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Mechanistic Evidence for Intermolecular Radical Carbonyl Additions Promoted by Samarium Diiodide

2006; American Chemical Society; Volume: 128; Issue: 30 Linguagem: Inglês

10.1021/ja060553v

1943-2984

Anna Mette Hansen, Karl B. Lindsay, P. K. Sudhadevi Antharjanam, Jakob Karaffa, Kim Daasbjerg, Robert A. Flowers, Troels Skrydstrup,

Oxidative Organic Chemistry Reactions

In this work, mechanistic studies were performed to understand the SmI2/H2O-mediated coupling of N-acyl oxazolidinones with acrylates and acrylamides, providing γ-keto esters and amides, respectively. Our results provide experimental evidence that C−C bond formation via intermolecular radical addition reactions to carbonyl substrates can be promoted by samarium diiodide. Coupling reactions with N-cyclopropylcarbonyl-2-oxazolidinone suggest the α,β-unsaturated esters/amides are reduced by the low-valent lanthanide reagent and not the N-acyl oxazolidinones, as originially proposed (J. Am. Chem. Soc. 2005, 127, 6544). Rate measurements support the preferred reduction of an acrylate or acrylamide by SmI2/H2O in the presence of an N-acyl oxazolidinone. In the absence of the N-acyl oxazolidinone, SmI2/H2O promotes dimerization of the acrylates, whereas the CC bond of the acrylamides is reduced. In addition, coupling of the Pfp ester of Cbz-protected phenylalanine with an acrylamide leads only to reduction of the acrylamide and recovered ester, whereas the same coupling with the N-acyl oxazolidinone derivative provides the γ-keto amides. These results imply that a pathway involving nucleophilic acyl substitution cannot take place and that a radical mechanism must be invoked to explain the C−C bond formation. We propose that the acrylate/acrylamide is reduced to a conjugated ketyl radical that adds to the exocyclic carbonyl group of the N-acyl oxazolidinone, activated through bidentate coordination to a lanthanide ion.