bridged Co(III) dimer. In wet solvents, this can be hydrolyzed to give a monometallic aquocobalt(III) species that was shown to be catalytically relevant and engages with a silane in the turnover-limiting step. However, in our system, we have used anhydrous chlorobenzene as a solvent thus it is more probable that the fluoride-bridged Co(III) dimer reacts directly with the silane or it releases monometallic entities to facilitate this step. Although we did not extensively dry the starting materials, reagents, or catalyst, so we cannot completely exclude the involvement of a water-bound cobalt(III) intermediate. The subsequently formed transient Co(III) hydride species reacts with the allylic alcohol in a Markovnikov-selective hydrogen atom transfer process to form an alkyl radical that is in equilibrium with an organocobalt(III) species. The reduced propensity for migration of electron-poor arenes supports the notion that these intermediates do not undergo 1,2-aryl migration. Instead, the alkyl-Co(III) derivative is feasibly oxidized by another cobalt(III) complex to generate an organocobalt(IV) intermediate, which is in agreement with our stoichiometric experiment (Figure ). The high enantioselectivity observed in this transformation corroborates that the 1,2-aryl migration occurs via stereospecific cleavage of the configurationally defined carbon-cobalt bond rather than a free carbocation intermediate formed after heterolysis. [55-57] The r value (-2.57) obtained from the Hammett analysis confirms the intermediacy of a phenonium ion, which leads to an enantioenriched α-aryl ketone after the loss of a proton.