Pd(II)/Lewis acid catalyzed oxidative C–H olefination/annulation with dioxygen to construct dihydrophenanthridines and its mechanistic studies

Transition-metal-catalyzed oxidative C–H olefination/annulation reactions are among the most attractive topics in organic synthesis to construct heterocyclic compounds. The challenge is that, in order to achieve satisfied catalytic efficiencies, most of these syntheses employed stoichiometric oxidants. Herein, we report a Pd(II)/LA-catalyzed (LA: Lewis acid) oxidative C–H olefination/annulation reaction of N-Ts-2-arylanilines with activated olefins to synthesize dihydrophenanthridines by using dioxygen balloon as the sole oxidant source, and the presence of LA sharply improved the catalytic efficiency of Pd(OAc)2. The mechanistic studies by using in situ generated palladacycle compound as the catalyst for the substrate H/D exchange disclosed that the C–H activation by Pd(II) was a reversible step. However, in the catalytic C–H olefination/annulation reaction, whether deuterium from solvent being incorporated into the recycled substrate was highly dependent on the aromatic electronic effect of the substrates.