Rhodium(III)‐Catalyzed Asymmetric Synthesis of Silanes and Cyclopropanes From Diazo Compounds Using Chiral <i>α</i> ‐Pinene‐Derived Indenyl Ligands

Abstract

ABSTRACT Rhodium‐catalyzed asymmetric organic reactions are efficient tools for the synthesis of pharmaceuticals and natural compounds, but the search for readily available chiral catalysts remains a challenge. In this work, we describe a straightforward synthesis of a set of chiral tetrahydrofluorenyl rhodium(III) complexes bearing donor substituents on the benzene ring. These complexes are readily accessible via a simple four‐step procedure starting from naturally occurring (−)‐α‐pinene, without the need for multi‐step and time‐consuming techniques such as chiral HPLC or diastereomeric resolution. With the dimethyl‐substituted complex, the first asymmetric reactions of diazo compounds with hydrosilanes and electron‐rich alkenes catalyzed by Rh(III)‐complexes were performed, giving the corresponding chiral silanes and cyclopropanes (up to 96% ee). The complexes also showed excellent results in asymmetric C─H annulation of aryl hydroxamates with alkenes, affording various chiral dihydroisoquinolones (up to 97% ee). DFT calculations revealed that methoxy groups exert a crucial influence on the stereoselectivity of both the C─H activation step and the alkene insertion step. The developed protocol was used for the synthesis of precursors of protoberberine alkaloids ( S )‐tetrahydropalmatine and ( S )‐O‐methylbharatamine.