Radical Fluoromethylation Enabled by Cobalamin-Dependent Radical SAM Enzymes.

Fluorine is an important atom in many drugs because it can improve the efficacy and metabolic stability of many molecules. Strategies to incorporate monofluoromethyl groups in drugs have been limited and have received less attention than strategies for difluoromethylation or trifluoromethylation. Previously, we and others reported the enzymatic monofluoromethylation of several biologically relevant metabolites based on the transfer of a fluoromethyl group from analogs of S-adenosylmethionine (SAM) to various nucleophiles (carbon, oxygen, nitrogen, sulfur, and carbon) through a polar S_N2 mechanism. However, this strategy is limited to molecules containing nucleophilic target atoms. Inspired by a subset of enzymes within the radical SAM superfamily that can methylate inert carbon atoms, we developed an enzymatic strategy to transfer fluoromethyl groups to unactivated carbon atoms. This strategy leverages the ability of halide methyltransferase to generate a transient fluoromethyl-containing SAM analog. Our studies show that S-adenosyl-L-(fluoromethyl)-methionine can undergo reductive cleavage to a 5'-deoxyadenosyl 5'-radical, which initiates radical-dependent fluoromethylation through substrate hydrogen-atom abstraction. Adding fluoromethyl groups to unactivated C-H bonds using radical SAM enzymes is a powerful approach that can be used to derivatize molecules of interest where S_N2-based fluoromethylation is precluded.