Chiral Crown Conformation of Rh2(S-PTTL)4: Enantioselective Cyclopropanation with α-Alkyl-α-diazoesters

Abstract

Herein, we provide crystallographic and computational evidence that Hashimoto’s Rh₂(S-PTTL)₄ catalyst adopts a “chiral crown” conformation with a reactive chiral face and an unreactive achiral face. In Rh₂(S-PTTL)₄, all four tert-butyl groups are aligned on the same face of the catalyst, and each C−(t-Bu) bond is roughly parallel to the central Rh−Rh bond. This orients the four phthalimido groups on the opposite face of the catalyst. Also described is an enantioselective and diastereoselective protocol for intermolecular Rh₂(S-PTTL)₄-catalyzed cyclopropanation using α-alkyl-α-diazoesters. Intermolecular cyclopropanation is selective over two competing intramolecular pathways: C−H insertion and β-hydride elimination. On the basis of density functional theory calculations and the Davies−Singleton model for cyclopropanation, a model for asymmetric induction is proposed.