Dispersive Fluorine/Bromine Interactions as Key Selectivity Determinant: Asymmetric Cyclopropanations with 3,3,3-Trifluoro-2-diazopropionate Catalyzed by a Heterochiral-at-The-Metal Centers Dirhodium Paddlewheel Complex

Asymmetric cyclopropanation reactions of 3,3,3-trifluoro-2-diazopropionate have so far been largely beyond the scope of dirhodium catalysts. This significant gap in coverage is now closed with the aid of a new class of paddlewheel complex with "backbone chirality", in which an array of only three different achiral ligands (acetamidate, trifluoroacetate, calix[4]arene dicarboxylate) renders the two rhodium centers inequivalent and chiral-at-the-metal each. The -NH group of the acetamidate engages the ester carbonyl of the metal carbene intermediate into favorable interligand hydrogen bonding, which in turn controls the trajectory of the incoming olefinic reaction partner. In addition, a dispersive interaction between the -CF₃ group flanking the carbene center and a strategically placed bromine substituent on the calixarene dicarboxylate locks the conformation of the reactive intermediate in place within the chiral binding site, which translates into high diastereoselectivity and optical purity alike. The same type of interhalogen London dispersion interaction can also benefit [2 + 1] cycloadditions of other fluorinated carbene donors.