Chiral crown conformation of Rh(2)(S-PTTL)(4): enantioselective cyclopropanation with alpha-alkyl-alpha-diazoesters.

Herein, we provide crystallographic and computational evidence that Hashimoto's Rh(2)(S-PTTL)(4) catalyst adopts a "chiral crown" conformation with a reactive chiral face and an unreactive achiral face. In Rh(2)(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 Rh(2)(S-PTTL)(4)-catalyzed cyclopropanation using alpha-alkyl-alpha-diazoesters. Intermolecular cyclopropanation is selective over two competing intramolecular pathways: C-H insertion and beta-hydride elimination. On the basis of density functional theory calculations and the Davies-Singleton model for cyclopropanation, a model for asymmetric induction is proposed.