Chloroiridium Complexes of Biaryl-Based Diphosphines for Thermal Catalytic Transfer Dehydrogenation of Hindered 1,1-Disubstituted Ethanes

Thermal catalytic dehydrogenation of congested alkanes, such as the 1,1-disubstituted ethane (1,1-DSE) motif, is still a largely unsolved challenge. Herein, we report the investigation of biaryl-based atropisomeric diphosphines with various electronic and steric properties and their corresponding chloroiridium complexes for catalytic transfer dehydrogenation (CTD) of 1,1-DSEs using tert-butylethene as H-acceptor. The use of diphosphines of the biaryl backbone with a narrow dihedral angle was found to be essential to forming an effective catalytic species, cis-P2ClIr. A systematic investigation of three atropisomeric diphosphine classes, Segphos, MeO-Biphep, and Binap, revealed that the sterically hindered and electron-rich ligands are more efficient than the less hindered and electron-deficient ones. With an understanding of the structure–activity relationships, we developed two highly encumbered ligands 3,5‑TMS,4‑MeOMeO-Biphep (M12) and 3‑TIPS,5‑TMSMeO-Biphep (M13), which afforded >1400 turnovers at 150 °C in the cumene/TBE CTD reaction, representing the most efficient catalyst for thermal catalytic dehydrogenation of 1,1-DSEs. Moreover, these two ligands produced ∼420 turnovers in the CTD of 2-(1-adamantyl)­propane, demonstrating their ability in dehydrogenation of unactivated hindered dialkyl-substituted ethanes.