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

The selective transition-metal-mediated activation of C(sp(2))-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.

Automated summary

In this study, a Pd-catalyzed C-H alkenylation reaction of electronically unbiased allenes was reported, providing penta-1,2,4-triene products in up to 94% yield. The use of picolinamide directing group enabled the formation of allenyl-palladacycles, which subsequently participated in a turnover-limiting Heck-type reaction with electron-deficient alkene coupling partners. Additionally, picolinamide N,O-acetals were introduced as readily removable auxiliaries for C-H activation reactions, facilitating the efficient alkenylation of allenyl carbinol derivatives while maintaining the reactivity of the products.