Ligand-Accelerated Catalysis in Scandium(III)-Catalyzed Asymmetric Spiroannulation Reactions
Ligand-Accelerated Catalysis in Scandium(III)-Catalyzed Asymmetric Spiroannulation Reactions
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DOI：
10.1021/acscatal.1c05768
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发表时间：
2022-03
期刊：
影响因子：
12.9
通讯作者：
Nicolas R. Ball‐Jones;Angel A. Cobo;Brittany M. Armstrong;Benjamin Wigman;J. Fettinger;J. Hein;A. Franz
中科院分区：
文献类型：
--
作者：
Nicolas R. Ball‐Jones;Angel A. Cobo;Brittany M. Armstrong;Benjamin Wigman;J. Fettinger;J. Hein;A. Franz
通过反应热流量热法、核磁共振和原位红外光谱分析，提出了钪催化不对称烯丙基硅烷环化反应的机理。用反应量热法研究了钪(III) -PyBox /BArF催化剂的性质，揭示了溶液中和沉淀催化剂之间复杂的相互作用。在加入双齿亲电试剂之前，钪(III) -PyBox /BArF催化剂是可溶的。最佳反应速率取决于催化剂的预络合作用、催化剂组分的络合顺序和亲核试剂的延迟加成。活性催化剂的形成是通过钪（III）与BArF阴离子的双分子结合，然后与PyBox配体络合，其中观察到配体依赖的速率和选择性。值得注意的是，观察到配体加速催化作用，这归因于配体减少了烯丙基硅烷的非循环寡聚。讨论了BArF在反应速率和对映体选择性中的作用，重点讨论了反离子的来源。我们还报道了由烯丙基硅烷与烷基基氧吲哚反应生成的机制相关的熔融四氢吡喃吲哚的形成。原位红外光谱证明了配体依赖的加速度，其中立体要求配体以更快的相对反应速率进行。
A mechanism for the scandium-catalyzed asymmetric allylsilane annulation reaction is proposed and supported by reaction heat flow calorimetry, NMR, and in situ infrared spectroscopy. The nature of a scandium(III)–PyBox/BArF catalyst is probed using reaction calorimetric analysis, which reveals a complex interplay between in-solution and precipitated catalyst species. The scandium(III)–PyBox/BArF catalyst is minimally soluble until the addition of a bidentate electrophile. The optimal reaction rate is dependent on precomplexation of the catalyst, order of complexation of the catalyst components, and delayed addition of nucleophile. The formation of the active catalyst proceeds through a bimolecular combination of scandium(III) with a BArF anion, followed by complexation with PyBox ligand, where the ligand-dependent rate and selectivity are observed. Notably, ligand-accelerated catalysis is observed, attributed to the ligand reducing off-cycle oligomerization of allylsilane. The role of BArF is discussed with a specific focus on the source of counterion in the reaction rate and enantioselectivity. We also report the formation of a mechanistically relevant fused tetrahydropyranindole produced upon the reaction of an allylsilane with alkylidene oxindole. In situ infrared spectroscopy demonstrates ligand-dependent acceleration where sterically demanding ligands perform with a faster relative reaction rate.