Asymmetric Cyclopropanation Using Dimethyl Diazomalonate

chiral dienes - cyclopropanation - rhodium

Significance

<P>Many well-developed dirhodium(II) carboxamidates and carboxylates exist for highly enantioselective cyclopropanations of alkenes with diazo compounds (see Review below). Due to often low reactivity and enantioselectivity no highly asymmetric cyclopropanation using diazo­malonates exists. The authors discovered that chiral cyclopropane gem-diesters can be prepared in good yields and enantioselectivies by ­using a cationic chiral rhodium-diene complex.</P><P> Review: H. Pellissier Tetrahedron 2008, 64, 7041-7095.</P>

Comment

<P>The authors screened a number of existing ligands for the title transformation, but an effective catalyst was not found. By placing amide groups on the ortho position of their previously developed tetrafluorobenzobarrelene ligands, good yields and enantioselectivities were obtained. X-ray crystallographic analyses of the complexes show a coordination of one of the amide oxygens to rhodium in the neutral complex, and both amide oxygens coordinating in the in situ formed cationic species. This structural feature is proposed to be crucial for the high reactivity.</P>