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.