A Horseradish Peroxidase–Mediator System for Benzylic C–H Activation

Enzyme–mediator systems generate radical intermediates that abstract hydrogen atoms under mild conditions. These systems have been employed extensively for alcohol oxidation, primarily in biomass degradation, but they are underexplored for direct activation of C(sp3)–H bonds in alkyl groups. Here, we combine horseradish peroxidase (HRP), H2O2, and redox mediator N-hydroxyphthalimide (NHPI) for C(sp3)–H functionalization of alkylbenzene-type substrates. The HRP–NHPI system is >10-fold more active than existing enzyme–mediator systems in converting alkylbenzenes to ketones and aldehydes under air, and it operates from 0–50 °C and in numerous aqueous–organic solvent mixtures. The benzylic substrate radical can be trapped through a reaction with NHPI, demonstrating the formation of benzylic products beyond ketones. Furthermore, we demonstrate a one-pot, two-step enzymatic cascade for converting alkylbenzenes to benzylic amines. Overall, the HRP–NHPI system enables the selective benzylic C–H functionalization of diverse substrates under mild conditions using a straightforward procedure.