Although Pd(II)-catalyzed C−H activation in arenes has been widely successful in organic synthesis with many palladacycle compounds isolated as the intermediates in liganddirected C−H activation, direct identification of the reaction intermediates such as the π-complex prior to the C−H activation is still not successful because of their instability. In the present study, we introduce a Pd(II)/LA (LA: Lewis acid)-catalyzed oxidative olefination/annulation reaction between N-methoxybenzamides and acrylates with oxygen as the oxidant source, in which two intermediates, including an unsymmetrical η 6 -complex and a palladacycle species without the proton releasing to the environment, were identified through NMR characterizations. The in situ formation of the heterobimetallic Pd(II)/LA species such as Pd(II)/Sc(III) may have enhanced the electrophilic properties of the Pd 2+ cation, thus improving the stability of the π-complex, herein, an unsymmetrical η 6 -complex, and improving its catalytic efficiency. The observed insensitive electronic effect preferred the concerted metalation−deprotonation (CMD) mechanism for this C−H activation, and the detected palladacycle intermediate without the proton releasing to the environment offered an experimental clue to support the proposed CMD mechanism.

Transition-metal ion catalyzed intramolecular dual C−H activation to construct polycyclic heteroarene skeletons is merited for its step and atom-economic advantages in organic synthesis. However, in most cases, stoichiometric oxidants, elevated temperature, and other harsh conditions were commonly faced for this reaction, which apparently block the synthetic applications. Herein, we report a Pd(II)/LA (LA: Lewis acid) catalyzed intramolecular dual C−H activation to construct indoloquinolinone derivatives under mild conditions with dioxygen as the sole oxidant. It was found that adding LA such as Sc 3+ to Pd(OAc) 2 sharply improved its catalytic efficiency, whereas Pd(OAc) 2 alone was very sluggish. The activity improvement was attributed to the linkage of the Sc 3+ cation to the Pd(II) species through a diacetate bridge that significantly enhanced the electrophilic properties of Pd(II) for dual C−H activation.

Direct C−H activation and functionalization offer a convenient protocol for pharmaceutical and material syntheses. Although versatile mechanisms have been proposed to depict transition-metal-catalyzed C−H activation, to date, the shared key agostic hydrogen intermediate in several major mechanisms has not been observed yet, which apparently puzzles the mechanism-based catalyst design. This work reports the direct observations of this intermediate in Pd(II)/Sc(III)-catalyzed C−H activation of acetanilides, and its stability and reactivity in C−H activation are investigated. Remarkably, this intermediate is only observed in electron-rich acetanilides, and the metasubstituent with increased σ m constant generally accelerates C−H activation, a characteristic of the base-assisted C−H activation mechanism. This study has unveiled the masks of this intermediate with an understanding of its first-hand physicochemical properties, shedding new light on mechanism-based catalyst design.

The present work introduces Pd(II)/LA-catalyzed (LA: Lewis acid) olefination of arylacetamides with dioxygen as the oxidant source. This protocol tolerates with different functional groups on the substrates, and the catalytic efficiency is highly Lewis acidity-dependent on added LA, that is, a stronger LA provided a better promotional effect. The 1 H NMR studies of the semireaction between the arylacetamide and the Pd(II)/Sc(III) catalyst in HOAc-d 4 disclosed the formation of a palladacycle intermediate, and the C−H activation step was reversible, which led to the formation of the deuterated arylacetamide substrate and the palladacycle intermediate. Further semireaction between the palladacycle intermediate and the olefin disclosed that it was a clean and much faster reaction than the C−H activation step, thus revealing multiple mechanistic information for Pd(II)-catalyzed C−H activation.

Transition metal ions catalyzed indole olefination through C-H activation has offered a convenient protocol to synthesize versatile bioactive vinylindole compounds, however, in most cases, stoichiometric oxidants were employed to achieve...