SET from CO2 •-to the substrate must be sufficiently fast to outcompete SET from CO2 •-to the oxidized form of the mediator. Note that this does not require that the rate constant for SET from CO2 •-to the substrate is larger than the rate constant for SET from CO2 •-to the oxidized form of the mediator. Rather, the rate constant for substrate reduction by CO2 •-relative to the rate constant for reduction of the oxidized form of the mediator by CO2 •-must be sufficiently large. Specifically, optimized values in this work find that the ratio kf 17 /kf 15 = 4 x 10 -2 is suitable for oxidative reduction to proceed. Alternatively, if kf 17 = 1 x 10 5 , as is the case for one of the simulations shown in Figure , then kf 17 /kf 15 = 1 x 10 -3 and 4-BrBN reduction is not observed. Further qualitative evidence of this can be seen by comparison of Figures and where debromination products are obtained in higher yield than dechlorination products, where SET from CO2 •-to a C-Cl bond is expected to be slower than SET from CO2 •-to a C-Br bond.