Accessing planar chiral ferrocenes via transient directing group-enabled C–H alkenylation under Pd(II) catalysis

Pd-catalyzed asymmetric C–H functionalization is a privileged method for the synthesis of planar chiral ferrocenes. Previous examples under this category virtually rely on a preinstalled strong directing group. The transient directing strategy has witnessed considerable success in recent years. However, only limited examples have been reported for the synthesis of planar chiral ferrocenes. Herein, we report an asymmetric C–H alkenylation of ferrocenes with an array of electron-deficient olefins under Pd catalysis with l-tert-leucine as the transient-directing auxiliary. This reaction exhibits a wide substrate scope, and the target planar chiral ferrocenes are prepared in good yields (up to 85%) with exceptional enantioselectivity (up to >99% ee). Comprehensive mechanistic studies suggest that the storage of ring strain in a seven-membered palladacycle after C–H activation is the key to the success of our reaction design. It guarantees the migratory insertion as a kinetically feasible step that occurs in a strain-releasing manner.