A palette of commonly used directing groups, including various pharmaceutically relevant nitrogen-containing heterocycles, are quantitatively ranked based on the results of intermolecular hydrogen isotope exchange competition reactions using two iridium complexes: [Ir(COD)(IMes)(PPh3)][BArF24] and [IrCl(COD)(IMes)]. The directing group power scales that have been constructed from these data reveal a wide range of reactivity covering four orders of magnitude. Intramolecular competition experiments have demonstrated that the obtained reactivity scale provides accurate predictions of regioselectivity within molecules with multiple competing directing groups. This work contributes to our understanding and control of regioselectivity in metal-catalysed C-H activation reactions.
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