Mechanistic Study and Development of Catalytic Reactions of Sm(II).

Samarium diiodide (SmI) is one of the most widely used single-electron reductants available to organic chemists because it is effective in reducing and coupling a wide range of functional groups. Despite the broad utility and application of SmI in synthesis, the reagent is used in stoichiometric amounts and has a high molecular weight, resulting in a large amount of material being used for reactions requiring one or more equivalents of electrons. Although few approaches to develop catalytic reactions have been designed, they are not widely used or require specialized conditions. As a consequence, general solutions to develop catalytic reactions of Sm(II) remain elusive. Herein, we report mechanistic studies on catalytic reactions of Sm(II) employing a terminal magnesium reductant and trimethylsilyl chloride in concert with a noncoordinating proton donor source. Reactions using this approach permitted reductions with as little as 1 mol % Sm. Mechanistic studies provide strong evidence that during the reaction, SmI transforms into SmCl, therefore broadening the scope of accessible reactions. Furthermore, this mechanistic approach enabled catalysis employing HMPA as a ligand, facilitating the development of catalytic Sm(II) 5- exo- trig ketyl olefin cyclization reactions. The initial work described herein will enable further development of both useful and user-friendly catalytic reactions, a long-standing, but elusive goal in Sm(II) chemistry.