Samarium Diiodide-mediated Enantioselective Radical Additions

3 Samarium Diiodide-mediated Enantioselective Radical Additions 

Recently, enantioselective additions of samarium diiodide-generated ketyl radicals to olefins have been demonstrated [15]. As illustrated in Eq. (10), the reductive coupling of acetophenone with methyl acrylate (31a) in the presence of chiral phosphine ligand 29 (i -BINAPO) gives somewhat low yields (mostly under 50%) but moderate to good levels of enantioselectivity (60-70% ee) for the y-butyrolactone products 34. 

Since samarium diiodide is only a one electron donor, two equivalents of the metal are required in order for the reaction to proceed. The first electron donated from the samarium produces a chiral ketyl radical 30 which undergoes enantioselective addition to the acrylate according to the chelated transition state shown in 32. The second electron donation then provides a chiral samarium enolate intermediate 33 that can potentially undergo stereoselective proton transfer in the formation of a second chiral center. 

Chiral samarium (II) complexes have also been applied towards the hydrodimerization of acrylic acid amides [16]. Such reactions involve the ligand-controlled dimerization of conjugated ketyl radicals in the enantioselective formation of 3,4-tra .y-disubstituted adipamides (Eq. 11). Yields were mainly low, often under 40% and enantiocontrol was modest with selectivities ranging from around 50-85% ee. A nine-membered chelated transition state 37 is used to rationalize the stereoselectivity of the dimerization where the ligand-bound conjugated ketyl radicals are oriented cis to each other on the metal assuming an octahedral geometry. 

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S.2.3 Samarium Diiodide-mediated Enantioselective Radical Additions... 

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