Catalyst- and Silane-Controlled Enantioselective Hydrofunctionalization of Alkenes by Cobalt-Catalyzed Hydrogen Atom Transfer and Radical-Polar Crossover
Catalyst- and Silane-Controlled Enantioselective Hydrofunctionalization of Alkenes by Cobalt-Catalyzed Hydrogen Atom Transfer and Radical-Polar Crossover
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DOI：
10.1021/jacs.0c05017
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发表时间：
2020-08-05
影响因子：
15
通讯作者：
Shigehisa, Hiroki
中科院分区：
文献类型：
--
作者：
Ebisawa, Kousuke;Izumi, Kana;Shigehisa, Hiroki
本文描述了通过过渡金属催化的氢原子转移（TM-HAT）和自由基极性交叉（RPC）机制催化对映选择性合成四氢呋喃（存在于许多具有生物活性的天然产物的结构中）。使用合适的手性钴催化剂N-氟-2，4，6-可力丁四氟硼酸盐和二乙基硅烷有效地进行非共轭烯烃的氢烷氧基化，具有优异的对映选择性（高达94%ee）。令人惊讶的是，产物的绝对构型高度依赖于硅烷的空间位阻。缓慢添加的硅烷，分子氧对溶剂的影响，热依赖性，和DFT计算结果支持两个竞争的选择性机制的前所未有的情况。对于较少受阻的二乙基硅烷，高浓度的扩散的碳为中心的自由基调用非对映体富集的烷基钴（III）中间体通过自由基链反应，这最终确定的绝对配置的产品。另一方面，更受阻的硅烷导致自由基链反应的机会较少，而不是促进对映选择性动力学决议在后期阶段的亲核取代的烷基钴（IV）中间体。
The catalytic enantioselective synthesis of tetrahydrofurans, which are found in the structures of many biologically active natural products, via a transition-metal-catalyzed hydrogen atom transfer (TM-HAT) and radical-polar crossover (RPC) mechanism is described herein. Hydroalkoxylation of nonconjugated alkenes proceeded efficiently with excellent enantioselectivity (up to 94% ee) using a suitable chiral cobalt catalyst, N-fluoro-2,4,6-collidinium tetrafluoroborate, and diethylsilane. Surprisingly, the absolute configuration of the product was highly dependent on the steric hindrance of the silane. Slow addition of the silane, the dioxygen effect on the solvent, thermal dependence, and DFT calculation results supported the unprecedented scenario of two competing selective mechanisms. For the less-hindered diethylsilane, a high concentration of diffused carbon-centered radicals invoked diastereoenrichment of an alkylcobalt(III) intermediate by a radical chain reaction, which eventually determined the absolute configuration of the product. On the other hand, a more hindered silane resulted in less opportunity for a radical chain reaction, instead facilitating enantioselective kinetic resolution during the late-stage nucleophilic displacement of the alkylcobalt(IV) intermediate.