Details

Autor(en) / Beteiligte

• Lou, Yan
• Horikawa, Manabu
• Kloster, Robin A
• Hawryluk, Natalie A
• Corey, E. J

Titel

A New Chiral Rh(II) Catalyst for Enantioselective [2 + 1]-Cycloaddition. Mechanistic Implications and Applications

Ist Teil von

• Journal of the American Chemical Society, 2004-07, Vol.126 (29), p.8916-8918

Ort / Verlag

Washington, DC: American Chemical Society

Erscheinungsjahr

2004

Quelle

Alma/SFX Local Collection

Beschreibungen/Notizen

• A novel chiral Rh(II) catalyst (1) is introduced for the [2 + 1]-cycloaddition of ethyl diazoacetate to terminal acetylenes and olefins with high enantioselectivity. The catalyst 1 consists of one acetate bridging group and three mono-N-triflyldiphenylimidazoline-2-one bidentate ligands (DPTI) spanning the Rh(II)−Rh(II) metallic center in a structure that was determined by single-crystal X-ray diffraction analysis. A rational mechanism is advanced that provides a straightforward explanation for the enantioselectivity and absolute stereochemical course of the [2 + 1]-cycloaddition reactions. A key element in this explanation is the cleavage of one of the Rh−O bonds of the bridging acetate group in the intermediate Rh−carbene complex to form a new pentacoordinate Rh carbene complex (formally 1.5 valent Rh) that can undergo [2 + 2]-cycloaddition with the C−C π-bond of the acetylenic or olefinic substrate. Reductive elimination of the resulting adduct affords the cyclopropene or cyclopropane product. The C 2-symmetry of the two DPTI ligands orthogonal to the bridging acetate also contributes to the high observed enantioselectivity and mechanistic clarity. The catalyst 1, which functions effectively at 0.5 mol %, can be recovered efficiently for reuse. Its ready availability, robustness, and effectiveness suggest it as a useful addition to the list of practical chiral Rh(II) catalysts for synthesis.