Once 11 is the lowest of four low-spin square-pyramidal Cu(III) isomers that all lie with 4 kcal/mol; all these structures are at least 17 kcal/mol less stable when computed as a high-spin triplet. Ph-I bond forming reductive elimination from 11 then proceeds with a minimal barrier of 1.1 kcal/mol to form an initial h 2 -PhI complex at +10.5 kcal/mol (see Figure ). Dissociation of the PhI product forms Cu(phen)(OAc), 12, at which AcO -/I -substitution and dimerization yields [Cu(µ-I)(phen)]2 6, at -7.9 kcal/mol. The DFT modelling studies indicate that bis-phen species are implicated both prior to ([3] + ) and after ([8] + ) the transmetalation step and that it is the disproportionation step that forms a mono-phen Cu(III) intermediate, 11. Both the transmetalation and the disproportionation steps are slightly endergonic, but the kinetically facile Ph-I reductive elimination from 11 drives the reaction to completion once coupled with the thermodynamically favorable formation of Cu(I) dimer 6. The AcOB(OH)2 side-product formed in the computed mechanism would be readily hydrolyzed under the reaction conditions releasing AcOH. AcOH facilitates the Cu(I) oxidative turnover step with O2 to re-form [3] + (see Figure ) 15-17 and releasing AcO -, which is then available for the boronate formation that enables the transmetalation step under catalytic turnover.