To investigate the ability of TDAE to serve as a mediator, TDAE 2+ (PF6 -)2 was prepared by aerobic oxidation of TDAE in the presence of TMSBr (see SI). CV of TDAE 2+ (PF6 -)2 in MeCN revealed two freely-diffusing reversible single-electron redox features at -1.05 V and -1.13V, consistent with previous literature reports (Figure , ii). Upon addition of L7•Cr III , the cathodic peaks corresponding to TDAE 2+ reduction increase in current and there is concomitant loss of the anodic features associated with the TDAE 0 / TDAE +• and TDAE +• / TDAE 2+ oxidations, consistent with loss of TDAE +• by chemical reaction with L7•Cr III (EC mechanism) (Figure , iv, v). The reduction of TDAE 2+ is less cathodic than both substrates (1 and 2) and L7•Cr III , consistent with a scenario where TDAE 2+ undergoes preferential cathodic reduction. Additional CV studies were carried out to evaluate whether TDAE can also mediate reduction of either RAE 2 or aldehyde 1. Addition of up to 10 equivalents of RAE 2 to TDAE 2+ (PF6 -)2 in the absence of TESCl led to a negligible current increase (Figure , vi). An increase in current was observed in the presence of TESCl (Figure , vii); however, this feature disappeared after the first scan in a manner consistent with a trace impurity in the TESCl, which we ascribe to HCl. In a recent review, Waldvogel notes challenges of CV studies of silyl halides to due to their facile hydrolysis to generate HCl. We have previously reported that the combination of TDAE and silyl halides induces reductive decarboxylation of NHP esters, but that TDAE/TESCl was determined to reduce benzylic NHP esters at rates that are slow relative to other silyl halides. No significant current increase were observed upon addition of aldehyde 1 (100 equiv) to TDAE 2+ (PF6 -)2 (Figure , viii). This mechanistic scheme was further supported by finite element simulations of the voltammetry (Figure , ix). Simulations of both TDAE 2+ and L7•Cr III in solution with no mediation step provided a simulated CV with a clear shoulder at -1.13 V corresponding to the TDAE + /TDAE 0 couple, a feature completely absent in the experimental CVs. Incorporation of an association step between TDAE + and L7•Cr III into the simulation provided a voltammogram with no associated TDAE + /TDAE 0 wave, providing evidence of a reaction between the reduced TDAE + and the Cr complex. Finally, incorporation of a turnover step (generating the reduced L7•Cr II and regenerating TDAE 2+ ) once again resulted in the TDAE + /TDAE 0 wave, leading to the conclusion that dissociation of TDAE + is slow, but still orders of magnitude faster than the direct reduction of L7•Cr III (full simulation details can be found in the SI). TDAE is known to form chargetransfer complexes with organic molecules and metal surfaces. Collectively, these results are consistent with TDAE serving as an electrochemical mediator to reduce L7•Cr III . It is also possible that TDAE can scavenge trace impurities such as HCl or O2 that could decompose intermediates in the catalytic cycle. The latter observation is corroborated by the generally improved performance of TDAE over TDAE 2+ in the reaction, which may result from the capability of TDAE to scavenge trace impurities before electrolysis is commenced. In principle, if TDAE mediates reduction of L7•Cr III , then it should be possible to use stoichiometric TDAE to drive the reaction L7•Cr II in the absence of current. Indeed, TDAE has been used as the stoichiometric reductant for Cr-catalyzed addition of alkenyl bromides and allyl bromides to aldehydes. 42 However, during the optimization process, <10% yield 3 was observed using stoichiometric TDAE and no electricity (see Table , entry 10). Based on a recent report by Wenger and coworkers in which TDAE +• was invoked as an H-atom source, we hypothesized that with high concentrations of TDAE +• (as under the stoichiometric conditions), hydrogen atom transfer (HAT) to the primary alkyl radical derived from 2 outcompetes addition of this species to L7•Cr II to generate the alkyl Cr III species. In contrast, prior work from the Reisman lab showed that benzylic radicals undergo radical-radical dimerization faster than HAT in the presence of TDAE +• . We ascribed this difference in reactivity to the difference in stability of the primary and benzylic radicals. This also highlights the enabling nature of using catalytic TDAE under electrochemical conditions: while TDAE +• can mediate reduction of L7•Cr III , its presence in high concentrations can intercept the radical generated from the RAE and prevent productive coupling. This is a distinct challenge for the alkyl NHK, which proceeds via formation of highly reactive alkyl radicals, relative to prior work.