functionalization process under the reaction conditions, that is the turnover limiting step. To further understand the reaction mechanism, we employed DFT calculations using gaussian 16 (Figure ). The mechanism of reoxidation by MnO2 is poorly understood, and is likely a heterogeneous oxidant, therefore only a single turnover of Cu was considered to examine the elementary steps. Imine formation between Int-1 and 2-hydroxynicotinaldehyde was slightly endergonic, followed by an exergonic complexation with copper acetate to give Int-3 as the lowest energy intermediate prior to the turnover limiting C-H activation step. The C-H activation is calculated to proceed via a 5-coordinate inner sphere transition state (TS-3/4) in which an axial acetate ligand mediates C-H activation, with a barrier of 24.1 kcalmol -1 . This CDM process leads to the formation of cupracycle Int-4 which is converted to Int-5 by release of acetic acid. Oxidation of the sulfinate salt is calculated to occur readily by an SET process, mediated by copper acetate. CV studies indicate the sulfinate salt can be oxidized in the redox window of the reaction (CVs in HFIP vs Fc/Fc*: MeSO2Na, Epa = +1.02V; TolSO2Na Epa = +1.06V). Association of the sulfinyl radical to the copper center occurs a barrierless process converting Int-5 to Int-7. Reductive elimination via TS-7/8 forms the C-S bond and a Cu I species, with a barrier of +21.2 kcal mol -1 . The de-coordination of copper from Int-8 is slightly uphill in energy, releasing the product imine which can then be hydrolyzed to produce the product sulfonyl amine Int-10. The free amine directed pathway was also calculated with an additional acetate bound to Cu in place of the TDG, to indicate the influence of the TDG. Noticeably the energy barrier of the CMD process is significantly lowered in the presence of the TDG when compared to the free amine directed process (TDG: +24.1 kcalmol -1 ; free amine: +32.2 kcalmol -1 , see supporting information).