While iodide and bromide are thermodynamically capable of reducing Ir(IV) to Ir(III), chloride is outside of the redox window for Ir(IV) reduction (0.98 V vs SCE; See SI, Table ). Because of this, we hypothesized that using tetrabutylammonium chloride (TBACl) as an additive would give no increase in conversion relative to the additive free reaction. To our surprise, the coupling with TBACl as an additive significantly outperformed the other additive screens, reaching 96% conversion in 18 hours (entry 6). The same increase in reactivity was observed when using TBACl with 4-BrBzCF3 as the substrate (entry 7), giving 89% conversion in 3 hours compared to 42% (entry 1) without additive. It is also worth noting that fluoride gave no conversion in a similar screen (See SI, Section 6.4). To further understand the role of Kinetics and Nickel Speciation with Halide Additives. In Rovis and coworker's recent deaminative cross-coupling, chloride significantly improved the system. They hypothesized that one potential role of chloride additive was stopping catalyst death by preventing aggregation. This was supported by the reaction going to high conversion (~90%) with chloride while plateauing at low conversion (~50%) with no additive. However, we did not see this same trend in our kinetics studies. For 4-BrBzCF3, the rate was pseudo-first order with no additive and reached high conversion (Fig. ), suggesting that prevention of catalyst death is not the main role of chloride in our system. When running the reaction with 1 eq. TBACl, there was a 4.7-fold increase in kobs and a further 2.2-fold increase at elevated temperature (35 °C vs 50 °C), likely due to faster thermal oxidative addition. Thus, further studies were performed to elucidate the role of the additive.