Extending functional group tolerance to include precursors not amenable to SNAr has been a focus of dierent research groups[116, 117]. To this end, diaryliodonium salts have been investigated heavily in the past 20 years, as these species can be employed for direct aromatic radiouorination. Diaryliodonium salts are typically designed with an unreactive aryl auxiliary, and selectivity is generally observed for the most electron-decient aryl substituent. However, regioselectivity issues can be encountered, and the auxiliary ring of electron-rich meta -substituted substrates can inadvertently uorinate, decreasing the reaction eciency[116, 118]. To overcome this, a method using (2-thienyl)aryliodonium salts has been reported (Scheme8.10)[116]. In this study, electron-rich aryl moieties were radiouorinated selectively when the corresponding diaryliodonium precursor contained an electron-rich 2-thienyl group, and this selectivity had previously been unattainable. However, the true reaction selectivity can be dicult to determine accurately due to the volatility of 2-uorothiophene. In general, o - and p- substituted substrates can display good reactivity toward radiouorination, while the functionalization of meta-substituted substrates is generally low yielding[119]. Electron-decient substrates can be slow to radiouorinate, although increased yields can be observed if a microreactor is used[118]. Issues of selectivity in unsymmetrical diaryliodonium salts are inuenced by steric con­gestion around the iodine center, which has been termed the “ortho-eect”[119–123].