Accessing Unusual Reactivity through Chelation-Promoted Bond Weakening

[Image: see text] Highly reducing Sm(II) reductants and protic ligands were used as a platform to ascertain the relationship between low-valent metal-protic ligand affinity and degree of ligand X–H bond weakening with the goal of forming potent proton-coupled electron transfer (PCET) reductants. Among the Sm(II)-protic ligand reductant systems investigated, the samarium dibromide N-methylethanolamine (SmBr(2)-NMEA) reagent system displayed the best combination of metal–ligand affinity and stability against H(2) evolution. The use of SmBr(2)-NMEA afforded the reduction of a range of substrates that are typically recalcitrant to single-electron reduction including alkynes, lactones, and arenes as stable as biphenyl. Moreover, the unique role of NMEA as a chelating ligand for Sm(II) was demonstrated by the reductive cyclization of unactivated esters bearing pendant olefins in contrast to the SmBr(2)-water-amine system. Finally, the SmBr(2)-NMEA reagent system was found to reduce substrates analogous to key intermediates in the nitrogen fixation process. These results reveal SmBr(2)-NMEA to be a powerful reductant for a wide range of challenging substrates and demonstrate the potential for the rational design of PCET reagents with exceptionally weak X–H bonds.