Exclusively Ligand-Redox-Promoted C–H Tertiary Alkylation of Heteroarenes

Herein, we describe a predominant role of the redox-active formazanate ligand backbone in steering base metal iron catalysis. The iron complex of the chosen formazanate ligand exhibits speciation comprising two species with high spin, S = 5/2 Fe(III), which have been probed thoroughly by zero-field Mössbauer and X-band electron paramagnetic resonance (EPR) spectroscopies at low temperatures. The one-electron oxidation of the bulk sample proves a completely ligand-based process, as examined by these spectroscopic techniques. The ligand-redox process has been exploited to develop an iron catalyst used for C–H tertiary alkylation for a host of heterocycles and styrenes. The efficiency of such ligand-promoted catalysis is further attested by only 1 mol % catalyst loading, which affords products in high yields. Plausibly, the vacant site at Fe(III) helps in substrate binding, leading to reductive bond cleavage of a substrate C–Br bond, while the electron for this purpose is entirely provided by the formazanate backbone. Several key intermediate isolations support the radical process and delineate the mechanism for C–H alkylative transformation, proving the great utility of the ligand redox in executing such a process.