Enantioselective Access to γ‑All-Carbon Quaternary Center-Containing Cyclohexanones by Palladium-Catalyzed Desymmetrization
journal contributionposted on 05.12.2019 by Qiang Wei, Jinhui Cai, Xu-Dong Hu, Jing Zhao, Hengjiang Cong, Chao Zheng, Wen-Bo Liu
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An efficient desymmetrization of γ-quaternary carbon-containing cyclohexanones using readily available Pd/(S)-tBuPhox and benzylamine as dual catalysts is reported. We describe herein the development of the reaction, exploration of the substrate scope, and studies on the reaction mechanism. The intramolecular coupling reaction leads to the formation of bicyclo[3.3.1]nonanones with a quaternary carbon bridgehead in synthetically useful yields (up to 98%) with high enantioselectivities (up to 98:2 er) and good functional group tolerance (>30 examples). Significantly, aryl and alkenyl bromides as well as less reactive triflates are all compatible substrates for this process. The synthetic versatility of this strategy is demonstrated by scale-up synthesis and diverse transformations of the products into valuable building blocks, including quaternary center-containing dihydronaphthalenes, ring-fused indoles and lactones, tetralones, and 6,6,5-tricycles. Mechanistic studies by computational calculations provide insights into the role of benzylamine in accelerating the reaction rate and enhancing the enantioselectivities.