We propose that the low concentration of Fe III (acac)3 forms the active iron(III)-ethoxide catalyst Fe-3 (Step C), which reacts with PhSiH3 in the turnover-limiting step to form iron-hydride Fe-4 (Step D). Following HAT to the olefin I, an alkyl radical II is generated (Step E). PhSH then coordinates to the resulting iron(II) intermediate Fe-5 to form an iron(II) thiol complex Fe-6 (Step F). Alkyl radical II is quenched via PCET from Fe-6, facilitated by the lowered S-H BDFE upon binding to iron, to generate the hydrogenated product III and the iron(III)-thiolate complex Fe-7 (Step G). Finally, Fe-7 undergoes ligand exchange to regenerate PhSH and Fe-3. This mechanism is supported by observation of Fe II (acac)2 and an iron(III)-thiolate species by Mössbauer spectroscopy and stoichiometric reactions shown in Figure . Evaluating the Potential for Enantioinduction. In the catalytic cycle shown in Scheme 4, a stereogenic center is set when radical II reacts with a formal H-atom donor. Accordingly, we envisioned that use of a chiral thiol may allow Step G to serve as an enantiodetermining step. To assess the possibility of enantioselective catalysis, we designed a small set of prochiral substrates. In the design, we took inspiration from multiple sources, opting for a heterocyclic substrate with exocyclic olefin modeled after the lactone substrates utilized by Roberts and coworkers. 18a, The exocyclic olefins that lead to the products shown in Scheme 5 also would proceed through intermediate radicals that would bear structural homology to the alkyl radical intermediate accessed in recent work of Miller, Knowles, and coworkers, in which enantioselective HAT from a thiol was integral to substrate deracemization. With synthetic accessibility in mind, we thus prepared the N-aryl cyclic ureas and N-aryl thiazolidines 3ad. Of note, thiazolidines are biologically-active structural scaffolds with various documented applications. Intriguingly, we found minimal to low conversion with the cyclic ureas 3a-b, potentially arising from inhibition through coordination with the iron catalyst. However, the thiazolidines were well tolerated, affording higher conversion to product. In particular, the methylated thiazolidine 3d reached 92% conversion to product 4d within 48 hours and was therefore prioritized for further study.