Excitation of aryl cations as the key to catalyst-free radical arylations

While Sandmeyer-type brominations/iodinations and Meerwein-type arylations are useful methodologies, they are not without their drawbacks. Here, we report an alternative catalyst- and additive-free access to Sandmeyer-type brominations/iodinations and to a diverse set of Meerwein-type arylations. The key for the highly selective transformations is the excitation of slowly and thermally formed aryldiazonium-derived aryl cations by blue LEDs, conditions that differ from the known photodissociation pathway. An excited singlet state (diradical character) of the cation is shown to be the key intermediate, which interacts with the solvent MeOH to eventually form the sp2-centered chain-initiating radical. Our results demonstrate a facile, mild, atom-economic, sustainable, and economically attractive route for a broad spectrum of aryl radical arylations, with methanol as the solvent and blue LED irradiation being the only restrictions. In addition, this methodology can be directly pursued from the abundant feedstock, the anilines.