C(sp

The “magic methyl” effect describes the change in potency, selectivity, and/or metabolic stability of a drug candidate associated with addition of a single methyl group. Here, we report a synthetic method that enables direct methylation of C(sp(3))–H bonds in diverse drug-like molecules and pharmaceutical building blocks. Visible-light-initiated triplet energy transfer promotes homolysis of the O–O bond in di-tert-butyl or dicumyl peroxide under mild conditions. The resulting alkoxyl radicals undergo divergent reactivity, either hydrogen-atom transfer from a substrate C–H bond or generation of a methyl radical via beta-scission. The relative rates of these steps may be tuned by varying the reaction conditions or peroxide substituents to optimize the yield of methylated product arising from nickel-mediated cross-coupling of substrate and methyl radicals.