The Role of Additive and Solvent Coordination in Sm(II) Reactions

Abstract The tunable reactivity of Sm(II) has fascinated chemists over the last few decades and has led to a plethora of Sm(II)-based reagent combinations. Although many mechanistic studies have been performed to date, a complete understanding of the principles governing the interaction of Sm(II) with solvent, additive, and substrate has remained evasive. The series of mechanistic studies described in this dissertation were aimed to isolate and observe the role of each individual reaction component. The impact of solvent on the reactivity of Sm(II) was examined by measuring the rate of reduction of a series of substrates in both coordinating and non-coordinating solvents using the highly soluble reductant {Sm[N(SiMe3)2]2(THF)2}. When SmI2 is combined with a proton source, such as water or ethylene glycol, the reagent combination is capable of reducing substrates well outside the reducing power of SmI2. Until recently, it was proposed that these reactions took place through sequential electron-proton transfer, but recent mechanistic studies have demonstrated that many of these reductions occur through a concerted process of proton-coupled electron-transfer (PCET). Through PCET, high-energy intermediates are bypassed, enabling highly endergonic reactions to proceed. Mechanistic studies were performed to examine the reduction of both coordinating and non-coordinating substrates as well as reductive cyclizations using the SmI2-H2O reagent combination and its ability to promote formal hydrogen atom transfer. Then, the use of alternative proton donors was employed to further elucidate the mechanism behind the unique reactivity of H2O. Finally, using the information gathered from the previous studies, new reagent combinations have been realized. Ideally, these studies will provide an even more useful reagent for the synthesis of complex organic molecules and contribute to a more complete understanding of the reactivity of low valent metals.