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Pd/Xu-Phos-Catalyzed Synthesis of Alkenyl Allenes via β-H Elimination of 1,3-Dien-1-yl Triflates
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DOI：10.1002/cjoc.70652.png)
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Abstract
En 中文
Alkenyl allenes represent a privileged subclass of allenes; the conjugation between the cumulated diene and the adjacent alkenyl group imparts distinctive chemical behaviors, rendering them valuable precursors for complex molecular architectures via transformations such as Diels–Alder cycloadditions and Nazarov cyclizations. Palladium-catalyzed β-hydride (β-H) elimination represents a fundamental and well-established strategy in organic synthesis, extensively utilized for the preparation of allenoates and aryl allenes. However, extending this powerful transformation to the synthesis of conjugated alkenyl allenes has historically proven to be a formidable challenge. This synthetic difficulty arises primarily from the intrinsic thermodynamic instability of the alkenyl allene motif, which is notoriously prone to rapid isomerization into more stable conjugated polyenes, often resulting in complex mixtures of byproducts and preventing the isolation of the desired compound. To address this long-standing limitation and achieve high selectivity, we herein report a robust and highly efficient protocol for the selective synthesis of alkenyl allenes via the Pd/Xu-Phos-catalyzed β-H elimination of 1,3-dien-1-yl triflates. Central to the success of this method is the identification and application of the sterically tuned Xu-Phos ligand. This bulky, electron-rich phosphine ligand effectively facilitates the challenging elimination step, while simultaneously suppressing the competitive hydropalladation and subsequent isomerization pathways that have plagued previous attempts. The reaction is proposed to proceed through an acetate-assisted intramolecular deprotonation manifold. Consequently, this methodology features exceptionally mild reaction conditions and demonstrates broad functional group compatibility. It provides general access to diverse di- and trisubstituted alkenyl allenes in high yields and excellent selectivities. These products serve as pivotal precursors for the construction of complex bicyclic scaffolds via subsequent cyclization reactions, thereby opening new avenues for the synthesis of bioactive natural products and pharmaceuticals.
Keywords:
Palladium
Alkenyl allene
β-H elimination
Enol triflate
Catalysis
Xu-Phos
Alkenyl palladium
Selective synthesis
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