Pd-Catalyzed Umpolung of π–Allylpalladium Intermediates: Assembly of All-Carbon α‑Vinyl Quaternary Aldehydes through C(sp³)–C(sp³) Coupling

Construction of sterically congested all-carbon quaternary centers represents a formidable challenge in synthetic chemistry. The method described herein provides direct and facile access to a series of structurally diverse and synthetically useful aliphatic aldehydes, bearing an all-carbon α-vinyl quaternary center and a 1,5-diene functionality, through Pd-catalyzed umpolung of vinylethylene carbonates (VECs). The reaction features electrophilic-to-nucleophilic reactivity reversal of the VEC-derived π-allyl-palladium intermediate via an unusual β-hydride elimination process, and the resultant enolate is chemoselectively coupled with allylic acetate to form an α-vinyl aldehyde embedded with an all-carbon quaternary center.空间位阻拥挤的全碳季碳中心的构建，是合成化学领域极具挑战性的研究课题。本文所述合成方法可通过钯（Pd）催化的碳酸乙烯烯丙酯（vinylethylene carbonates, VECs）极性反转过程，直接且便捷地获得一系列结构多样、合成应用价值优异的脂肪醛类化合物；此类脂肪醛均带有α-乙烯基全碳季碳中心与1,5-二烯官能团。该反应的核心特征在于：经由一种非常规的β-氢消除过程，实现了VEC衍生的π-烯丙基钯中间体（π-allyl-palladium intermediate）的反应活性从亲电性向亲核性的反转；所得烯醇负离子可与烯丙基乙酸酯发生化学选择性偶联，从而得到嵌入全碳季碳中心的α-乙烯基醛类产物。