We propose the Fe(III) salt forms octahedral PyBOX complex 10 in the presence of the ligand L4 (see supplementary information S90 for mass spectrometry data consistent with this complex). This can undergo ligand exchange to form a complex in which the naphthol binds in a bidentate fashion (see supplementary information S91 for mass spectrometry evidence for this species). Oxidation to Fe(IV) complex 11 occurs with di-tert-butylperoxide, (which also liberates a tert-butoxy radical); subsequent reversible single electron transfer (SET) generates an Fe(III) ligated naphthoxy radical 12. We propose that indole radicals 13 can be generated from indoles in the presence of Fe(III) complex 10 and an external oxidant (Figure ) or by SET from Fe(IV) complex 11. This radical may be complexed (reversibly) to the Fe(III) center which would confer extra stability to this species and potentially render it persistent ; this, in conjunction with the extremely low concentration of this species, is consistent with our observation that the homocoupled 3,3'-bisindole is not a product of this reaction. We believe that it is likely this oxidation does not play a significant role in the cross-coupling reaction. Katsuki has previously noted the necessity of two cis-sites on the Fe centre being available to enable binding of two naphthols for cross-coupling to generate BINOLs 46 . In our system, binding both an indole and a bidentate naphthol on the same metal center makes the requirement for close approach of the indole to its naphthoate partner extremely challenging from a geometric perspective. As such, we tentatively propose that the addition of the indole could occur via an outer sphere mechanism in which the key facially selective addition to the Fe(III) naphthoxy radical species is directed by the C2-symmetric ligand as in 14. The resultant radical could subsequently undergo hydrogen atom abstraction or oxidation to afford 15 , followed by ligand exchange to enable release of the enantioenriched heterobiaryl system 3 and an Fe(III) complex able to continue the catalytic cycle. Although we have proposed an outer sphere coupling between a chiral Fe-bound naphthoxy radical and an indole, this does not preclude the possibility of an alternative cycle involving an indole radical being operative in the catalytic reaction, and further mechanistic studies are underway.