Allene C(sp2)–H Activation and Alkenylation Catalyzed by Palladium

The selective transition-metal-mediated activation of C­(sp2)–H bonds of allenes is a formidable challenge because of the competitive, intrinsic reactivity of cumulated double bonds. Herein, we report a Pd-catalyzed C–H alkenylation of electronically unbiased allenes, affording penta-1,2,4-triene products in up to 94% yield. A picolin­amide directing group enables the formation of putative allenyl-pallada­cycles, which subsequently participate in a turnover-limiting Heck-type reaction with electron-deficient alkene coupling partners. This mechanistic proposal is consistent with experimental and computational investigations. Additionally, we report for the first time the use of picolin­amide N,O-acetals as readily removable auxiliaries for C–H activation reactions, allowing the efficient alkenylation of allenyl carbinol derivatives. Successful removal of the directing groups without affecting the reactive penta-1,2,4-triene substructure of the products is demonstrated.