Copper-Catalyzed\nSite-Selective C(sp<sup>2</sup>)–H\nAlkynylation of Allenes

Conjugated allenynes serve as prominently featured structural\nunits\nin various natural products. However, the absence of an efficient\nand adaptable synthesis has hindered their broader application in\nsynthetic endeavors and biological investigations. In terms of atom-\nand step-economy, there is an urgent need for an efficient strategy\nwith a broader substrate scope compared to previous reports. Herein,\nwe report a copper-catalyzed allenic C(sp<sup>2</sup>)–H alkynylation\nof an array of allenes with exceptional site selectivity. While the\nalkynylation of aliphatic C(sp<sup>3</sup>)–H bonds has been\nextensively studied, radical-mediated regioselective alkynylation\nof allenyl C(sp<sup>2</sup>)–H bonds via direct hydrogen atom\ntransfer (HAT) from the C(sp<sup>2</sup>)–H bond of allenes\nremains an unresolved challenge in synthetic chemistry. This difficulty\narises from inherent issues such as the addition of radical intermediates\nto allenes or alkynes and the regioselectivity of the HAT process.\nOur protocol addresses these challenges and demonstrates a broader\nsubstrate scope, tolerance toward various functional groups, including\nactive sp<sup>3</sup> C–H bonds such as benzyl groups and ethers,\nand results in a high diversity of products.