A Cascade C(sp3)-H Annulation Involving C(alkyl),C(alkyl)-Palladacycle Intermediates.

ABSTRACT

C-H bond functionalization involving C,C-palladacycle intermediates provides a unique platform for developing novel reactions. However, the vast majority of studies have been limited to the transformations of C(aryl),C-palladacycles. In sharp contrast, catalytic reactions involving C(alkyl),C(alkyl)-palladacycles have rarely been reported. Herein, we disclose an unprecedented cascade C(sp3)-H annulation involving C(alkyl),C(alkyl)-palladacycles. In this protocol, alkene-tethered cycloalkenyl bromides undergo intramolecular Heck/C(sp3)-H activation to generate C(alkyl),C(alkyl)-palladacycles, which can be captured by α-bromoacrylic acids to afford tricyclic fused pyridinediones. In addition, this strategy can also be applied to indole-tethered cycloalkenyl bromides to construct pentacyclic fused pyridinediones via suquential Heck dearomatization/C(sp3)-H activation/decarboxylative cyclization. Notably, the removal of α-bromoacrylic acids in the reaction of alkene-tethered cycloalkenyl bromides can build an interesting tricyclic skeleton containing a four-membered ring. Preliminary mechanistic experiments indicate that five-membered C(alkyl),C(alkyl)-palladacycles serve as the key intermediates. Meanwhile, density functional theory (DFT) calculations have provided insights into the reaction pathway.