Reversible Formation of Alkyl Radicals at [Fe4S4] Clusters and Its Implications for Selectivity in Radical SAM Enzymes

All kingdoms of life use the transient 5'-deoxyadenosyl radical (5'-dAdo center dot) to initiate a wide range of difficult chemical reactions. Because of its high reactivity, the 5'-dAdo center dot must be generated in a controlled manner to abstract a specific H atom and avoid unproductive reactions. In radical S-adenosylmethionine (SAM) enzymes, the 5'-dAdo center dot is formed upon reduction of SAM by an [Fe4S4] cluster. An organometallic precursor featuring an Fe-C bond between the [Fe4S4] cluster and the 5'-dAdo group was recently characterized and shown to be competent for substrate radical generation, presumably via Fe-C bond homolysis. Such reactivity is without precedent for Fe-S clusters. Here, we show that synthetic [Fe4S4]-alkyl clusters undergo Fe-C bond homolysis when the alkylated Fe site has a suitable coordination number, thereby providing support for the intermediacy of organometallic species in radical SAM enzymes. Addition of pyridine donors to [(IMes)(3)Fe4S4-R](+) clusters (R = alkyl or benzyl; IMes = 1,3-dimesitylimidazol-2-ylidene) generates R center dot, ultimately forming R-R coupled hydrocarbons. This process is facile at room temperature and allows for the generation of highly reactive radicals including primary carbon radicals. Mechanistic studies, including use of the 5-hexenyl radical clock, demonstrate that Fe-C bond homolysis occurs reversibly. Using these experimental insights and kinetic simulations, we evaluate the circumstances in which an organometallic intermediate can direct the 5'-dAdo center dot toward productive H-atom abstraction. Our findings demonstrate that reversible homolysis of even weak M-C bonds is a feasible protective mechanism for the 5'-dAdo center dot that can allow selective X-H bond activation in both radical SAM and adenosylcobalamin enzymes.