Can the Radical Channel Contribute to the Catalytic Cycle of N-Heterocyclic Carbene in Benzoin Condensation?
Can the Radical Channel Contribute to the Catalytic Cycle of N-Heterocyclic Carbene in Benzoin Condensation?
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DOI：
10.1021/acs.joc.8b02483
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发表时间：
2018-12-21
影响因子：
3.6
通讯作者：
Yu, Jen-Shiang K.
中科院分区：
文献类型：
--
作者：
Hsieh, Ming-Hsiu;Huang, Gou-Tao;Yu, Jen-Shiang K.
NHC可以通过关键的Breslow中间体催化安息香缩合反应。EPR光谱最近证实了自由基物种的存在，但其催化作用仍不清楚。在这里，我们使用密度泛函方法来研究自由基相关的途径与非自由基的机制之前报道的比较。理论研究表明，非自由基路径（Δ G（双匕首）= 18.7千卡/摩尔）比自由基路径（Δ G（双匕首）= 27.6千卡/摩尔），这是由苯甲醛从Breslow中间体的氢提取，导致自由基对初始化的动力学更有利。因此，产物的形成主要是由非自由基途径。此外，Breslow中间体的稳定性不及酮基，阻碍了苯偶姻缩合反应的进行，自由基物种在协助互变异构反应和促进催化反应中起着重要的作用.
NHC can catalyze benzoin condensation via the key Breslow intermediate. EPR spectroscopy recently confirmed the existence of the radical species, but its catalytic role is still unclear. Herein, we use density functional approaches to study the radical-associated pathway in comparison with the nonradical mechanism reported previously. Theoretical investigations show that the nonradical path (Delta G(double dagger) = 18.7 kcal/mol) is more kinetically favorable than the radical route (Delta G(double dagger) = 27.6 kcal/mol), which is initialized by the hydrogen abstraction from the Breslow intermediate by benzaldehyde, leading to a radical pair. The product formation is thus dominated by the nonradical pathway. In addition, the Breslow intermediate is less stable than its keto form, which blocks the benzoin condensation, and the radical species could play an important role in assisting the tautomerization and promoting the catalytic reaction.