In the presence of the Fe(III) catalyst, a different species also consistent with an indole radical can be observed (g = 2.00265). This lacks the 14 N hyperfine structure observed previously, most likely due to reduced nitrogen character in the wavefunction and rapid relaxation as a consequence of being proximal to the metal centre no other radical species apart from Fe(III) were observable (see supplementary information S96-97). We were also able to capture the indole radical species by the addition of trapping agents triethylphosphite and 5,5-dimethylpyrroline-Noxide. This led to the observation of adducts of the proposed indole radical (by HRMS ESI); the corresponding naphthol adducts were not observed. (see supplementary information S92-93). We also determined the oxidation potentials of the indoles used in the synthesis of 3r-3v (Figure ) and found that the presence of electron withdrawing groups had a significant impact: the oxidation potential of the unsubstituted indole is 0.71 V, whereas this value rises to 1.22 V for the 5-cyano derivative. This is consistent with the electronic nature of the 5-substituent on the indole limiting the ease of oxidation, which would impact on the rate of formation of indole radicals. However, the spin concentration was very low throughout our ESR experiments, and we considered that our observation of the indole radical was potentially an off-cycle event occurring independently of the cross-coupling. This is consistent with the lack of formation of the homocoupled indole product under the reaction conditions in the absence of the phenol component, where the indole starting material can be isolated unchanged. In contrast, we were able to isolate the homocoupled BINOL derivative in 66% yield when naphthol 1 was treated under the reaction conditions in the absence of indole 2. The formation of this product likely occurs via the reaction of a ligated naphthoxy radical, which is trapped by a naphthol as a p-nucleophile , and we considered whether this mechanism could be operative for our observed heterocoupling. This led us to consider whether the divergent reactivity of 5-substituted indoles observed previously might be explained by the relative nucleophilicity of these substrates. Mayr has determined nucleophilicity parameters for indoles, which demonstrate that electron withdrawing groups in the 5-position lead to a significant reduction in rates of attack upon a standardized electrophile . This is coherent with observations from Baran who showed that reactions between indoles and ketone-derived radicals were less efficient with electron-deficient indoles . To probe this further, we performed a Hammett analysis of the coupling reactions that yield 3p-3v. The Hammett plot (ratio of the initial reaction rate (k s /k u ) vs sp parameters) gave a linear graph with a negative slope (r = -0.49, R 2 = 0.99; see supplementary information S102). This is indicative of the build-up of positive charge on the indole during the rate-determining transition state and is consistent with its proposed role as a p-nucleophile. In cases where the conversion to the heterobiaryl is low (3r-3v), we were able to recover both unreacted indole and naphthol. This is consistent with slow trapping of the ligated naphthoxy radical with indole limiting the rate of reaction where the nucleophilicity is relatively low and is also reflected in the (incrementally) lower ratio of heterocoupled:homocoupled products when electron deficient indoles are used.