Origins of Enantioselectivity in Asymmetric Radical Additions to Octahedral Chiral-at-Rhodium Enolates: A Computational Study

The origin of asymmetric induction in the additions of carbon- and nitrogen-centered radicals to octahedral centrochiral rhodium enolates has been investigated with density functional theory calculations. Computed free energies of activation reproduce the preference for the experimentally observed major enantiomer. Good levels of enantioselectivity are maintained upon replacement of the bulky tert-butyl substituents on the ligands with methyl groups. Distortion interaction analysis indicates that for both carbon- and nitrogen-centered radicals, which have relatively early and late transition states, respectively, the difference in the distortion energies controls the enantioselectivity. In the enolate derived from the Lambda-configured catalyst, the tert-butyl group that shields the si face of the substrate plays the most sterically significant steric role by directly hindering access to the enolate double bond. Exploration of the effect of the N substituent size and shape on the imidazole substrate shows that compared to N-Me, N-iPr and N-Ph variants, the N-o-tolyl variant of the rhodium enolate results in the most substantial improvement in stereodiscrimination, a finding that is in agreement with experimental ee values.