The Photocatalyzed Aza-Henry Reaction of N-Aryltetrahydroisoquinolines: Comprehensive Mechanism, H•-versus H+-Abstraction, and Background Reactions
The Photocatalyzed Aza-Henry Reaction of N-Aryltetrahydroisoquinolines: Comprehensive Mechanism, H•-versus H+-Abstraction, and Background Reactions
复制标题
DOI：
10.1021/jacs.6b06658
复制
发表时间：
2016-09-14
影响因子：
15
通讯作者：
Gschwind, Ruth M.
中科院分区：
文献类型：
--
作者：
Bartling, Hanna;Eisenhofer, Anna;Gschwind, Ruth M.
N-芳基四氢异喹啉（THIQ）的交叉偶联（CDC）反应是最受关注的光催化转化反应之一，也是各种催化剂的测试反应。然而，其机制仍不清楚涉及的中间体，胺自由基阳离子的反应途径和氧气和光源的影响。因此，核磁共振（NMR），电子自旋共振（ESR）和合成方法相结合，提供了一个全面的图片使用Ru（bpy）（3）Cl-2作为光催化剂在有氧和厌氧条件下的反应机理。通过NMR光谱监测和分析反应曲线和涉及的中间体。鉴定了几种有助于产物形成的中间体，亚胺离子、氢过氧化物和THIQ的二聚物以及一种新的开环中间体;在苄基C-N键处裂解。给出了在厌氧条件下优先的α-氨基自由基是由去质子化形成的，相反，通过H-中心点提取在氧的存在下形成的亚胺离子的机械证据。此外，在没有催化剂的情况下，光诱导的背景反应进行了详细研究，揭示了产物形成速率与光源的强度和波长相关，并且氧是有效转化所必需的。反应速率和效率与先前报道的光催化系统相当，该系统在有氧条件下与强蓝色光源组合进行。因此，众多的反应参数的研究揭示了在光反应中的氢原子或质子抽象的偏好，并允许评估实验条件对机械途径的影响。
The cross-dehydrogenative coupling (CDC) reaction of N-aryltetrahydroisoquinolines (THIQ) is one of the most exploited photocatalytic transformation and a test reaction for an exceptional Variety of catalysts. However, its mechanism remained unclear concerning involved intermediates, reactive pathways of the amine radical cation and the influence of oxygen and the light source. Therefore, nuclear magnetic resonance (NMR), electron spin resonance (ESR) and synthetic method's were combined to provide a comprehensive picture of the reaction mechanism using Ru(bpy)(3)Cl-2 as a photocatalyst under aerobic and anaerobic conditions. The reaction profiles and involved intermediates were monitored and analyzed by NMR spectroscopy. Several intermediates contributing to product formation were identified, the iminium ion, the hydroperoxide and dimes of THIQ and a new ring opened intermediate; cleaved at the benzylic C-N bond. Mechanistic evidence is given that under anaerobic conditions preferentially the alpha-amino radical is formed by deprotonation, in contrast to the formation of iminium ions via H-center dot-abstraction in the presence of oxygen. Further, the light-induced background reaction in the absence of the catalyst was studied in detail, revealing that the product formation rate is correlated to the intensity and wavelength of the light source and that oxygen is essential for an efficient conversion. The reaction rate and efficiency is comparable to previously repotted photocatalytic systems, performed under aerobic conditions in combination with intense blue light sources. Thus, the multitude of reaction parameters investigated reveals the preference for hydrogen atom or proton abstraction in photoreactions and allows to assess the influence of experimental conditions on the mechanistic pathways.