Palladium(II)/Lewis Acid-Catalyzed Oxidative Olefination/Annulation of <i>N</i>-Methoxybenzamides: Identifying the Active Intermediates through NMR Characterizations

Abstract

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 ligand-directed 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 η6-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 Pd2+ cation, thus improving the stability of the π-complex, herein, an unsymmetrical η6-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.