Rhodium(III)-Catalyzed Asymmetric Synthesis of Silanes and Cyclopropanes From Diazo Compounds Using Chiral α-Pinene-Derived Indenyl Ligands.

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.