Palladium-Catalyzed Oxidative Allene-Allene Cross-Coupling.

Direct cross-coupling reactions between two similar unactivated partners are challenging but constitute a powerful strategy for the creation of new carbon-carbon bonds in organic synthesis. [4]Dendralenes are a class of acyclic branched conjugated oligoenes with great synthetic potential for the rapid generation of structural complexity, yet the chemistry of [4]dendralenes remains an unexplored field due to their limited accessibility. Herein, we report a highly selective palladium-catalyzed oxidative cross-coupling of two allenes with the presence of a directing olefin in one of the allenes, enabling the facile synthesis of a broad range of functionalized [4]dendralenes in a convergent modular manner. Specifically, the selective allenic C-H activation of an allene with an allyl substituent as the assisting group gives rise to a vinylpalladium intermediate, which reacts with a less substituted allene in a carbopalladation, followed by a β-hydride elimination. The reaction sequence leads to a new C(sp)-C(sp) bond between two diene units. Remarkably, this protocol provides an unconventional strategy for the site-selective and stereoselective construction of C(vinyl)-C(vinyl) bonds without using any halogenated and organometallics olefin precursors. Furthermore, the practical transformations of the synthesized [4]dendralenes and late-stage modifications of biorelevant molecules demonstrate their potential in the total synthesis of natural products and drug discovery.