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

Supporting Info

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

Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts, 02138

J. Am. Chem. Soc., 2004, 126 (29), pp 8916–8918

DOI: 10.1021/ja047064k

Publication Date (Web): June 30, 2004

In papers with more than one author, the asterisk indicates the name of the author to whom inquiries about the paper should be addressed.

, corey@chemistry.harvard.edu

Abstract

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 C₂-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.

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