On the Mechanism of Cyclopropanation Reactions Catalyzed by a Rhodium(I) Catalyst Bearing a Chelating Imine-Functionalized NHC Ligand: A Computational Study

Abstract

A computational study at the DFT level using BP86 and dispersion-corrected D-BP86 methods has been performed on the mechanism of a highly cis-diastereoselective cyclopropanation reaction between ethyl diazoacetate and styrene, catalyzed by a Rh(I) complex bearing a chelating imine-functionalized NHC ligand. The key steps in the mechanism have been elucidated. The favored mechanistic pathway was found to be a stepwise mechanism involving the formation of a Rh metallacyclobutane intermediate. The results from the theoretical study indicate that the diastereoselectivity is determined in the step where styrene coordinates to the Rh(I) carbenoid and that the high cis-diastereoselectivity can be attributed to an unfavorable steric interaction between styrene and the substituents on the N-aryl ring in the ligand system, which disfavors the formation of the trans cyclopropanation product.