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 Mossbauer 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.